VARIOUS GENERAL CONSIDERATIONS CONNECTED WITH LIGHTHOUSES.

In the course of supplying the numerous wants of navigation, it will often be found necessary to cut off, on a given bearing, the beam proceeding from a Lighthouse, as a guide to the seaman to avoid some shoal, or as a hint to put about and seek the opposite side of a channel. This is attended with some little practical difficulty, especially in lights from reflectors arranged externally on a circle, because a certain portion of light, chiefly due to the divergence caused by the size of the flames, and partly from the effects of the diffraction or inflexion of the light, spreads faintly over a narrow sector between the light arc and the dark one. Masking Lights. It becomes necessary, of course, to make allowance for this penumbral arc by increasing the masked portion of the lantern; and, where a very sharp line of demarcation is required, a board is sometimes placed on the outside of the Lightroom, in such a position, and of such length, that while it does not enter the boundaries of the luminous sector, it prevents the more powerful part of the penumbral beam from reaching the observer’s eye. This effect is, of course, more conveniently produced, where the circumstances admit of its adoption, by distributing the reflectors round the concave side of the lantern, towards the land; but such an arrangement is inapplicable when the illuminated sector exceeds the dark one. I have found, by observation, that the sector intercepted between the azimuth on which the lantern is masked and that on which total darkness is produced to an observer, at moderate distances, may be estimated at not less than 3° for dioptric, and 7° for catoptric lights of the highest class.[79]

[79] The method which I adopted for determining those quantities, was to mask a certain portion of the lantern of a lighthouse subtending an horizontal sector of about 30° or 40°, and at night to fix, by actual observation, at the distance of 5 or 6 miles, two points on the coast between which the light so masked was obscured. The angle included between the lines joining those points and the centre of the lantern was then determined by triangulation next day, and half the difference between the observed angle (which is always the lesser of the two) and the computed subtense of the masked sector of the lantern, is, in each case, the amount of the allowance stated in the text.

Those quantities may therefore serve to guide the Lighthouse engineer to approximate more rapidly to his object, as he will generally be safe in increasing the dark sector, by one or other of the above constants, according to the kind of apparatus employed. I need not add, that in a matter of this kind, a final appeal to actual observation is, in all cases, indispensable.

A few words on the subject of double lights, naturally spring out of what has been said about the masking of lights. Double Lights. The term double lights is properly and distinctly confined to lights on different levels, but not necessarily (as leading-lights are) in separate towers. The sole object of using double lights is for distinction from neighbouring lights; and they are unquestionably most effective in this respect, when they are placed in the same tower. In this point of view, therefore, I shall speak of them; and it is obvious that all that peculiarly belongs to them is, that the difference of level between them shall be sufficiently great to present the lights as separate objects to the eye of the seaman, when placed at the greatest distance at which it may be desirable that he should be able to recognise their characteristic appearance. In many cases it is not necessary (but it is certainly always desirable) that the lights should, from the first moment of their being seen, be known as double lights; but in others, it may well consist with safety, that two lights, which appear as a single light when first seen at the distance of 20 miles, shall at 15 or 10 miles distance be discovered to be double. Now we should at first be apt hastily to imagine, that all that is required to produce that effect, is to raise the one light above the other to such an extent, that the distance between them shall be somewhat more than a minimum visibile at the most distant point of observation, or, in other words, that the difference of the height of the lights should be such as to subtend to the eye at the point of observation, an angle greater than 13″·02, which is the subtense of a minimum visibile during the day.[80] But the effect of irradiation, to which I have already alluded, tends to blend together the images of the lights long before their distance apart has become so low a fraction of the observer’s distance from the Lighthouse, as to subtend so small an angle; and I have accordingly found by experiments, conducted under various circumstances, and at various distances, that repeated observations gave me 3′ 18″ as the mean of the subtenses calculated in reference to the distances at which the lights began to be blended into one.

[80] This quantity is deduced from observations made by my friend Mr James Gardner, while engaged on the Ordnance Survey, and may be regarded as the extreme limit of visibility, under the most favourable circumstances as to the state of the atmosphere and also the contrast of colours. The observed object, also, was a pole, not a round disc; and it is familiar to every one accustomed to view distant objects, that vertical length is an important constituent in their visibility.

Adopting this as the smallest angle which the two lights should subtend at the observer’s eye, we may find the least vertical distance between them which will cause them to appear as separate objects by the following formula:

H = 2 Δ . tan θ

in which Δ is the observer’s distance in feet; θ, half the subtense, = 1′ 39″; and H the required height of the tower between the two lights in feet. The following Table gives the height in feet corresponding to the distance in nautic miles, from 1 to 20 inclusive: the heights, which are the bases of similar isosceles triangles, increase, of course, in an arithmetical series:

Distance
of the
observer
in
Nautic
Miles.		Vertical
distance
in feet
between
the Lights.		Distance
of the
observer
in
Nautic
Miles.		Vertical
distance
in feet
between
the Lights.
 1 6·0211 66·22
 212·0412 72·24
 318·0613 78·26
 424·0814 84·28
 530·1015 90·30
 636·1216 96·32
 742·1417102·34
 848·1618108·36
 954·1819114·38
1060·2020120·40

Akin to the subject of Double Lights, is that of Leading Lights. Leading Lights, the object of which is to indicate to the mariner a given line of direction by their being seen in one line. In most instances, this line of direction is used to point out the central part of a narrow channel; and the alternate opening of the lights, on either side of their conjunction, serves to indicate to the mariner (who ought to conjoin with his watching of the lights the observation of the elapsed time and also frequent soundings) the proper moment for changing his tack. In some places, the line of conjunction of the lights is placed nearer to one side of a channel than the other, according as the set of the tides, or the position of shoals, may seem to require. In other situations, this line only serves as a cross-bearing to shew the mariner his approach to some danger, or to indicate his having passed it, and thus to assure him of his entry on wider sea-room. Similar considerations to those which determine the difference of elevation for double lights regulate the choice of the distance between two leading lights; but the question is less narrow, and may be generally solved graphically by simply drawing the lines on an accurate chart of the locality. In some few situations, the configuration of the coast does not admit of a separation between the lights, sufficient to cause what is called a sharp intersection; but, in most cases, there is room enough to place them so far apart, that but a few yards of deviation in the vessel’s course, from the exact line of the conjunction of the lights in one, produces a distinct opening between them on the opposite side of that line. In order to insure the requisite sharpness of intersection, the distance between the lights, wherever attainable, should be not less than one-sixth of the distance between the more seaward of the two Towers and that point at which the seaman begins to use the line of conjunction as his guide. I have only to add, that in situations where the land prevents a considerable separation between leading lights, they should be placed as nearly on one level as is consistent with their being seen as vertically separated, so as in some measure to compensate for their horizontal nearness, by rendering their intersection more sharp and striking than it can be where the observer must draw from the upper light an imaginary perpendicular in his mind, and then estimate the separation of the lights by the sine of an angle, which decreases as the difference of their apparent elevations increases.

Distribution of Lights on a Coast. The considerations which enter into the choice of the position and character of the Lights on a line of coast, are either, on the one hand, so simple and self-evident as scarcely to admit of being stated in a general form, without becoming mere truisms; or are, on the other hand, so very numerous and often so complicated as scarcely to be susceptible of compression into any general laws. I shall not, therefore, do more than very briefly allude to a few of the chief considerations which should guide us in the selection of the sites and characteristic appearance of the Lighthouses to be placed on a line of coast. Perhaps those views may be most conveniently stated in the form of distinct propositions:—

1. The most prominent points of a line of coast, or those first made on over-sea voyages, should be first lighted; and the most powerful lights should be adapted to them, so that they may be discovered by the mariner as long as possible before his reaching land.

2. So far as is consistent with a due attention to distinction, revolving lights of some description, which are necessarily more powerful than fixed lights, should be employed at the outposts on a line of coast.

3. Lights of precisely identical character and appearance should not, if possible, occur within a less distance than 100 miles of each other on the same line of coast, which is made by over-sea vessels.

4. In all cases, the distinction of colour should never be adopted except from absolute necessity.

5. Fixed lights and others of less power, may be more readily adopted in narrow seas, because the range of the lights in such situations is generally less than that of open sea-lights.

6. In narrow seas also, the distance between lights of the same appearance may often be safely reduced within much lower limits than is desirable for the greater sea-lights; and there are many instances in which the distance separating lights of the same character need not exceed 50 miles, and there are peculiar cases in which even a much less separation between similar lights may be sufficient.

7. Lights intended to guard vessels from reefs, shoals, or other dangers, should in every case be placed, where practicable, to the seaward of the danger itself, as it is desirable that seamen be enabled to make the lights with confidence.

8. Views of economy in the first cost of a Lighthouse should never be permitted to interfere with placing it in the best possible position; and, when funds are deficient, it will generally be found that the wisest course is to delay the work until a sum shall have been obtained sufficient for the erection of the lighthouse on the best site.

9. The elevation of the lantern above the sea should not, if possible, for sea-lights, exceed 200 feet; and about 150 feet is sufficient, under almost any circumstances, to give the range which is required. Lights placed on high headlands are subject to be frequently wrapped in fog, and are often thereby rendered useless, at times when lights on a lower level might be perfectly efficient. But this rule must not, and indeed cannot, be strictly followed, especially on the British coast, where there are so many projecting cliffs, which, while they subject the lights placed on them to occasional obscuration by fog, would also entirely and permanently hide from view lights placed on the lower land adjoining them. In such cases, all that can be done is carefully to weigh all the circumstances of the locality, and choose that site for the lighthouse which seems to afford the greatest balance of advantage to navigation. As might be expected, in questions of this kind, the opinions of the most experienced persons are often very conflicting, according to the value which is set on the various elements which enter into the inquiry.

10. The best position for a sea-light ought rarely to be neglected for the sake of some neighbouring port, however important or influential; and the interests of navigation, as well as the true welfare of the port itself, will generally be much better served by placing the sea-light where it ought to be, and adding, on a smaller scale, such subsidiary lights as the channel leading to the entrance of the port may require.

11. It may be held as a general maxim, that the fewer lights that can be employed in the illumination of a coast the better, not only on the score of economy, but also of real efficiency. Every light needlessly erected may, in certain circumstances, become a source of confusion to the mariner, and, in the event of another light being required in the neighbourhood, it becomes a deduction from the means of distinguishing it from the lights which existed previous to its establishment. By the needless erection of a new Lighthouse, therefore, we not only expend public treasure, but waste the means of distinction among the neighbouring lights.

12. Distinctions of lights, founded upon the minute estimation of intervals of time between flashes, and especially on the measurement of the duration of light and dark periods, are less satisfactory to the great majority of coasting seamen, and are more liable to derangement by atmospheric changes, than those distinctions which are founded on what may more properly be called the characteristic appearance of the lights, in which the times for the recurrence of certain appearances differ so widely from each other as not to require for their detection any very minute observation in a stormy night. Thus, for example, flashing lights of five seconds interval, and revolving lights of half a minute, one minute, and two minutes, are much more characteristic than those which are distinguished from each other by intervals varying according to a slower series of 5″, 10″, 20″, 40″, &c.

13. Harbour and local lights, which have a circumscribed range, should generally be fixed instead of revolving; and may often, for the same reason, be safely distinguished by coloured media. In many cases also, where the purpose of guiding into a narrow channel is to be gained, the leading lights which are used, should, at the same time, be so arranged as to serve for a distinction from any neighbouring lights.

14. Floating lights, which are very expensive and more or less uncertain from their liability to drift from their moorings, as well as defective in power, should never be employed to indicate a turning point in a navigation in any situation where the conjunction of lights on the shore can be applied at any reasonable expense.

Height of Lighthouse Tower, and its relation to range of Light. The spheroïdal form of the Earth requires that the height of a Lighthouse Tower should increase proportionally to the difference between the Earth’s radius and the secant of the angle intercepted between the normal to the spheroïd at the Lighthouse and the normal at the point of the light’s occultation from the view of a distant observer. The effect of atmospheric refraction, however, is too considerable to be neglected in estimating the range of a light, or in computing the height of a Tower which is required to give to any light a given range; and we must, therefore, in accordance with the influence of this element, on the one hand increase the range due to any given height, and vice versa reduce the height required for any given range, which a simple consideration of the form of the globe would assign. In considering this height, we may proceed as follows:—

Fig. 92.

Referring to the accompanying figure ([No. 92]), in which S′ d L′ is a segment of the ocean’s surface, O the centre of the earth, L′L a Lighthouse, and S the position of the mariner’s eye, we obtain the value of LL′ = H′, the height of the tower in feet by the formula,

H′ = 2 l² 3 (1.)

in which l = the distance in English miles L′d at which the light would strike the ocean’s surface. We then reduce this value of H′ by the correction for mean refraction, which permits the light to be seen at a greater distance, and which =

2 l² 21, (2.)

So as to get, H = 2 l²3 - 2 l²21 = 4 l²7 (3.)

an expression which at once gives the height of the tower required, if the eye of the mariner were just on the surface of the water at d, where the tangent between his eye at S and the light at L would touch the sea. We must, therefore, in the first instance, find the distance d S = l′, which is the radius of the visible horizon due to the height SS′ = h of his eye above the water, and is, of course, at once obtained conversely by the expression:—

l′ = √7 h 2 (4.)

Deducting this distance from SL, the whole effective range of the light, we have L d = l, and operating with this value in the former equation,

H = 4 l²7

we find the height of the tower which answers the conditions of the case.[81] From the above data the following Table has been computed.

[81] In the above expressions l and l′ are given in English miles, which in Scotland may be considered as bearing to nautical miles the ratio of 5280 to 6088. In order to convert a distance given in nautical miles to English miles, all that is needful is to add the log of the number of nautical miles to log 5280, and subtract log 6088.

Hλλ′Hλλ′Hλλ′
Heights
in Feet.		Lengths
in
English
Miles.		Lengths
in
Nautical
Miles.		Heights
in
Feet.		Lengths
in
English
Miles.		Lengths
in
Nautical
Miles.		Heights
in
Feet.		Lengths
in
English
Miles.		Lengths
in
Nautical
Miles.
 5 2·9582·565 7011·067 9·598 25020·91618·14 
10 4·1843·628 7511·456 9·935 30022·91219·87 
15 5·1234·443 8011·83210·26  35024·74821·46 
20 5·9165·130 8512·19610·57  40026·45722·94 
25 6·6145·736 9012·54910·88  45028·06224·33 
30 7·2456·283 9512·89311·18  50029·58025·65 
35 7·8266·78710013·22811·47  55031·02426·90 
40 8·3667·25511013·87412·03  60032·40328·10 
45 8·8747·69612014·49012·56  65033·72629·25 
50 9·3548·11213015·08313·08  70035·00030·28 
55 9·8118·50914015·65213·57  80037·41632·45 
6010·2468·88615017·20114·91  90039·83634·54 
6510·6659·24920018·70816·22 100041·83336·28 

If the distance at which a light of given height can be seen by a person on a given level be required, it is only needful to add together the two numbers in the column of lengths λ or λ′ (according as Nautical or English miles may be sought) corresponding to those in the column of heights H, which represent respectively the height of the observer’s eye and the height of the lantern above the sea. When the height required to render a light visible at a given distance is required, we must seek first for the number in λ or λ′ corresponding to the height of the observer’s eye, and deduct this from the whole proposed range of the light, and opposite the remainder in λ or λ′ seek for the corresponding number in H.

A considerable practical defect in all the lighthouse lanterns which I have ever seen, with the exception of those recently constructed for the Scotch Lighthouses, consists in the vertical direction of the astragals, which, of course, tend to intercept the whole or a great part of the light in the azimuth which they subtend.[82] The consideration of the improvement which I had effected in giving a Diagonal Lantern. diagonal direction to the joints of the fixed refractors, first led me (as stated at [p. 266], ante), to adopt a diagonal arrangement of the framework which carries the cupola of zones and afterwards for the astragals of the lantern. Not only is this direction of the astragals more advantageous for equalising the effect of the light; but the greater stiffness and strength which this arrangement gives to the frame-work of the lantern make it safe to use more slender bars and thus also absolutely less light is intercepted. The panes of glass at the same time become triangular, and are necessarily stronger than rectangular panes of equal surface. This form of lantern is extremely light and elegant, and is shewn, with detailed drawings of some of its principal parts, in [Plate XXVI.] To avoid the necessity of painting, which, in situations so exposed as those which lighthouses generally occupy, is attended with many inconveniences and no small risk, the framework of the lantern is now formed of gun-metal and the dome is of copper; so that a first order lantern of 12 feet diameter and 10 feet height of glass costs, when glazed, about L.1260. In order to give the lightkeepers free access to cleanse and wash the upper panes of the lantern (an operation which in snowy weather must sometimes be frequently repeated during the night), a narrow gangway, on which they may safely stand, is placed on the level of the top of the lower panes, and at the top of the second panes, rings are provided of which the lightkeepers may lay hold for security in stormy weather. A light trap-ladder is also attached to the outside of the lantern by means of which there is an easy access to the ventilator on the dome.

[82] I must also except the small pier light at Kirkcaldy, erected (I believe in 1836) by my friend Mr Edward Sang.

Glazing of the Lantern. Great care is bestowed on the glazing of the lantern, in order that it may be quite impervious to water, even during the heaviest gales. When iron is used for the frames, they are carefully and frequently painted; but gun-metal, as just noticed, is now generally used in the Scotch Lighthouses. There is great risk of the glass plates being broken by the shaking of the lantern during high winds; and as much as possible to prevent this, various precautions are adopted. The arris of each plate is always carefully rounded by grinding; and grooves about ¹⁄₂ inch wide, capable of holding a good thickness of putty, are provided in the astragals for receiving the glass, which is ¹⁄₄ inch thick. Small pieces of lead or wood are inserted between the frames and the plates of glass against which they may press, and by which they are completely separated from the more unyielding material of which the lantern-frames are composed. Panes glazed in frames padded with cushions, and capable of being temporarily fixed in a few minutes, in the room of a broken plate, are kept ready for use in the Store-room. Those framed plates are called storm-panes, and have been found very useful on several occasions, when the glass has been shattered by large sea-birds coming against it in a stormy night, or by small stones violently driven against the lantern by the force of the wind.

Ventilation of the Lanterns. The ventilation of the lanterns forms a most important element in the preservation of a good and efficient light. An ill-ventilated lantern has its sides continually covered with the water of condensation, which is produced by the contact of the ascending current of heated air; and the glass thus obscured obstructs the passage of the rays, and diminishes the power of the light. In the Northern Lighthouses, ventilators, capable of being opened and shut at pleasure, so as to admit from without a supply of air when required, are provided in the parapet-wall on which the lantern stands; the lantern roof also is surmounted by a cover which, while it closes the top of an open cylindric tube against the entrance of rain, and descends over it only so far as is needful for that purpose, still leaves an open air-space between it and the dome. This arrangement permits the current of heated air, which is continually flowing from the lantern through the cylindric tube, to pass between it and the outer cover, from which it finally escapes to the open air through the space between the cover and the dome. The door which communicates from the lightroom through the parapet to the balcony outside, is also made the means of ventilating the lightroom; and, for that purpose, it is provided with a sliding bolt at the bottom, which, being dropped into one or other of the holes cut in the balcony for its reception, serves to keep the door open at any angle that may be found necessary. A useful precaution was introduced by my predecessor, as Engineer to the Northern Lights Board, in order to prevent the too rapid condensation of heated air on the large internal surface of the lantern roof, which consists in having two domes with an air-space between them, as shewn in the enlarged diagrams in [Plate XXVI.]

An important improvement in the ventilation of Lighthouses was some years ago introduced by Dr Faraday into several of the Lighthouses belonging to the Trinity House, and has since been adopted in all the dioptric lights belonging to the Commissioners of Northern Lighthouses. After mentioning several proofs of extremely bad ventilation in Lighthouses, Dr Faraday thus describes his apparatus:[83]

[83] Minutes of Institution of Civil Engineers, vol. i., p. 207.

“The ventilating pipe or chimney is a copper tube, 4 inches in diameter, not, however, in one length, but divided into three or four pieces; the lower end of each of these pieces for about 1¹⁄₂ inch is opened out into a conical form, about 5¹⁄₂ inches in diameter at the lowest part. When the chimney is put together, the upper end of the bottom piece is inserted about ¹⁄₂ inch into the cone of the next piece above, and fixed there by three ties or pins, so that the two pieces are firmly held together; but there is still plenty of air-way or entrance into the chimney between them. The same arrangement holds good with each succeeding piece. When the ventilating chimney is fixed in its place, it is adjusted so that the lamp-chimney enters about ¹⁄₂ inch into the lower cone, and the top of the ventilating chimney enters into the cowl or head of the lantern.

“With this arrangement, it is found that the action of the ventilating flue is to carry up every portion of the products of combustion into the cowl; none passes by the cone apertures out of the flue into the air of the lantern, but a portion of the air passes from the lantern by these apertures into the flue, and so the lantern itself is in some degree ventilated.

“The important use of these cone apertures is that when a sudden gust or eddy of wind strikes into the cowl of the lantern, it should not have any effect in disturbing or altering the flame. It is found that the wind may blow suddenly in at the cowl, and the effect never reaches the lamp. The upper, or the second, or the third, or even the fourth portion of the ventilating flue might be entirely closed, yet without altering the flame. The cone junctions in no way interfere with the tube in carrying up all the products of combustion; but if any downward current occurs, they dispose of the whole of it into the room without ever affecting the lamp. The ventilating flue is in fact a tube, which, as regards the lamp, can carry everything up but conveys nothing down.”

The advantages of this arrangement, as applied to the Northern Lighthouses, were much less palpable than those which are described in the beginning of Dr Faraday’s paper, because their ventilation was very good before its introduction; and the flame in particular was perfectly steady, being by no means subject to derangement from sudden gusts of wind from the roof in the manner noticed above.

Arrangements and internal management of a Lighthouse. All the Lighthouses in the district of the Commissioners are under the charge of at least two Lightkeepers, whose duties are to cleanse and prepare the apparatus for the night illumination, to mount guard singly after the light is exhibited, and to relieve each other at stated hours, fixed by the printed regulations and instructions, under which they act. The rule is, that no keeper on watch shall, under any circumstances, leave the Lightroom until relieved by his comrade; and, for the purpose of cutting off all pretext for the neglect of this universal law, the dwelling-houses are built close to the Light Tower, and means are provided for making signals directly from the Lightroom to the sleeping apartments below. These signals are communicated by air-tubes, through which, by means of a small piston, or a puff of wind from the mouth, calls can be exchanged between the keepers, enabling the man on guard in the Lightroom, at the end of the watch, or on any sudden emergence, to summon his comrade from below, who, on being thus called, answers by a counter-blast, to shew that the summons has been heard and will be obeyed. For the purpose of greater security, in such situations as the Bell Rock and the Skerryvore, four keepers are provided for one lightroom; one being always ashore on leave with his family, and the other three being at the Lighthouse, so that, in case of the illness of one lightkeeper, an efficient establishment of two keepers for watching the light may remain. At all the land-lighthouses also, an agreement is made with some steady person residing in the neighbourhood, who is instructed in the management of the light and cleansing of the apparatus, and comes under an obligation to be ready to do duty in the light-room when called upon, in the event of the sickness or absence of one of the lightkeepers. This person is called the occasional keeper, and receives pay only while actually employed at the Lighthouse; but in order to keep him in the practice of the duty, he is required to serve in the lightroom for a fortnight annually in the month of January. The details of the lightkeeper’s duty may be seen by referring to the [instructions] already alluded to, which will be found in the Appendix.

Each of the two lightkeepers has a house for himself and family, both being under a common roof, but entering by separate doors, as shewn in [Plates XXVII.] and [XXVIII.], which exhibit the buildings for the new Lighthouse at Ardnamurchan Point, on the coast of Argyllshire. The principal keeper’s house consists of six rooms, two of which are at the disposal of the visiting officers of the Board, whose duty in inspecting the Lighthouse, or superintending repairs, may call them to the station; and the assistant has four rooms, one of which is used as a barrack-room for the workmen, who, under the direction of the Foreman of the lightroom works, execute the annual repairs of the apparatus.

The early Lighthouses contained accommodation for the lightkeepers in the Tower itself; but the dust caused by the cleaning of those rooms in the Tower was found to be very injurious to the delicate apparatus and machinery in the lightroom. Unless, therefore, in situations such as Skerryvore, where it is unavoidable, the dwellings of the lightkeepers ought not to be placed in the Light Tower, but in an adjoining building.

Great care should be bestowed to produce the utmost cleanliness in everything connected with a Lighthouse, the optical apparatus of which is of such a nature as to suffer materially from the effect of dust in injuring its polish. For this purpose covered ash-pits are provided at all the dwelling-houses, in order that the dust of the fire-places may not be carried by the wind to the lightroom; and for similar reasons, iron floors are used for the lightrooms instead of stone, which is often liable to abrasion, and all the stonework near the lantern is regularly painted in oil.

If, in all that belongs to a lighthouse, the greatest cleanliness be desirable, it is in a still higher degree necessary in every part of the lightroom apparatus, without which the optical instruments and the machinery will neither last long nor work well. Cleansing of Apparatus. Every part of the apparatus, whether lenses or reflectors, should be carefully freed from dust before being either washed or burnished; and without such a precaution, the cleansing process would only serve to scratch them. For burnishing the reflectors, prepared rouge (tritoxide of iron) of the finest description, which should be in the state of an impalpable powder of a deep orange-red colour, is applied, by means of soft chamois skins, as occasion may require; but the great art of keeping reflectors clean consists in the daily, patient, and skilful application of manual labour in rubbing the surface of the instrument with a perfectly dry, soft, and clean skin, without rouge. The form of the hollow paraboloid is such, that some practice is necessary in order to acquire a free movement of the hand in rubbing reflectors; and its attainment forms one of the principal lessons in the course of the preliminary instruction, to which candidates for the situation of a light-keeper are subjected at the Bell Rock Lighthouse. For cleansing the lenses and glass mirrors, spirit of wine is used. Having washed the surface of the instrument with a linen cloth steeped in spirit of wine, it is carefully dried with a soft and dry linen rubber, and finally rubbed with a fine chamois skin, free from any dust which would injure the polish of the glass, as well as from grease. It is sometimes necessary to use a little fine rouge with a chamois skin, for restoring any deficiency of polish which may occur from time to time; but in a well-managed lighthouse this application will seldom, if ever, be required.

The machinery of all kinds, whether that of the mechanical lamp or the revolving apparatus, should also be kept scrupulously clean, and all the journals should be carefully oiled.

Mode of measuring the relative intensity and power of Lights. As I have had frequent occasion to speak of the comparative power of lights, it will not be out of place to present the reader with a few practical observations, chiefly drawn from the excellent work of M. Peclet to which I have so often referred, on the measurement of the intensity of lights by the method of shadows.

Fig. 93.

The intensity of light decreases as the observer recedes from the luminous body, in proportion to the square of his distance. Suppose a [beam of light] to proceed from a radiant at F, and we shall have the rays which, of course, move in straight lines, gradually receding from each other, as b, b′, b″, b‴, and c, c′, c″, c‴, so that the section of the beam will increase with the distances F b, and F c; and the same number of rays, being thus spread over spaces continually increasing, will illuminate the surfaces with a less intensity. This decrease of intensity will, therefore, be in the inverse ratio of the extent of the transverse parallel sections of the luminous cones at b and c, which, we know, increase as the square of their distances from the apex of the cone at F. Hence we conclude, that the intensity of any section of a divergent beam of light decreases as the square of its distance from the radiant. This law furnishes us with a simple measure of the comparative intensity of lights. If we suppose two lights so placed that they may separately illuminate adjacent portions of a vertical screen of paper, we may, by repeatedly comparing the luminousness of those surfaces, and moving one of the lights farther from, or nearer to the screen, at length cause the separate portions of the paper to become equally luminous. This arrangement, however, has many practical difficulties, which I shall not wait to specify; but shall at once indicate a more simple and equally correct mode of obtaining the same result, by means of the shadows cast by the lights from an opaque rod, in a vertical position at O ([fig. 94]), placed between them, and a screen covered with white paper on which the shadows fall. It is obvious that the light at F would cause the object O to cast a shadow at SS, while the light at F′ would cast a shadow at S′S′. But while the shadow at S would still receive light from F′, S′ would receive light from F, so that those two shadows are, in fact, the only portions of the screen which are each illuminated only by one of the lights, while every other portion of its surface receives light from both the radiants at F and F′. If we suppose F to be the weaker light, we can bring it nearer the screen, until the shadow S′S′, shall become similar in appearance to the shadow SS; and we shall have the ratio of the intensity of the light at F to that of the light at F′, as (FS′)² is to (F′S)², which distances must be measured with the greatest exactness. Such is the mode commonly used in estimating the comparative intensities of two lights; but there are various precautions which are needful in order to prevent errors in comparing the deepness of the shadows, and to insure the greatest attainable accuracy in the estimate of the power of the lights, which I shall endeavour briefly to describe.

Fig. 94.

More accurate comparison of the intensity of Lights. The difficulties of estimating the deepness or sharpness of the shadow is very great, and many persons seem quite incapable of arriving at any right judgment in this matter. The same person also will discover such unaccountable variations in his decision after observations made at short intervals of time, as, one would think, can only arise from a sudden change of the intensity of one or both lights. M. Peclet, in his Traité de l’éclairage, gives, as the result of his experience (and I can fully confirm his result by my own), that those differences depend less frequently on any real difficulty of estimating the deepness of the shadows, than on variations in the position of the observer, or rather in the angle at which he views the shadows, and that, consequently, in proportion to the distance between the two shadows, this source of error is increased. Any thing like a glossy texture of the surface of the screen, which then, of course, becomes a reflector, also tends to aggravate this evil. Thus, if the two lights which are to be compared be placed on a table, in such situations as to spread pretty far apart on the screen the shadows of a vertical rod placed between them; and if the shadow nearer to the observer seem to be a little deeper or sharper than the other, let the observer look at them from the other side of the table, and their difference will be reversed, and that which seemed the paler, will become the deeper. Again, if the difference between the two shadows be very great when seen from the right side of the screen, it may happen that, on viewing them from the left hand, the difference may still be in favour of the same shadow, but in a much less degree.

“When I observed this effect,” says M. Peclet,[84] “I tried to view the shadows through a transparent screen, but I remarked the same variations. They were indeed even more sensible; for a variation in the distance of the eye of a few centimètres, made a prodigious change in the deepness of the shadows. I observed also that the shadow was much deeper when seen in the line of the light, and that in every other direction, it became paler in proportion as the eye receded from that direction.

[84] Traité de l’éclairage, p. 214.

“In all the cases which I have just described, the differences of the tints when the position is changed, increase in proportion as the shadows are farther separate; and they grow very minute when the shadows are almost touching each other.

Fig. 95.

“Let AB ([fig. 95]) be a white opaque surface, a, a luminous body, and m, a black opaque body, then the shadow b′ cast on AB, will appear deeper when observed from P, than as seen from Q. This is a fact which may be easily verified, and the cause of which is easily conceived. In fact, the surface AB, although it disperses the light, must still reflect more of it, in the directions in which the regular reflection takes place; and hence the rays which are reflected round about the shadow, must have a greater intensity in the direction of P than in that of Q, and, consequently, the shadow b′ must appear deeper from the point P than from Q.

Fig. 96.

“If we now place ([fig. 96]) two lights in front of the screen AB, at such distances that the two shadows a′ and b′ should have equal intensities, it is evident that if the eye be placed at P, the shadow b′ must appear more intense than the shadow a′, and that the reverse will take place if the eye be at Q. But the difference which is then observed, arises not only from the difference in the brightness of the parts surrounding the shadows, but also from a difference in the intensity of the shadows themselves; for the shadow b′ is illuminated by b, and radiates much more towards Q than towards P; and, on the contrary, the shadow a′, which is illuminated by a, radiates much more towards P than towards Q. We perceive also why the differences of the tints increase with the separation of the two shadows, and why they become very small when the shadows touch each other; it is because, in proportion as the shadows are farther apart, each of them is illuminated more obliquely, and a greater quantity of light is radiated (by reflection) in the regular direction. When they touch each other, on the contrary, they are illuminated almost perpendicularly, and consequently the shadows radiate light almost equally on either side.

“Those anomalies of a like kind which are observed when the shadows are viewed through a translucent body, such as paper or linen, may be referred to a similar cause. We know, in fact, that, in looking through a translucent medium, we always, more or less, distinctly perceive the luminous body behind it, and, also, that there is a very large proportion of the rays which traverse the body, which stray but a little from the direction which they would follow if the substance were absolutely transparent. Consequently, the space which surrounds the shadow is more luminous in proportion as we come nearer to the direction of the shadow; and as the absolute intensity of the shadows diminishes as we come nearer to the direction of the rays which light them, those two effects concur to increase the intensity of that shadow to which the eye is nearer.

“As the dispersion by reflection is much more complete than by refraction, the variations of which we have just spoken are much greater with a transparent screen, through which the shadows are viewed, than with an opaque screen (from which they are reflected).

“This, then, is the mode of observing which has appeared to me the best, and by means of which we may obtain very great precision in measuring the intensity of two lights. I view, first, the two shadows in such a manner that both of them may be seen in succession from either side of the body which produces them, and at equal distances. For this purpose I use a good opera-glass. I alter the distance of the flames until in those two positions I perceive the differences (of the intensity in the shadows) to be in opposite directions. The distances of the lamps may then be considered as very nearly in the proper proportion for producing equal shadows, and to make them exactly so, the differences, which are observed on either side (of the centre line between them), should be equal; and, of course, the two shadows themselves, seen at one moment from either side of the opaque body, should be perfectly equal also.[85] These three observations, which mutually serve to verify or correct each other, will lead, with a little practice, to very great precision in the result. We may, also, by using a narrow screen, bring the shadows sufficiently near to touch each other; the variations of the tints then become very small by any change of our position, and we may, in this case, rest content with observing them from one point. To get rid of large penumbrae which are always an obstacle in forming a right estimate of the tints of the shadows, I place the opaque body very near the screen.

[85] I prefer to view the exterior portions of both shadows from the central line itself, in which case the opaque rod stands between them, because, in this manner, I obtain a more correct comparison by the direct contrast of the surfaces than by successive views of them, however quickly taken.

Fig. 97.

“When we wish to make a great many observations, it is very convenient to mark divisions on the table (which carries the lights), in order to read off, by means of them, the distance of the lamps from the shadows which they illuminate. By this means, each observation need not occupy more than two minutes. I generally use a table CC DD ([fig. 97]), about two mètres long (6 feet 6 inches), by 80 centimètres wide (2 feet 8 inches). At one end I place the screen AB, covered with white paper, dull (or not glazed), and kept in a vertical plane by two small pieces P and Q. Through the point M, the centre of the opaque body, I draw two lines Mf and Mg, equally inclined to the central line x y, whose extremities b′, a′ are the axes of the two shadows. These lines must be inclined in such a manner that the distance of the shadows may be a little less than the diameter of the opaque body, or so that they may actually touch each other, according to the mode of observing which you wish to follow. These lines M f, M g I divide into decimètres and centimètres, starting from the points a′, b′ and over these lines I place the centres of the flames; the distance between the shadows remains always the same, whatever may be the distance of the lamps: to determine the distance of each lamp from the shadow which it illuminates, we ought, strictly speaking, to take the distance of the centre of the flame b from the point a′; but as the distance from the point b to the point a′ differs little from the distance between the points b and b′, we assume the latter for the former, without causing any sensible error. That distance may be obtained very conveniently by taking the half of the sum of the distances of the two extremities z and z′ of the diameter of the pedestal of the lamp. When the burner is not placed over the centre of the pedestal, we may suspend from it a small plummet, whose point will touch some division and indicate the distance between the centre of the burner and the shadow.

“When the lights are coloured, the shadows are coloured also, and it is then far more difficult to judge accurately of their intensity. They may in that case be much better seen from the point x, as the black opaque body which is interposed between them renders the difference of colour less sensible to the eye.

“The opaque body M is a cylindric rod of iron, whose upper part is blackened in the flame of a lamp, in order to prevent the reflection which might interfere with the sharpness (netteté) of the shadows, and to make them more distinct when they are viewed from the point x.”[86]

[86] Those who feel a curiosity to look farther into this subject may consult Count Rumford’s elaborate paper in the Phil. Trans. for 1794, p. 67.

I shall make a few trifling additions to M. Peclet’s clear description of his excellent mode of measuring the intensity of lights. It is, of course, presumed throughout, that the centres of the flames should be on one level; and I have found it most convenient to place the lamps on small carriages with rollers, which are guided by means of fine strips of wood nailed along the table in the directions gM and fM, and carrying the divided scales of centimètres. This affords the means of making any slight change in the position of the lamps so easily, as entirely to avoid the disturbance of the flame which ensues from lifting the lamp and readjusting it in another position; and will, in practice, be found very convenient when many observations are to be made. I have already said that my own experience has satisfied me that, with the aid of a good opera-glass, the central observation of the two shadows, with the opaque rod between them, is by far the best, and conducts, at once, to a result which is confirmed by the observations of two assistants who watch the shadows at the same time on opposite sides of the table, and at equal distances from them. I have found it convenient in comparing lights, to cover the table with dull black linen cloth, and to surround it with curtains of the same material, hung from slender brackets, in such a manner as to leave space for the observer to move freely round the table within them. The curtains prevent reflection from the walls of the chamber in which the experiments may be conducted, and also lessen the disturbing effects of currents of air. When a comparison of the intensity, and not of the aggregate power of two flames, is to be made, it is necessary to adopt the precaution of inclosing the lights in opaque boxes, with slits of equal area in each, placed on the same level, and so arranged, in reference to the flames, as to be directly opposite the brightest portion of each. After what has been said, it will be almost needless to add that the quotient of the square of the greater observed distance divided by the lesser, is the ratio of the illuminating power of the two flames. The most convenient mode of registering observations, and that which is generally practised, is in the form of a Table like the following:—

Trials.Distance.Squares of
Distances.		Illuminating Power,
or
Quotient of Squares.
Lamp A.Lamp B.Lamp A.Lamp B.Lamp A.Lamp B.
114314020,44919,7001·000·958
211711413,68912,9961·000·949

As a standard lamp by which to test others, I believe few will be found superior to the best Carcel lamp, which has a clockwork movement, and whose flame continues to increase in power for about four hours after it is lighted; after which it maintains its state permanently, until the supply of oil fails. This fact was verified by M. Peclet with the greatest care. “I took,” says he, “two similar lamps. They were lighted at the same time, and their relative intensities were measured. One was then extinguished, without touching the wick, and its clockwork movement was stopped. One hour afterwards, I set the clockwork in motion and relighted the lamp, but without touching the wick. It was found in the same state as at the first comparison, and I measured its intensity in reference to the first. Those experiments I repeated every hour, and these are the results which I obtained. The lamp which I call No. 1, is that which remained continually burning; No. 2, is that which was only lighted during the continuance of the (successive) observations.”

Times of
Observation.		Intensities.
Lamp,
No. 1.		Lamp,
No. 2.
H.M.
 530100100
 630103100
 730106100
 830110100
 930117100
1030117100
1130117100
1230117100

This curious scale of increase in power, seems to be solely due to a peculiarity of the manner in which the lamp, that derives its supply of oil by clockwork, becomes heated; and the effect may be described as follows: The heating of the wicks, the chimney, and the oil in this burner, as in that of all other lamps, tends to increase the light; but, in an ordinary lamp, acting by a constant pressure, this maximum of heat is soon attained; whereas in the clockwork-lamp, into the burner of which the oil is thrown up by a pump, the whole of the oil in the cistern must reach its maximum temperature before the best effect of that lamp is produced. After this state has been reached, there is no disturbing influence at work, and the lamp burns steadily as long as the oil lasts.

I have myself tried what may naturally appear to be the most simple mode of obtaining an unvarying standard-light, by employing a gas-burner, supplied from a gasometer under a constant pressure; but I found it very difficult to obtain satisfactory proof of the constancy of the pressure; and in a large town, where there are many burners around one, their lighting or extinction is found to exercise a material influence in changing the condition of the flame. I must confess that I have always been disappointed in attempting to use a gas-flame as a standard of comparison.

There are various dangers on the shores of Britain, more especially at the entrance of the great estuaries of England and also in Ireland, whose position is such as to put them beyond the reach of regular lighthouses. Sand-banks which are too soft to sustain a solid structure, and have too deep water on them to admit of the erection of screw-pile lighthouses, are often the sites for Floating Lights. mooring light-vessels, to guide the mariner into the entrance of some estuary, or enable him to thread his way through the mazes of gats and channels, which, even during the daytime, baffle the mariner, who sees no natural object on the low sandy shores of the neighbouring coast to help him to guess at his true position. The first Light-vessel moored on the coast of Great Britain, was that at the Nore in 1734. There are now no fewer than 26 floating lights on the coast of England.

By the kindness of the Elder Brethren of the Corporation of Trinity House of Deptford Strond, I am enabled to give the following brief sketch of the nature and peculiarities of Floating Lights which was communicated to me by Mr Herbert, the secretary of the Corporation:—

“The annual expense of maintaining a Floating Light, including the wages and victualling of the crew, who are eleven in number, is, on an average, L.1000; and the first cost of such a vessel, fitted complete with lantern and lighting apparatus, anchors, cables, &c., is nearly L.5000. The lanterns are octagonal in form, 5 feet 6 inches in diameter; and, where fixed lights are exhibited, they are fitted with eight Argand lamps, each in the focus of a parabolic reflector of twelve inches diameter; but, in the revolving lights, four lamps and reflectors only are fitted. The greatest depth of water in which any light-vessel belonging to the Corporation of Trinity House of Deptford Strond at present rides, is about 40 fathoms (which is at the station of the Seven Stones between the Scilly Islands and the coast of Cornwall).

“The Corporation’s light-vessels are moored with chain-cables of 1¹⁄₂ inch diameter, and a single mushroom anchor of 32 cwt., in which cases the chain-cables are 200 fathoms in length; some of the said vessels are moored to span-ground moorings, consisting of 100 fathoms of chain to each arm, and a mushroom anchor of similar weight at the end of each; a riding cable of 150 fathoms being in such cases attached to the centre ring of the ground chain. The tonnage and general dimensions of the light-vessel are given on the drawing of the lines.” (See [Plate XXIX.])

Still lower in the scale of “signs and marks of the sea,” are Beacons and Buoys. Beacons and Buoys, which are used to point out those dangers which, either owing to the difficulty and expense that would attend the placing of more efficient marks to serve by night as well as by day, are necessarily left without lights, or which, from the peculiarity of their position, in passages too intricate for navigation by night, are, in practice, considered to be sufficiently indicated by day-marks alone. Beacons, as being more permanent, are preferred to Buoys; but they are generally placed only on rocks or banks which are dry at some period of the tide. On rocks, in exposed situations, the kind of Beacon used is generally that of squared masonry, secured by numerous joggles (as shewn at [Plate XXXII.]); and in situations difficult of access, and in which works of uncompleted masonry could not be safely left during the winter season, an open framework of cast-iron pipes, firmly trussed and braced, and secured to the rock with strong louis-bats, is preferred. The details of this framework are shewn at [Plates XXX.] and [XXXI.] A stone Beacon of the form and dimensions shewn in [Plate XXXII.], may be erected for about L.700, and the iron Beacon shewn at [Plate XXX.], for about L.640. In less exposed places, where the bottom is gravel or hard sand, a conical form of Beacon, composed of cast-iron plates, united with flanges and screws, with rust-joints between them, is sometimes used. A Beacon of this kind is shewn at [Plate XXXII.], which can be erected for about L.400.

Lastly, Buoys, which may be regarded as the least efficient kind of mark, and as bearing the same relation to a Beacon that a Floating-light does to a Lighthouse, are used to mark by day dangers which are always covered even at low water, and also to line out the fair-ways of channels. They are, for the most part, of one of the three forms shewn in [Plate XXXIII.], viz., the Nun-buoy, in the form of a parabolic spindle, generally truncated at one end, so as to carry a mast or frame of cage-work, and loaded at the other end, so as to float in a vertical position; the Can-buoy, which is a conoid floating on its side; and, lastly, the Cask-buoy, which is a short frustum of a spindle truncated at both ends, but almost exclusively used for carrying the warps of vessels riding at moorings. Those buoys are of various sizes and differ in cost. Mast-buoys, from 10 to 15 feet in length, cost from L.23, 15s. to L.48; and those of the Ribble and the Tay, which are 21 and 24 feet long, cost respectively L.105 and L.79; the Can-buoys are from 5 to 8 feet long, and cost from L.13, 13s. to L.20, 5s. Large buoys are often built on kneed frames resembling the timbers of vessels. The Cask-buoy is generally 6 feet long, and costs L.22, 15s. All these buoys are formed of strong oaken barrel-staves, well hooped with iron rings, and shielded with soft timber; and the nozzle-pieces at the small end of the Nun and Can buoys are generally solid quoins of oak, formed with a raglet or groove to receive the ends of the staves. Much skill, on the part of the cooper, is required in heating and moulding the staves to the required form; and great care must be taken that they be of well-seasoned timber. Buoys are not caulked with oakum, but with dry flags closely compressed between the edges of the staves, which swell on being wet; and they are carefully proved by steaming them like barrels, to see if they be quite tight. Buoys are also formed of sheet-iron, in which case they are sometimes protected with fenders of timber; but they have been found more troublesome for transport, and, for most situations, are considered less convenient than those of timber.

In the beginning of 1845, I suggested the idea of rendering Beacons and Buoys useful during night, by coating them with some phosphorescent substance, or surmounting them with a globe of strong glass filled with such a preparation, whose combustion is very slow, and emits a dull whitish light and little heat. Some experiments were accordingly made by my suggestion; but I cannot add that any practically useful result has been obtained.

In laying down Beacons or Buoys, their position is fixed, as may be seen in the [Table] in the Appendix, either by the intersection of two lines drawn through two leading objects on the shore (the magnetic bearings of which are given for the sake of easy reference on the spot, in finding out the marks), or by means of the angles contained between lines drawn to various objects on the shore, which meet at the Beacon or Buoy from which they are measured by means of a sextant. In the latter case, the angles are always measured around the whole horizon, thus affording a check by the difference of their sum from 360°. The magnetic bearing of one of those lines is afterwards carefully ascertained, by means of the prismatic compass (if possible from one of the objects on shore, and if not, conversely from the Beacon or Buoy), so as to afford the means of translating the whole into magnetic bearings for the use of seamen. The buoys are moored, as shewn in [Plate XXXIII.], by means of chains and iron sinkers, with a sufficient allowance in the length of the chain to permit them to ride easily.