By this arrangement each lens transmits to all the points of the horizon in succession a light equivalent to that of from three to four thousand lamps with double currents, and eight times greater than the light produced by the silver parabolic reflectors; it is, according to Arago, the same amount of light as would be obtained if it were possible to bring together the third of the whole number of gas-lights which illumine the streets, the shops, and the theatres of Paris; and this wonderful result is obtained from a single lamp.
This lamp has four concentric burners, which are defended from the action of the excessive heat produced by their united flames, by means of a superabundant supply of oil, which is thrown up from the cistern below by a clock-work movement, and constantly overflows the wicks. A very tall chimney is necessary in order to supply fresh currents of air to each wick with sufficient rapidity to support the combustion. The carbonization of the wicks is not very rapid; and after they have been burning a long time, the flame is not sensibly diminished, as the great heat evolved from the mass of flame promotes the rising of the oil in the cotton.
In the year 1820, in the course of some investigations connected with the Trigonometrical Survey of Great Britain, and conducted by a deputation of scientific persons from London and Paris, M. Fresnel exhibited from the French side of the channel, by means of his lens and a large lamp, a powerful light which was observed by the English across the channel. The brilliancy of this light so struck Lieut.-Col. Colby, of the Royal Engineers, who was engaged in these observations, that he immediately corresponded with Mr. Stevenson as to its probable use upon the Scottish coast. A considerable time was occupied in inquiry and negotiation, when at length, on the 26th October, 1836, the light at the Isle of May was changed from the catoptric to the dioptric system, and a committee of the Royal Society of Edinburgh met at Dunbar, a distance of thirteen miles from the lighthouse, to make observations on the two lights, which were exhibited in contrast. In their report they conclude:—
‘1. That at a distance of thirteen miles the mean effect of the new light is very much superior to the mean effect of the old light (perhaps in the ratio of two to one). 2. That at all distances the new light has a prodigious superiority to the old, from the equality of its effects in all azimuths. 3. That the new light fulfils rigorously the conditions required for the distribution of light to the greatest advantage. 4. That at distances much exceeding thirteen miles, the new light must still be a very effective one, though to what extent the committee have not observed. The light is understood to be still a good one, when seen from Edinburgh at a distance of about thirty miles.’
On a further comparison of results, it was found that the light of one of the great annular lenses, used in the revolving lights of the first order, was equal to the united effect of about eight of the large reflectors employed in the revolving lights on the Scottish coast. At the Isle of May and Inchkeith the quantity of sperm-oil consumed by the great lamp is equal to that burned by fourteen of the Argand lamps used in the Scotch lights. Hence by dioptric means the consumption of oil necessary for the fourteen reflectors will produce almost as powerful a light as that which would require the oil of twenty-four reflectors in the catoptric system, and consequently there is an excess of oil equal to that consumed by ten reflectors, or four hundred gallons in the year against the Scotch system.
The Dutch were the first to adopt Fresnel’s system. In the year 1834 the Commissioners of Northern Lighthouses sent Mr. Alan Stevenson to Paris to inspect the system, and his report was so favourable, that the reflecting apparatus of the revolving light at Inchkeith was removed, and the dioptric instruments substituted. The new light was exhibited on the evening of the 1st of October, 1835, and so great was the satisfaction afforded, that a similar change was made at the fixed light of the Isle of May. The Trinity-House of London followed next in adopting the improved system, and a revolving dioptric light of the first order was erected at the Star Point in Devonshire.
In the lighthouses of this country sperm-oil is the most usual fuel. In France[6] an oil is burned called Colza oil, expressed from the seeds of a species of wild cabbage. In the lighthouses on the Mediterranean olive-oil is used. In a few lighthouses near large towns coal-gas has been advantageously adopted. Much also has been said in favour of the Drummond and Voltaic lights, which, on account of their prodigious intensity would appear to be most desirable; but the uncertainty which attends their exhibition renders it at present impossible to adopt them: but there is a yet more fatal objection—the smallness of the flame renders them wholly inapplicable to dioptric instruments, which require a great body of flame in order to produce a degree of divergency sufficient to render the duration of the flash in revolving lights long enough to answer the purpose of the mariner.
In the year 1835, Mr. Gurney proposed a lamp of great power in which the flame of oil or wax was sustained by streams of oxygen gas, a method said to be more economical than the combustion of oil in atmospheric air. The Trinity House entertained the proposal, and instituted a number of experiments. In applying this light to reflectors it is intended to use three small flames, each about three-eighths of an inch in diameter, productive, it is said, of an effect equal to that of ten Argand lamps. But for lenses the burner has seventeen films of flame, and is said to possess six times the power of the Fresnel lamp.
In the year 1840, Captain Basil Hall instituted a series of experiments to ascertain whether the well-known superior brilliancy of a revolving light could not be obtained for a fixed or continuous light, that is, for one equally visible in all directions at the same moment. His idea was, that by giving a certain velocity of revolution to a series of lenses round a fixed light, as in Fresnel’s arrangement, a continuity of illuminative power, equal almost in brilliancy to that of a slowly revolving light, might be produced. The apparatus was arranged so as to cause a series of eight lenses one foot in diameter and three feet focal distance to revolve with any velocity up to sixty revolutions per minute round a central lamp. The light from this lamp being concentrated by refraction through the eight lenses into eight pencils, having a divergence of about eight degrees each, illuminated when at rest not quite fifty degrees of the horizon; but when this system of lenses was put into rapid motion, every degree of the three hundred and sixty degrees of the horizon became illuminated, so that to spectators placed all round the horizon the light would appear continuous and equally brilliant in every direction. The only question would be, whether or not this continuous light is essentially less intense than the light seen through the lenses at intervals when in slow motion; and this is a point which further inquiry must decide.
One of the causes which has tended to improve the brilliancy of lighthouses, has produced inconveniences, which long existed without remedy. During the combustion of a pound of oil, the union of its hydrogen with the oxygen of the air produces more than a pound of water in the state of vapour. When a cold wind is blowing upon the lantern of the lighthouse from without, this vapour is condensed into water upon the inner surface of the glass, and in very severe weather forms a crust of ice, in some cases, as much as four inches thick in the course of one night. This not only very much dims the brilliancy of the light to the sailor, but also entails a great amount of labour on the light-keepers, and injury to the lantern. The combustion of the oil also produces a large quantity of carbonic acid gas, which is of a very deleterious nature, and in many cases rendered the light-keepers’ rooms almost uninhabitable. Under these circumstances, the Trinity House made application to Dr. Faraday to investigate the subject, with a view to the discovery of some remedy. With his usual skill and sagacity, Dr. Faraday instituted a number of inquiries and experiments, and visited some of the principal lighthouses. The result was the contrivance of a complete method of ventilating lighthouses. On the dioptric system, the remedy was simple: it was merely to erect a tall chimney over the central lamp, and lead it out at the roof; by which means, the draught of the lamp was improved, and all the products of combustion carried off. On the catoptric system, with revolving lights, each lamp was furnished with a chimney, which passed out at its upper extremity, through a small hole in the reflector into a fixed central hollow shaft, which served the purpose of a ventilating chimney to all the lamps. These plans are said to have been eminently successful in removing the inconveniences, which rendered the light less efficient, and the lighthouse an unwholesome and even dangerouse place of abode.