TABLE.

Barometer
Inches.		Height in
feet.		Barometer
Inches.		Height in
feet.		Barometer
Inches.		Height in
feet.
31·0026·8382922·78201
30·98526·7392722·68317
30·817026·6402522·58434
30·725526·5412422·48551
30·634126·4422322·38669
30·542726·3432322·28787
30·451326·2442322·18906
30·360026·1452422·09025
30·268726·0462521·99145
30·177425·9472621·89266
30·086225·8482821·79388
29·995025·7493021·69510
29·8103825·6503321·59632
29·7112625·5513621·49755
29·6121525·4524021·39878
29·5130425·3534421·210002
29·4139325·2544821·110127
29·3148225·1555321·010253
29·2157225·0565820·910379
29·1166224·9576320·810506
29·0175324·8586920·710633
28·9184424·7597620·610760
28·8193524·6608320·510889
28·7202724·5619020·411018
28·6211924·4629720·311148
28·5221124·3640520·211278
28·4230324·2651420·111409
28·3239624·1662320·011541
28·2248924·0673319·911673
28·1258223·9684319·811805
28·0267523·8695319·711939
27·9276923·7706419·612074
27·8286423·6717519·512210
27·7295923·5728719·412346
27·6305423·4739919·312483
27·5314923·3751219·212620
27·4324523·2762519·112757
27·3334123·1772919·012894
27·2343823·0785418·912942
27·1353522·9796918·813080
27·0363322·8808518·713219
26·93731

MARINE BAROMETER,
ADOPTED BY
HER MAJESTY'S GOVERNMENT,
On the recommendation of the Kew Observatory Committee of the
British Association for the Advancement of Science.

This instrument should be suspended in a good light for reading, but out of the reach of sunshine or the heat of a fire or lamp. It should be as nearly amidships, and exposed as little to sudden changes of temperature, gusts of wind, or injuries, as possible. In a ship of war it should be below the lowest battery or gun-deck. Light should have access to the back of the tube, to admit of setting the index so as to have its lower edge a tangent to the surface of the mercury—the eye being on the same level, which is known by the back and front edges of the index being in one line with the mercury surface. White paper or card will reflect light for setting the vernier correctly. The height of the cistern above or below the ship's water-line should be ascertained, and entered on the register.

It is desirable to place the barometer in such a position as not to be in danger of a side blow, and also sufficiently far from the deck above to allow for the spring of the metal arm in cases of sudden movements of the ship.

If there is risk of the instrument striking anywhere when the vessel is much inclined, it will be desirable either to put some soft padding on that place, or to check movement in that direction by a light elastic cord; in fixing which, attention must be paid to have it acting only where risk of a blow begins, not interfering otherwise with the free swing of the instrument: a very light cord attached above, when possible, will be least likely to interfere injuriously.

The vernier, as usual in standard barometers, reads to the two thousandth (·002) part of an inch. Every long line cut on the vernier corresponds to ·01 part; each small division on the scale is ·05; the hundredth parts on the vernier being added to the five when its lower edge is next above one of the short lines; or written down as shown by the figures on the vernier only, when next above one of the divisions marking tenths.

In placing this barometer, it is only necessary to fix the instrument carefully, as indicated in the above directions, and give a few gentle taps with the fingers on the bottom, to move the mercury. Without further operation it will usually be ready for observation in less than an hour.

When moving the barometer, or replacing it in its case, the mercury should be allowed to run gently up to the top of the tube, by holding the instrument for a few minutes inclined at an angle. The vernier should be brought down to the bottom of the scale. No other adjustment for portability is required. During carriage, it ought to be kept with the cistern end uppermost, or lying flat, the former position being preferable.

If the mercury should not descend at first by a few gentle taps, use sharper (but of course without violence), by which, and two or three taps, with the finger ends, on the tube—between the scale and the tangent screw—the mercury will be made to begin to descend.

In reading off from a barometer, it should hang freely, not inclined by holding, or even by touch.

Sometimes, though rarely, at sea the mercury seems stopped. If so, take down the instrument (after sloping), reverse it, tap the tube gently while the cistern end is upwards, and then replace as before.

Testing Barometers, Hydrometers, and Thermometers.

In the year 1853 a conference of maritime nations was held at Brussels, on the subject of meteorology at sea. The report of this conference was laid before Parliament, and the result was a vote of money for the purchase of instruments and the discussion of observations, under the superintendence of the Board of Trade. Arrangements were then made, in accordance with the views of the Royal Society and the British Association for the Advancement of Science, for the supply of instruments properly tested.

In the barometers now in general use by meteorologists on land, the diameters of the tubes are nearly equal throughout their whole length, and a provision is made for adjusting the mercury in the cistern to the zero point, previous to reading the height of the top of the column. The object of the latter arrangement, it is well known, is to avoid the necessity of applying a correction to the readings for the difference of capacity between the cistern and the tube. At sea, barometers of this construction cannot be used. Part of the tube of the marine barometer must be very much contracted to prevent "pumping," and the motion of the ship would render it impracticable to adjust the mercury in the cistern to the zero point. In the barometer usually employed on shore, the index error is the same throughout the whole range of scale readings, if the instrument be properly made; but in nearly all the barometers which have till recently been employed at sea, the index correction varies through the range of scale readings, in proportion to the difference of capacity between the cistern and the tube. To find the index correction for a land barometer, comparison with a Standard at any part of the scale at which the mercury may happen to be, is generally considered sufficient. To test the marine barometer is a work of much more time, since it is necessary to find the correction for scale readings at about each half inch throughout the range of atmospheric pressure to which it may be exposed; and it becomes necessary to have recourse to artificial means of changing the pressure of the atmosphere on the surface of the mercury in the cistern.

The barometers intended to be tested are placed, together with a Standard, in an air-tight chamber, to which an air pump is applied, so that, by partially exhausting the air, the Standard can be made to read much lower than the lowest pressure to which marine barometers are likely to be exposed; and by compressing the air it can be made to read higher than the mercury ever stands at the level of the sea. The tube of the Standard is contracted similarly to that of the marine barometer, but a provision is made for adjusting the mercury in its cistern to the zero point. Glass windows are inserted in the upper part of the iron air-chamber, through which the scales of the barometers may be seen; but as the verniers cannot be moved in the usual way from outside the chamber, a provision is made for reading the height of the mercury independent of the verniers attached to the scales of the respective barometers. At a distance of some five or six feet from the air-tight chamber a vertical scale is fixed. The divisions on this scale correspond exactly with those on the tube of the Standard barometer. A vernier and telescope are made to slide on the scale by means of a rack and pinion. The telescope has two horizontal wires, one fixed, and the other moveable by a micrometer, screw so that the difference between the height of the column of mercury and the nearest division on the scale of the Standard, and also of all the other barometers placed by the side of it for comparison, can be measured either with the vertical scale and vernier or the micrometer wire. The means are thus possessed of testing barometers for index error in any part of the scale, through the whole range of atmospheric pressure to which they are likely to be exposed, and the usual practice is to test them at every half inch from 27·5 to 31 inches.

In this way barometers of various other descriptions have been tested, and their errors found to be so large that some barometers read half an inch and upwards too high, while others read as much too low. In some cases those which were correct in one part of the scale were found to be from half an inch to an inch wrong in other parts. These barometers were of the old and ordinary construction. In some the mercury would not descend lower than about 29 inches, owing to a fault very common in the construction of the marine barometer till lately in general use, that the cistern was not large enough to hold the mercury which descended from the tube in a low atmospheric pressure.

The practice which has long prevailed of mounting the marine barometer in wood is objectionable. The instrument recently introduced agreeably to the recommendation of the Kew Committee, is greatly superior to any other description of marine barometer which has yet been tested, as regards the accuracy with which it indicates the pressure of the atmosphere. The diameter of the cistern is about an inch and a quarter, and that of the tube about a quarter of an inch. The scale, instead of being divided into inches in the usual way, is shortened in the proportion of about 0·04 of an inch for every inch. The object of shortening the scale is to avoid the necessity of applying a correction for difference of capacity between the cistern and the tube. The perfection with which this is done may be judged of from the fact, that of the first twelve barometers tested at the Liverpool Observatory with an apparatus exactly similar to that used at Kew (whence these instruments were sent by railway, after being tested and certified), the index corrections in the two pressures of 28 and 31 inches in three of them were the same; two differed 0·001 of an inch; and for the remainder the differences ranged from 0·002 to 0·006 of an inch. The corrections for capacity were therefore considered perfect, and, with one unimportant exception, agreed with those given at Kew.

In order to check the pumping of the mercury at sea, the tubes of these barometers are so contracted, through a few inches, that, when first suspended, the mercury is perhaps twenty minutes in falling from the top of the tube to its proper level. When used on shore, this contraction of the tube causes the marine barometer to be always a little behind an ordinary barometer, the tube of which is not contracted. The amount varies according to the rate at which the mercury is rising or falling, and ranges from 0·00 to 0·02 of an inch. As the motion of the ship at sea causes the mercury to pass more rapidly through the contracted tube, the readings are almost the same there as they would be if the tube were not contracted, and in no case do they differ enough to be of importance in maritime use.

The method of testing thermometers is so simple as scarcely to require explanation. For the freezing point, the bulbs and a considerable portion of the tubes of the thermometers, are immersed in pounded ice. For the higher temperatures, the thermometers are placed in a cylindrical glass vessel containing water of the required heat; and the scales of the thermometers intended to be tested, together with the Standard with which they are to be compared, are read through the glass. In this way the scale readings maybe tested at any required degree of temperature, and the usual practice is to test them at every ten degrees from 32° to 92° of Fahrenheit. For this range of 60° the makers who supply Government are limited to 0·6 of a degree as a maximum error of scale reading; but so accurately are these thermometers made, that it has not been found necessary to reject more than a very few of them.

Hydrometers are tested by careful immersion in pure distilled water; of which the specific gravity is taken as unity.

In water less pure, more salt, dense, and buoyant, the instrument floats higher, carrying more of the graduated scale out of the fluid.

The zero of the scale should be level with the surface of distilled water, and rise above it in proportion as increase of density causes less displacement.

The scale is graduated to thousandths—as far as ·040 only—because the sea water usually ranges between 1·014 and about 1·036. Only the last two figures need be marked.

LONDON:
Printed by George E. Eyre and William Spottiswoode,
Printers to the Queen's most Excellent Majesty.
For Her Majesty's Stationery Office.