Chapter 3

Such a diagram may be filled up by _uncorrected_ observations, its object being to serve as a weather guide for immediate use, rather than for future investigation. If closely kept up, it will prove to be of utility, and will in some degree reward the trouble of keeping a regular record. For purely scientific objects much more nicety and detail are required.

* * * * *

HESITATION is sometimes felt by young seamen, at first using the vernier of a barometer, for want of some such familiar explanation as the following:--

The general principle of this moveable dividing scale is, that the total number of the smallest spaces or subdivisions of the vernier are made equal, taken altogether, to one less than that number of the smallest spaces in an equal length of the fixed scale.

For example: ten spaces on the vernier being made equal to nine on the scale, each vernier space is one tenth less than a scale space; and if the first line or division of the vernier agree exactly with any line of the scale, the next line of the vernier must be one tenth of a tenth (or one hundredth) of an inch from agreement with the next _scale_ division; the following vernier line must be two hundredths out, and so on: therefore, the number of such differences (from the next tenth on the scale) at which a vernier line agrees with a scale line, when set, is the number of hundredths to be added to the said tenth; (in a common barometer, reading only to hundredths of an inch).

The vernier of a barometer reading to thousandths of an inch, is on a similar principle, though differently divided. In this application of it, generally, twenty-five vernier spaces equal twenty-four of the scale spaces, which are each half a tenth, or five hundredths of an inch; therefore, the difference between one of the vernier and one of the scale is two-tenths of a hundredth, or two thousandths of an inch [25)·050(·002].

This is the usual graduation of scientific barometers; but for ordinary purposes, as weather-glasses, a division, or reading, to the hundredth of an inch is sufficient.

When set properly, the vernier straight edge, the top of the mercury, and the observer's eye, should be on the same level; the edge (or pointer) just _touching_[36] the middle and uppermost point of the column.

Great care should be taken to look thus square, or at right angles to the scale.

Light, or something white, at the _back_ of the tube, assists in accurately setting the vernier, and may be shifted about to aid in reading off.

* * *

THE ANEROID has been recommended, in these pages, as a weather-glass; but it may increase its usefulness to append a table for measuring heights (approximately) by this, or any barometer, which can be compared with another, or itself, at a higher or lower station.

If the measure of a height rather greater than the aneroid will commonly show, be required, it may be _re-set_ thus--When at the upper station (_within its range_), and having noted the reading carefully, touch the screw behind so as to bring back the hand a few inches (if the instrument will admit), then read off and start again. _Reverse the operation when descending._ This may add some inches of measure _approximately_.

* * * * *

In the following Table, the difference between the number of feet opposite the height of a barometer, at one station, and that at another station, is their approximate difference of height.

TABLE.

+-----------++-----------+-----------++-----------+----------- Barometer | Height in || Barometer | Height in || Barometer | Height in Inches. | feet. || Inches. | feet. || Inches. | feet. -----------+-----------++-----------+-----------++-----------+----------- 31·0 | 0 || 26·8 | 3829 || 22·7 | 8201 30·9 | 85 || 26·7 | 3927 || 22·6 | 8317 30·8 | 170 || 26·6 | 4025 || 22·5 | 8434 30·7 | 255 || 26·5 | 4124 || 22·4 | 8551 30·6 | 341 || 26·4 | 4223 || 22·3 | 8669 30·5 | 427 || 26·3 | 4323 || 22·2 | 8787 30·4 | 513 || 26·2 | 4423 || 22·1 | 8906 30·3 | 600 || 26·1 | 4524 || 22·0 | 9025 30·2 | 687 || 26·0 | 4625 || 21·9 | 9145 30·1 | 774 || 25·9 | 4726 || 21·8 | 9266 30·0 | 862 || 25·8 | 4828 || 21·7 | 9388 29·9 | 950 || 25·7 | 4930 || 21·6 | 9510 29·8 | 1038 || 25·6 | 5033 || 21·5 | 9632 29·7 | 1126 || 25·5 | 5136 || 21·4 | 9755 29·6 | 1215 || 25·4 | 5240 || 21·3 | 9878 29·5 | 1304 || 25·3 | 5344 || 21·2 | 10002 29·4 | 1393 || 25·2 | 5448 || 21·1 | 10127 29·3 | 1482 || 25·1 | 5553 || 21·0 | 10253 29·2 | 1572 || 25·0 | 5658 || 20·9 | 10379 29·1 | 1662 || 24·9 | 5763 || 20·8 | 10506 29·0 | 1753 || 24·8 | 5869 || 20·7 | 10633 28·9 | 1844 || 24·7 | 5976 || 20·6 | 10760 28·8 | 1935 || 24·6 | 6083 || 20·5 | 10889 28·7 | 2027 || 24·5 | 6190 || 20·4 | 11018 28·6 | 2119 || 24·4 | 6297 || 20·3 | 11148 28·5 | 2211 || 24·3 | 6405 || 20·2 | 11278 28·4 | 2303 || 24·2 | 6514 || 20·1 | 11409 28·3 | 2396 || 24·1 | 6623 || 20·0 | 11541 28·2 | 2489 || 24·0 | 6733 || 19·9 | 11673 28·1 | 2582 || 23·9 | 6843 || 19·8 | 11805 28·0 | 2675 || 23·8 | 6953 || 19·7 | 11939 27·9 | 2769 || 23·7 | 7064 || 19·6 | 12074 27·8 | 2864 || 23·6 | 7175 || 19·5 | 12210 27·7 | 2959 || 23·5 | 7287 || 19·4 | 12346 27·6 | 3054 || 23·4 | 7399 || 19·3 | 12483 27·5 | 3149 || 23·3 | 7512 || 19·2 | 12620 27·4 | 3245 || 23·2 | 7625 || 19·1 | 12757 27·3 | 3341 || 23·1 | 7729 || 19·0 | 12894 27·2 | 3438 || 23·0 | 7854 || 18·9 | 12942 27·1 | 3535 || 22·9 | 7969 || 18·8 | 13080 27·0 | 3633 || 22·8 | 8085 || 18·7 | 13219 26·9 | 3731 || | || | -----------+-----------++-----------+-----------++-----------+-----------

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.

FOOTNOTES:

[1] In South latitude the South wind corresponds to our North wind in its nature and effects. The Easterly and Westerly winds retain their respective peculiarities in both hemispheres.

[2] Exclusive of local land and sea breezes of hot climates.

[3] Glass, barometer, column, mercury, quicksilver, or hand.

[4] Or atmosphere, or the atmospheric fluid which we breathe.

[5] Or exhaustion.

[6] A vacuum.

[7] See pages 24 and 25.

[8] Thirty-two degrees is the point at which water begins to freeze, or ice to thaw.

[9] Evaporation.

[10] The two thus combined making a hygrometer: for which some kinds of hair, grass, or seaweed may be a make-shift.

[11] It stands lower, about a tenth of an inch for each hundred feet of height directly upwards, or vertically, above the sea; where its average height, in England, is 29·94 inches (at 32°).

[12] In an Aneroid, a metallic, or a wheel barometer, the hand's motion should correspond to that of mercury in an independent instrument.

[13] Southerly in South latitude.

[14] In the best columns, those of standards for example, no concavity is seen, at any time: but it is otherwise with many barometers, which do show a concavity.

[15] In these cases there is usually a combination or a contest of currents in the atmosphere, horizontally, _or_ one _above_ the other, or diagonally.

[16] Thunder clouds sometimes rise and spread against the wind (lower-current). It is probable that there is a meeting, if not a contest of air currents, electrically different, whenever lightning is seen. Their concurrence, when the new one advances from _polar_ regions, does not depress the barometer, except in oscillations of the mercury, which are very remarkable at some such times.

[17] Aneroids, metallic barometers, and oil sympiesometers, seem to be much more affected than mercurial barometers by electrical changes.

[18] Southerly, in North latitude; the reverse in the Southern hemisphere.

[19] A "high dawn" is when the first indications of daylight are seen above a bank of clouds. A "low dawn" is when the day breaks on or near the horizon. The first streaks of light being very low.

[20] Indications of weather, afforded by colours, seem to deserve more critical study than has been often given to the subject. Why a rosy hue at sunset, or a grey neutral tint at that time, should presage the reverse or their indications at sunrise;--why bright yellow should foretell wind at either time, and pale yellow, wet;--why clouds seem soft, like water colour; or hard edged, like oil paint, or Indian ink on an oily plate;--and why such appearances are infallible signs--are yet to be shown satisfactorily to practical men.

[21] In the trade winds of the tropics there is usually a counter current of air, with light clouds,--which does not indicate any approaching change. In middle latitudes such upper currents are not so evident, except before a change of weather.

[22] _Much_ refraction is a sign of Easterly wind. _Remarkable_ clearness is a bad sign.

[23] The "young moon with the old moon in her arms" (Burns, Herschel, and others) is a sign of bad weather in the temperate zones or middle latitudes, because (probably) the air is then exceedingly clear and transparent.

[24] Even in ordinary changes of weather it is interesting, as well as useful, to mark the formation or disappearance of clouds, caused by colder and warmer currents of air mixing: or intermingling.

[25] Depending on pressure and temperature.

[26] Sir James Ross--M. Daussy.

[27] Williwaw (Whirl-awa?) of the old sealers and whalers.

[28] Seamen call the light sails, used only in very fine weather, "flying kites."

[29] Herschel.

[30] Dové.

[31] For a barometer of this kind, Admiral Milne has invented self-registering mechanism, that answers well.

[32] A small turnscrew being applied gently to the screw head at the back. This is often necessary, on receiving or first using an aneroid that has long been lying by, or that has been shaken by travelling.

[33] It is a good weather glass--to be suspended on or near the upper deck, for easy reference;--and is unlikely to be injured by mere concussion of air, or vibration of wood, when guns are fired.

[34] Allowing 0,0011 of an inch for each foot.

[35] The manufacture of these useful auxiliary instruments (all French originally) has increased much latterly: and now the patent has expired. They might be so improved so to be worth more than double their present value.

[36] Like the sun's edge or limb, touching the sea horizon, as seen inverted when using a sextant.

End of Project Gutenberg's Barometer and Weather Guide, by Robert Fitzroy