Midland Naturalist/Volume 01/On an Improved Aneroid Barometer

by William Jerome Harrison

On an Improved Aneroid Barometer.

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By W. J. Harrison, Esq., F.G.S.

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An instrument which shall accurately indicate differences in level or height above the sea is much needed by practical men of science. The geologist requires it to ascertain the varying heights of his beds of rock, thy zoologist and the botanist to know the limits of the zones of animal and vegetable life, and it is of not less service to the meteorologist, the surveyor, the engineer, and the traveller.

The most accurate instrument for this purpose is the ordinary mercurial barometer, arranged in as compact a form as possible, and swinging by the centre from a tripod stand, when it is knows as a Mountain Barometer. Even in this form it is, however, of considerable weight, of awkward form, and liable to break.

The Hypsometer is a simple instrument, consisting of a thermometer inserted in a partly closed vessel containing multiple water, which is made to boil by means of a spirit lamp. Now on a mountain top, the water being under less pressure, will boil at a lower temperature than at the sea level. Thus Tyndall found the boiling point of water on the top of Mont Blanc to be 185° F., showing a lowering of the boiling paint of 1° F. for every 596ft. of assent.

The Sympiesometer, invented by Adie, of Edinburgh, measures pressure by means of a glass tube some 18in. long, closed at one end and bent round at the other. The lower part is filled with glycerine, and by the varying pressure of the outside air on this fluid, the air in the upper part of the tube is compressed or allowed to expand as the case may be, the amount being shown by a scale marked on the tube.

Lastly we have the Aneroid Barometer, which consists of a thin, hollow corrugated metallic box, almost exhausted of air, and the lid of which is prevented from sinking too far in by a strong spring which is attached to it. The spring is connected with levers, which move a pointer over a graduated dial. When the pressure of the air increases, the lid of the exhausted box is forced further in and the pointer moves in one direction, and vice versa should the air become lighter. All aneroids are graduated by comparison with a standard Mercurial Barometer. and they vary in size from 2in. to 12in. in diameter.

A few months ago it was proposed to attempt the construction of an accurate topographical model of Leicestershire, commencing with the Charnwood Forest district, and gradually adding square by square of the region around until the whole county was shown in relief. The model once executed several casts could be taken from it, and if is intended to colour one of these geologically, and to show river-basins, the distribution of plants, on a second. Such models would teach many important lessons, would be interesting and instructive to everyone, and the very task of construction could not fail to yield valuable results. The best method of executing such models I hope to lay before readers of this journal on some future occasion.

Having obtained a plain Ordnance map of the district we inhabit, if we desire to ascertain the height of the places named thereon, we must refer if possible to some standard, Now the Ordnance Survey published in 1861 an "Abstract of the principal lines of Spirit Levelling in England and Wales," giving to a fraction of a inch the heights of some thousands of stations. In this valuable work are given the heights of many points between Rugby and Leicester, between Leicester and Burton, and between Leicester and Nottingham, which heights are indicated upon public buildings. &c., by means of the well-known bench mark . The points thus marked would then serve as starting points or for reference, but it is necessary to ascertain the height to within a foot or so of a great many other places, and after a consideration of the various Instruments described in the early part of this article, it was determined to use a new form of aneroid, invented by Mr. Rogers Field and made by Casella, and a grant for the purpose was obtained from the Literary and Philosophical Society of Leicester.

Most aneroids have attached to them a scale of feet; in some this is moveable, the altitude being obtained by setting the zero of the scale at the lower station to correspond with the position of the hand, and then reading off at the upper station only. This cannot give true results, as the scale can only be correct when the zero is in the position in which it was graduated. Aneroids, with fixed scales, are correct only at one given temperature, and at one only. It is true that, by using the aneroid as an ordinary barometer, and reading off in inches, we can, by a long calculation, arrive at a correct result, but when some hundreds or thousands of observations have to he made, the process of reduction will evidently be a wearisome one; and if we can read off correctly in feet, at once, it will be a great advantage.

The accompanying woodcut shows Field's aneroid as an instrument 2¾in. in diameter, and a little more than an inch in thickness, thus fitting easily in the waistcoat pocket. Its novelty consists in the fact that the lid marked A A is moveable, and bears nicks, which lock with a pin on the fixed lower case, These nicks are marked from 30° F to 75° F., and the rim must be moved round until the nick bearing the temperature of the air at the time of observation is fixed by the lock pin. The scale of feet on the inside of the rim is then correct for that temperature. The altitudes are in all cases determined by taking two readings, one at the lower station and another at the upper, and then subtracting.

The principle, in fact, is this: an ordinary aneroid scale is correct at one temperature only, and is incorrect at every other: but "the very fact of the scale becoming inaccurate for the temperature for which it was graduated renders it practically accurate for some other temperature, so that the shifting of the scale into certain fined positions answers the same purpose as if the original scale were altered to suit various temperatures of the air."

Another improvement which has been introduced has been the jewelling of the working parts of the interior like a watch, the effect of which has been to increase its sensitiveness in a marked degree, and to render unnecessary the repeated tappings by which an ordinary aneroid must be subjected. It bas further been tested and verified at Kew Observatory. A certificate from Kew should be demanded by every purchaser of meteorological instruments, of whatever description. The aneroid thus constructed is not intended to measure very great elevations; indeed, its scale does not extend above 5,000ft., but this permits of reading with accuracy, by means of a pocket magnifier, to 2ft. or 3ft. To set it, we obtain the temperature of the air, by means of a small whirling thermometer, (thermometre fronde,) which is strung round the hand by a cord, thus giving true shade temperature even under the direct rays of the sun; but this is really hardly necessary, as if it be set approximately within 5° F. a nearly correct result will be obtained.

The following instances may be taken as a fair test of the accuracy and simplicity of the aneroid:—

Now, the true difference of the heights of these two points, according be the Ordnance Survey, is 667ft., the Leicester Museum being 237ft., and the Town Hell, Derby, 159ft. above mean or half-tide sea level at Liverpool, the error in determination being 2ft. only.

Adding this difference of 667ft. to the known height of the Museum above the sea (237ft.) we get 904ft as the height of Bardon Hill, while the height obtained by levelling is 902ft.

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