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 .jpg){kind=small}
. 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
