122 
MR. J. R. CHRISTIE ON THE USE OF THE BAROMETRIC 
and, calling (3' and b' two other corresponding values of (3 and b, we have 
•899 
log log ft— (& — &0- 
Substituting this value in Laplace’s formula for the determination of H, the number 
of English feet in vertical height between two stations, where the barometric pres- 
sures are (3 and (3 ! , and the mean temperature of the air at which is t, viz. 
H = 60345’6(log (3- log (3') { 1 -f(7— 32°)-00222 } , 
we finally get 
H = 547’99(6 — b'){l + (t- 32°)‘00222} II. 
That a very high degree of accuracy attaches to the formula I. is at once evident 
from the observations made, in test of it, by its author, and more especially from the 
one made by De Saussure on the summit of Mont Blanc with the same object. I 
proposed therefore, in undertaking a series of observations at elevated places, to re- 
strict myself to the question of the fitness of the barometric thermometer, in its pre- 
sent form, for affording trustworthy data; making observations at stations whose 
heights had been previously well-determined, and at as great elevations as circum- 
stances would admit: and, in case of feeling satisfied on this point, I considered 
that it would be interesting to determine the extent of accuracy to which such a 
series of observations would give the relative levels of successive places of observa- 
tion, without reference to the simultaneous observations at any fixed station. 
The instrument with which my observations were made consists of a brass cylin- 
drical boiler, 3 inches in depth and 2*25 inches in internal diameter, supported, when 
in use, upon a brass tripod stand ; in the upper disc of this cylinder a thermometer, 
graduated from 181° to 215° Fahr., is made to screw by means of a strong brass 
collar, one inch in diameter, surrounding the tube at the distance of two inches from 
the bulb, which is of a pear-shaped form, having the diameter of its largest circular 
section ’75 inch. The whole length of the scale is rather more than twelve inches, 
and each degree is *343 inch in length, subdivided into tenths, and reading, by means 
of a vernier, to hundredths ; the index at the zero point of the vernier by enveloping 
the tube prevents the intrusion of parallax. The water in the boiler is heated by 
means of a small spirit-lamp, which I have always found sufficient for that purpose, 
even in situations exposed to considerable force of wind, when protected by a shade 
formed by coiling up and pinning together the ends of a sheet of stiff paper. The 
escape of steam is at a small orifice, T5 inch in diameter, in the upper disc of the 
boiler, and also through the aperture left between the tube of the thermometer and 
its screw collar. In using the instrument I was careful to have the water as nearly 
as possible of the same depth on all occasions, and to make use of the purest I could 
procure, which was apparently always sufficiently free from any admixture of foreign 
matter likely to cause appreciable error in the boiling-point. 
It was necessary, before deducing any results from observations made, to deter- 
