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IX. On the Use of the Barometric Thermometer for the Determination of Relative 
Heights. Bi/ James R. Christie, Esq., of the Royal Military Academy. Com- 
municated hy S. Hunter Christie, Esq., Sec. R.S., 8$c. 
Received November 28, 1845, — Read January 29, 1846. 
A.LTHOUGH the observation of the temperature of boiling 1 water has been for 
some time, but not extensively, employed for the determination of relative heights, 
yet the only means which experiment has confirmed of reducing it to a measure of 
the atmospheric pressure as usually estimated by the height of an equiponderate 
column of mercury has, till very recently, been overlooked ; and it may perhaps be 
owing to this circumstance that the instrument for making the requisite observations 
remains to have fully developed in it the advantages it undoubtedly possesses, in por- 
tability and strength of construction, over the fragile and easily deranged baro- 
meter. 
My attention having been called to this subject by a remark made by Professor 
Forbes in his interesting work on the Alps, to the effect that he had found the tem- 
perature of boiling water to decrease uniformly with the increase in height of the 
place of observation, and at the rate of one degree of Fahrenheit for every 550 
feet of vertical ascent, I considered that it would be highly satisfactory to verify this 
result during an excursion over the Alps of Savoy and Piedmont which I then had 
in contemplation, and in the course of which I proposed to visit some localities at 
very considerable elevations above the sea level : and I was induced also to seek for 
some foundation for this very simple law. In prosecuting the latter inquiry, I soon 
found that, by assuming the truth of De Luc’s formula for the determination of the 
boiling-point from the barometric pressure, at all accessible heights, a corroboration 
of the law in question is at once arrived at. I have since found, by reference to a 
paper in Vol. xv. of the Transactions of the Royal Society of Edinburgh, that Professor 
Forbes had himself verified his original conjecture in the same manner. 
The formula alluded to, when reduced to the common English units of measure, 
becomes 
99 
b— *899 " lo §> 10 P — 6 °' 804 I. 
where h is the variable boiling-point on Fahrenheit’s scale, and (3 the corresponding 
barometric pressure in inches of mercury. From this we obtain at once 
«QQQ 
log 10/3= -gjj- (6 + 60-804) 
r 2 
