344 On Ifypsometrical Measurements. [No. 4. 



when the expansion of the mercury alone has to be taken into 

 account ; but when the scale of the barometer is brass and extends 

 from the cistern — 



Log B — log B' = log b — log V — 0.00003905 (t — t'). 



Example. 

 M M Bravais and Martins made the following observations : — On 



mm. 



M. St. Bernard 8114 feet above the level of the sea B = 568.03, 

 t = 7!*6Cent. or 45.7 F. and a = 0.59 ; and on M. Blanc B' = 



mm. 



424.29, t' = — 9.1 C. or 15.3 F. and a = 0.57. 

 Here 



B =22!i64.. log 1.34955 -§ (t + t') = 30°6 F. 



B' = 16.705. . log 1 22284 a + a' = 1.16 



Diff. 0.12671. . log 9.102811 ^/bF. . log 8.7138 - 10 



Table I,. Log A. 4.780555 Tab. I. log C 8.5078 



Table II, Log C. 0.000842 a + a. . log 0.0645 



Table III,. ... Log G-.— 0.000033 



a -f* a' 



O. = 7.2861 



\/B B' 



If — h ... 7659 ft. ..log 3.884175 



Then by Table IY. 7659 -f 12.9 ~ 3.2 -f 8114 = 15782.7 feet 

 the height. 



II. — Measurement of Heights by tlie Boiling Point of Water. 



1. It has long been known that the temperature of ebullition is 

 lowered, as the pressure under which a fluid boils is decreased. This 

 was first used by Archdeacon Wollaston, about 1817, as a means of 

 measuring altitudes. "Wollaston, however, in drawing up his table of 

 heights corresponding to different temperatures of boiling water, 

 made use of the empirical formula of Dr. Ure, for the elasticities of 

 steam of different temperatures, and which was not calculated to 

 give accurate results. 



In the Journal of the Asiatic Society for 1833, the late Mr. James 

 Prinsep discussed the subject, using Tredgold's formula for the 

 tension of steam, but modifying the results to make them agree 



