410 



PROFESSOR FORBES ON THE DETERMINATION OF HEIGHTS 



to be higher, under a given pressure, than the temperature of steam which has 

 the same tension. Thus, comparing DALTON'S two tables 



it is exactly at the part of the scale where the difference is most practically im- 

 portant that it is most conspicuous, namely, between 190 and 212. The method 

 of observation used by Dr DALTON, does not admit of any great accuracy in ob- 

 serving the boiling points, and the numbers he has given are evidently only ap- 

 proximate. Still, from observations made under naturally low pressures (the only 

 ones worthy of much confidence in this case), I have found the same nonconfor- 

 mity of the theoretical tension of steam and the atmospheric pressure. 



In 1817, Archdeacon WOLLASTON described a thermometer destined particu- 

 larly for the purpose of determining heights.* But he seems not to have been 

 aware of the progress which the subject had already made in the hands of DE- 

 LUC and DE SAUSSUKE. The latter used a thermometer indicating jggg of a de- 

 gree of REAUMUR. WOLLASTON'S instrument, though a neat laboratory one, has 

 almost every fault which a travelling instrument can have, excepting only its 

 small dimensions, to which everything is sacrificed. It is apt to break, and still 

 more apt to be deranged, the contrivance for extending the scale being excessively 

 incommodious ; finally, it is impossible to use it in windy weather, and its indi- 

 cations are in an arbitrary scale. No was the method of calculating the heights 

 more happy. At first he contented himself with assuming the progression of 

 height to be proportional to the fall of the boiling point, near 212;f but he 

 afterwards \ extended his calculation from Dr URE'S table of tensions of vapour, 

 expressly stating, that he had used the proportionality of 1 of FAHRENHEIT to 

 0.589 inches of the barometer, or 530 feet, merely as an approximation for small 

 heights. 



A reference in BOUE'S Guide du Geologue Voyageur, directed me to a paper 

 by Mr PRINSEP, in the Journal of the Asiatic Society of Bengal for April 1833. 

 I hoped there to have found a table of boiling temperatures observed at great 

 heights in India. But it only contains a modification of TREDGOLD'S Formula of 



* Phil. Trans, vol. cxx. p. 183. 



t Ibid. p. 192. 



1 Ibid. vol. ex. p. 295. 



