( 286 ) 
On the Determination of Heights by the Boiling Point of 
Water. By James D. Forsus, Esq., F.R.S., Sec. 
R.S. Ed., Corresponding Member of the Institute of 
France, and Professor of Natural Philosophy in the Uni- 
versity of Edinburgh. With Two Plates.* 
It was observed by Fahrenheit, that the boiling point of 
water depends on the height of the barometer, the pressure 
of the air hindering the conversion of water into steam by a 
resistance which must be overcome by an increase of heat. 
Deluct and De Saussuref contrived apparatuses for making 
the observation in the open air, and at great heights, and 
appear to have contemplated the substitution of the ther- 
mometer for the barometer upon occasion. They, as well as 
Dr Horsley,§ Sir George Schuckburgh,|| and Mr Cavendish,¥ 
seem to have regarded the question as one which concerned 
the fixity of the point used in graduating thermometers, and 
its requisite corrections, rather than as applicable to baro- 
metric purposes generally. Several of them have given em- 
pirical tables for correcting the boiling point within the 
limits of the usual barometric variations, but one only, M. 
Deluc, has given a formula for connecting the indications of 
the barometer with the boiling point of water throughout 
the range which the barometer has been observed to vary 
on the earth’s surface. This is the on/y formula immediately 
deduced from direct observations of the boiling point; and 
having been verified by De Saussure at a height greater than 
the limits for which it was constructed, and having else- 
where been declared by him to be so accurate as to super- 
sede farther experiment on the subject, it might have been 
expected to be generally adopted, or at least known. We 
find, however, that though it has been occasionally copied 
into the formal articles of Encyclopedias, as a correction in 
* From Transactions of the Royal Society of Edinburgh, vol. xv., 
part iii. 
+ Modificationsde l’ Atmosphere, tome ii. + Voyages, secs. 1275, 2011. 
§ Phil. Trans. vol. Ixiv. || Ibid., vol. Ixix. 
q Ibid., vol. Ixvii. p. 816. 
