on some of the leading doctrines of caloric, &c. 341 
generated, depresses more or less the barometric column. 
Hence, by subjecting the liquid to successive degrees of tem- 
perature, the corresponding depressions of the barometer, or 
elasticities of the vapour, are obtained. 
The only difficulty in this mode of operating, is to bring a 
considerable length of vertical tube to an uniform temperature 
Mr. Dalton, well aware of this source of error, obviated 
it in a great measure, by taking a series of different tubes, 
decreasing in their lengths with the increasing expansions of 
the vapour, and concomitant descent of the mercurial column. 
In several experiments conducted on this plan, I found it 
scarcely possible to obtain results rigidly corresponding with 
each other, when the column of vapour, exposed in the 
barometer tube to the influence of surrounding heat, exceeded 
two inches in length. 
M. Biot, in his system of physics recently published, while 
he adopts Mr. Dalton’s results as the basis of his reasoning, 
treats fully of this difficulty, and suggests an ingenious means 
of avoiding it. “ We have had occasion several times to 
“ remark/’ says he,“ that the temperature of a mass of liquid 
“ which cools in the air, is not entirely the same at the bottom, 
iC as it is at the top of the vessel; because the colder particles 
“ subside into the lower strata, by the excess of their weight. 
“ Thus the temperature of the column of hot water, which 
“ surrounds the tube in the preceding experiment, cannot 
“ be rigorously uniform throughout its whole height. We 
“ may endeavour to render it equal, by agitating and mingling 
“ the different strata of which it is composed ; but this would 
