40 New experimental Researches 
heats, and difficult to render air-tight in the lower temperatures, 
I abandoned it, after some unsatisfactory trials. At the low de- 
grees of heat, the vacant part of the barometer tube introduces 
errors, since it has not the,temperature of the boiler ; and the 
bulb of the barometer, used in high heats, occasions a similar 
fallacy in the determination of the true elasticities. 
Still, however, it was ingeniously conceived, and the results 
furnish good approximations, creditable to the celebrated experi- 
menter*, They agree nearly with those of Betancourt, being 
obtained probably in a similar way. The method adopted by 
Mr. Dalton is recommended by an elegart simplicity. It is 
merely a common barometer, into which a little of the vapour- 
giving liquid is introduced, so as to moisten, and float above the 
mercury. The vapour which is generated, depresses more or less 
the barometric column. Hence, by subjecting the liquid to suc- 
cessive degrees of temperature, 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 con- 
siderable 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 expe- 
riments 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 avoid- 
ing 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, 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 pre- 
ceding experiment, cannot be rigorously uniform throughout its 
whole height. We may endeavour to render it equal, by agi- 
tating and mingling the different strata of which it is composed ; 
but this would be attended with no little difficulty. It would 
be better to have several thermometers suspended at different 
heights, in the body of the water, and to take the arithmetical 
mean of their indications Or otherwise, which would probably be 
more exact, we might employ a thermometer having a cylindrical 
* See Encyclopedia Britannica, vol. xvii. p. 739, 2d edition. 
bulb y 
