IN THE PRESSURE OF THE ATMOSPHERE. 479 
function of the temperature; however, the correctness of this 
assumption is evinced for the first time by the reasons now 
advanced. 
As regards the motions of the atmosphere in vertical direction, 
by which the barometer is moved periodically during the day 
and the year, the nature of the district over which the air ascends, 
whether it be solid or liquid, will be of essential influence. 
Formerly, when we possessed only observations of the sea-cli- 
mate, the first case could only be imaginarily represented by 
calculation ; now, that we are able to avail ourselves of observa- 
tions of the continental climate, the abstraction formerly obtained 
so completely merges in reality, that this latter almost loses the 
charm of novelty. 
1. Annual Variations. 
As the dry air and the aqueous vapour mixed with it press 
conjointly upon the barometer, the raised mercurial column con- 
sequently consists of two parts, one of which is supported by 
the dry air, the other by the vapour; we shall easily understand 
that, as with rising temperature, the air increases in volume, and 
consequently ascends and flows off laterally above, while, on the 
other hand, at a higher temperature the evaporation increases, 
and therefore the elasticity of the vapour contained in the air is 
augmented, the periodical changes of the barometer cannot stand 
in a very simple connexion with the periodical changes of tem- 
perature; for as long as we are unacquainted with the quanti- 
tative proportion of both these changes, which occur at the same 
time but in opposite direction, we cannot even decide whether 
the total pressure will augment or diminish with an increase of 
heat ; whether perhaps during one portion of the period the pre- 
ponderance may not be on the side of the one change, while 
during the remainder of the period it is on the side of the other. 
It is evident therefore that we can only expect to obtain a know- 
ledge of these variations by cotemporaneous observations of the 
barometer and hygrometer. 
What is gained by this separation will be seen by a glance at 
the illustrations which accompany this memoir. The figures 
1, 2, 3, Plate VI. contain the variations of the total pressure 
during the annual period; fig. 4, the annual variations of the 
pressure of the dry air only. In the temperate zone the places 
having a continental climate are separated from those having a 
