Meteorological Essays and Observations. 337 
The density and temperature of this fluid have also a tendency to 
decrease, as its distance from the surface augments by a rate less 
rapid than that of the former: Its equipoise would be maintained 
by the adaptation of the upper parts of the medium, in which it 
moves, to the progression of its temperature, and by a current flow- 
ing from the hotter parts of the globe to the colder. Constant eva- 
poration on the line of greatest heat, and unceasing precipitation at 
every other situation, would be the necessary accompaniments of this 
balance. ‘The conditions of these two states of equilibrium, to which, 
by the laws of hydrostatics each fluid must be perpetually pressing, 
are essentially opposed to each other. The vapour or condensible 
elastic fluid is forced to ascend in a medium, whose heat decreases 
much more rapidly than its own natural rate : and, it is consequently 
condensed and precipitated in the upper regions. Its latent caloric 
is evolved by the condensation, and communicated to the air; and 
it thus tends to equalize the temperature of the medium in which 
it moves, and to constrain it to its ownlaw. ‘This process, the 
author considers, must evidently disturb the equilibrium of the per- 
manently elastic fluid, by interfering with that definite state of tem- 
perature and density which is essential to its maintenance. ‘The 
system of currents is unequally affected by the unequal expansion, 
and the irregularity extended, by their influence, much beyond the 
sphere of the primary disturbance. The decrease of this elasticity 
above, is accompanied by an extremely important re-action upon the 
body of vapour itself, being compelled to accommodate itself to the 
circumstances of the medium in which it moves, its own law of dene 
sity can only be maintained by a corresponding decrease of force 
below the point of condensation ; so that the temperature of the air, 
at the surface of the globe is far from the term of saturation; and 
the current of vapour which moves from the hottest to the coldest 
points, penetrates from the equator to the poles, without producing” 
that condensation in mass, which would otherwise cloud the whole 
depth of the atmosphere with precipitating moisture. The clouds 
are thereby confined to parallel horizontal planes, with intermediate, 
clear spaces, and thus arranged are presented to the influence of the 
sun, which dissipates their accumulation, and greatly extends the 
expansive power of the elastic vapour. ‘The power of each fluid 
being in proportion to its elasticity, Mr. Daniell considers that of the 
vapour compared with the air can never exceed at most 1.30; so 
that the general character of the mixed atmosphere is derived from 
the latter, which in its irresistible motions must hurry the former 
along with it. The influence, however, of the vapour upon the air, 
though slower in its action, is sure in its effects, and the gradual and 
silent procgsses of evaporation and precipitation govern the boisterous 
power of the winds. By the irresistible force of expansion unequally 
applied, they give rise to undulations in the elastic fluid; the return- 
