176 M. W. Siemens on the Admissibility of the 
not appear to play any essential part. The cause of the for- 
mation of clouds is, as a rule, to be sought in the ascending 
and descending motion of the air, to which not only this and 
the rain falling from the clouds, but also the letting loose of 
storms is almost exclusively to be ascribed. The views on this 
subject which still prevail to a great extent in meteorology need, 
in my opinion, correction in some points. Were the equili- 
brium of the aerial ocean not constantly disturbed by unequal 
heating and cooling of the air by radiation, the temperature 
and density of the air could not but be in so-called indifferent 
equilibrium up to the greatest altitude, and in such wise that 
the loss of temperature with increasing height would be every- 
where equivalent to the work of expansion of the gas. The 
higher temperature of the air of the lower latitudes would be 
equalized by slowly coursing whirlwinds with a horizontal 
axis of rotation, as is shown on a large scale by the trade 
winds, and finally the entire sea of air would possess equal 
temperature at equal bights. This indifferent or adiabatic 
equilibrium is now continually disturbed by extra heating 
of the earth's surface and of the lower strata of the air by 
solar radiation, by absorption of the same on passing through 
the atmosphere, and by the extra cooling of the higher strata 
by radiation outwards. Through this the lower strata become 
lighter and the upper heavier than the adiabatic equilibrium 
requires ; and this disturbance must be compensated by 
ascending and descending currents in the atmosphere. As 
the ascending air, which has become warmer on the ground, in 
correspondence with the adiabatic curve of temperature pre- 
serves this excess of heat on ascending, but the upward impulse 
increases with the increasing height of the ascending current 
because the succeeding layers of air at the ground have always 
the same excess of temperature, the upcurrent must continue, 
in the places where it has once been produced by favourable 
local conditions, until the difference of temperatures is equa- 
lized. The work done by the upcurrent of the relatively 
lighter air and the descent in other places of the relatively 
heavier air cooled by radiation must be converted into vis viva, 
as it puts the air into quicker motion. This is effected essen- 
tially by the volume of the ascending air being increased by 
the diminution of pressure. Since, the diameter of the terres- 
trial globe being great, the air-space becomes only impercep- 
tibly larger with the height, the velocity of the ascending air 
must even from this cause increase nearly proportionally to 
the decrease of pressure. In the highest regions of the air to 
which each upcurrent formed will reach, the velocity of the 
air must therefore be very considerable j and there with the 
