1884.1 
in Land and at Sea. 
195 
The efficacy of the other cause, which we must conse- 
quently acknowledge as the main cause of the entire 
phenomenon, is easily seen. The velocity of the movement 
of the air augments, generally speaking, from the earth’s 
surface to a great altitude, on account of the decrease of 
fridtion. When, therefore, masses of air arrive from the 
heights at the surface of the earth they bring with them, 
generally speaking, a greater horizontal velocity than the 
masses of air under the same gradient which have been in 
prolonged contadt with the earth’s surface. 
Since cold air, at the same external pressure, is denser — 
or, in other words, heavier — than warm air, there follows, 
when the magnitude of the decrease of temperature exceeds 
a certain value, an unstable equilibrium. A mass of air 
which has begun to sink becomes denser than its surround- 
ings, and thus is impelled to sink lower, and, inversely, a 
mass of air which rises is inclined to further ascent. The 
vertical decrease of temperature which corresponds to this 
unstable equilibrium is T C. per 100 metres for dry air, and 
between 5- 0 and T C. for moist air. If this value is reached 
everywhere, or partially over a large surface, there occurs a 
play of descending and ascending currents of air which in 
damp weather makes itself manifest by cumuli piled up like 
mountains with blue interstices. The clouds represent 
masses of air which are ascending, in consequence of the 
decrease of pressure on being refrigerated. Hence a part of 
their moisture is separated out in the shape of the minutest 
drops, forming clouds. The clear, cloudless intervals are 
formed by descending masses of air which are experiencing 
the opposite process, of increasing temperature and heat, 
and are consequently removing further from their point of 
saturation. 
The mean value of the decrease of temperature upwards 
is greatest in the warmest hours of the day. In the night 
the difference of temperature between above and below be- 
comes insignificant. In clear nights there is even, up to a 
certain altitude, a regular inversion of the difference, the 
temperature at the earth’s surface being lower than it is 
above. • This involves a very stable equilibrium of the air 
and a very trifling vertical circulation. In calm days, with 
bright sunshine, therefore especially in summer, the pre- 
viously mentioned unstable equilibrium sets in a few hours 
after sunrise, and ceases a short time before sunset. The 
daily period of the velocity of the wind has the greatest 
correspondence with the formation of cumuli ; but the con- 
nedtion^of the two phenomena is far from complete, since 
