58 
Proceedings of the Royal Society of Edinburgh. [Sess. 
It is a fact of universal observation that the atmosphere, as regards the 
vertical distribution of temperature, is almost always in a more stable 
state than the isentropic one. Hence if the temperature of any portion 
of it is altered, the change in its first stages will not produce convection 
at all ; and even considerable changes may occur without convection effects 
arising, provided the change of temperature is sufficiently continuous from 
top to bottom of the region affected to produce only a moderate change in 
the lapse rate. 
The entropy of any mass of air is maintained by a delicate balance 
between the absorption, radiation, convection, and internal production of 
heat. Each of these factors is liable to vary independently. 
The amount of heat absorbed by the mass depends upon the amount of 
radiation to which it is exposed and upon its coefficient of absorption. 
The former may be altered : — (a) By expansion or contraction of the mass. 
If this expansion is adiabatic, not only is the mass exposed to more solar 
radiation, but also, in consequence of the resulting diminution of temperature, 
it will itself radiate less actively. Thus its entropy is increased, and if it 
be again compressed adiabatically to its former volume it will be at higher 
temperature and pressure than before, (b) By the formation of cloud at 
higher levels, screening off a portion of the solar radiation ; or at lower level 
reflecting back a greater portion of the solar rays than would be returned 
from the earth through the intervening air. (c) By a general translation of 
the mass bringing it over terrestrial surfaces of a different reflecting or 
radiating power, (d) By a movement in latitude. A poleward movement 
of a mass of air in the lower reaches of the atmosphere must, in the winter 
hemisphere, be accompanied by a loss of heat. For not only has the solar 
radiation to pass through a greater thickness of absorbent air before reaching 
the mass under consideration, but it has also less time to operate owing to 
the longer night ; while, in addition, the underlying terrestrial surface will 
generally be cooler as the poleward movement continues. In the summer 
hemisphere, at any rate in the higher latitudes, it is probable that the 
increased duration of exposure overwhelms the other two effects and that 
the entropy increases with poleward movement ; but sufficiently exact data 
for the calculation are lacking. 
The coefficient of absorption may be altered by variation of density, 
temperature, humidity, dustiness, or cloudiness. It should be observed that 
any increase in the coefficient of absorption tends automatically to diminish 
the loss of heat by radiation. For the heat radiated from the central 
portions of the mass is absorbed to a greater extent than before by the 
outer portions, and so the total effective rate of radiation is diminished. 
