Woeikof — CrolVs Hypotheses of Geological Climates. 163 



ing 'point, and as this is reached, according to him, at such a 

 height that the air, returning toward the surface of the sea, 

 would be warmed to the freezing point, it is clear that, in his 

 opinion, the atmosphere is in a state of unstable equilibrium, be- 

 cause otherwise the temperature at which the upper strata arrive to 

 -sea level would not be lower than the existing temperature of the 

 lower strata, but, in Dr. Croll's opinion, it is by 48° F. lower, as 

 the mean temperature of the equator is about 80° F. and the 

 air of the higher regions, in sinking to sea level, would bring 

 with it a temperature of 32° F. 



Now if really the case was such, if there existed an unstable 

 -equilibrium, why do not convection currents of great magni- 

 tude arise in our atmosphere and bring a temperature of 32° to 

 the sea level at the equator ? 



The reason why such convection currents do notarise is, that 

 the normal state of the air is that of stable equilibrium, even a 

 very stable one, so that, if by any cause the higher strata be 

 forced clown, they would acquire by compression a much 

 higher temperature than the lower have. 



Am unstable equilibrium, a few cases excepted,* does exist in 

 the lower strata, but only to a few thousand feet, and this only 

 during daytime, when the surface of the ground is much heated 

 by the sun. It disappears about sunset, even somewhat earlier. 

 It does not happen in winter in high and even in higher middle 

 latitudes (say from 50° onward). All this has been established 

 by so numerous observations in mountain countries and on 

 balloons that it can scarcely be doubted. In any case, if Dr. 

 Croll is in doubt concerning such well known facts, that is, if 

 he doubts that between the lower strata of air and those some 

 tens of thousands feet high there exists a stable equilibrium, the 

 onus probandi rests with him. In fact, a temperature —50° F. 

 must be found at such a considerable height above sea level 

 that the air, if forced down, would arrive warmer and not 

 colder than the air existing at sea level. 



Besides, I maintain that the radiation of particles of air is 

 but a very trifling cause of loss of heat by our globe, and that 

 by far the principal causes of it are the radiation of the surface 

 of the ground (or snow) and that of the surface of the water. 

 If the lowering of temperature of the surface of land and of 

 the air be considered, the loss of heat by radiation of the sur- 

 face of the ground (and snow) is by far the most important. I 

 can much better agree with Dr. Croll in what he remarks about 

 the conservative character of snow once formed, though, as will 

 be seen further on, I do not agree as to the importance of winter 

 in aphelion during high eccentricity. 



Further, I think Dr. Croll does not well understand the 

 * For example, in thunder-storms, hail-storms, etc. 



