224 Dr. A. Woeikof s Examination of 



mobility of the particles, but only as causing the removal of 

 heat from the tropics by ocean-currents.) Dr. Croll seems to 

 think that the high temperature of the surface of the ocean is 

 caused by the difficulty with which water gets rid of its heat 

 by radiation, as if, here as in the case of land, loss of heat by 

 radiation caused a low temperature of the surface. 



Now this is evidently not the case, and the convection- 

 currents originating as soon as the surface temperature sinks 

 below that of the stratum immediately under it, bring the 

 latter to the surface, and thus maintain constantly a higher 

 temperature of the surface than of the other strata, although 

 they are rather conducive to a loss of heat by the whole 

 mass, as the colder water sinks to the bottom, where it is out 

 of the reach of the radiant heat of the sun, and receives heat 

 only by the slow process of conduction*. 



Dr. Croll does not see that, instead of " the difficulty of the 

 water to get rid of its heat," he really considers the cause of 

 the high temperature of the surface of the ocean ; and here 

 misses the most efficient cause. 



The next point I have to notice is this : " The quantity of 

 heat lost by expansion must therefore be trifling in compa- 

 rison of that lost by radiation ; and although the heat lost by 

 expansion is fully restored by compression, yet the air would 

 reach the earth nearly entirely deprived of the heat with which 

 it left the equator. All that it could possibly give back 

 would be the heat of compression ; and that would hardly be 

 sufficient to raise the air at —50° F. to the freezing-point/'t 



As before this, Dr. Croll considers the temperature of the 

 upper atmosphere, even under the equator, to be 80° F. below 

 freezing-point ; and as this is reached, according to him, at such 

 a height that the air, returning towards 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, 

 because otherwise the temperature at which the upper strata 

 arrive at sea-level could not be lower than the existing tempera- 

 ture of the lower strata ; but in Dr. Croll's opinion it is 48° 

 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 this was really 

 the case, if there existed an unstable equilibrium, why do not 

 convection-currents of great magnitude arise in our atmo- 

 sphere and bring a temperature of 32° to the sea-level at the 



* I consider here the case of a body of water having constantly a 

 higher temperature than that of the maximum density. The oceans are 

 certainly in that state. 



t Pp. 25 and 26. 



