162 



} 1 &dtyfar-CroW8 Hyjioth^styof Geological Climates. 



the land, Tjut even 'seas which receive no such cold water, and 

 are known as being very warm, such as the Mediterranean and 

 Eed seas, have a mean temperature considerably lower than the 

 land. 



It might, perhaps, be remarked, that Dr. C roll's startling 

 statement is a simple u lapsus pennce" and that he does not 

 consider the mean temperature of the whole column of ocean 

 water, but the mean annual temperature of the surface; yet it is 

 clear, especially from the statement on page 33, that such is not 

 the fact. Dr. Croll considers the difficulty in the sea of "get- 

 ting quit of its heat as rapidly as the land," and in this passage, 

 as in the former, he seems to have forgotten entirely the mobil- 

 ity of the particles of water, which is so extremely important a 

 fact and which so essentially affects the thermal relations of 

 water by the convection currents which it causes. A few pages 

 earlier he mentions the 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 sur- 

 face of the ocean is caused by the difficult}^ in the water to get 

 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 so, and the convection cur- 

 rents arising 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 that of the other strata, but are rather con- 

 ducive 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 in the 

 water's getting 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 compar- 

 ison of that lost by radiation ; and. although t^he 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."f 



As before, Dr. Croll mentions the temperature of the upper 

 atmosphere, even under the equator as being 80° F. below freez- 



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

 perature than that of the maximum density. The oceans are certainly in that 

 state. f Pages 25 and 26. 



