reply to curries. 23 



I am very much pleased to find that Professor 

 Newcomb draws a conclusion from Dulong and Petit's 

 law favourable to my theory of the cause of the Glacial 

 Epoch, which certainly did escape my notice. And it 

 is a curious circumstance that Mr. Hill* has likewise 

 deduced a conclusion even more favourable to glaciation 

 than that of Professor Newcomb. 



I am pleased to find that he agrees, in the main, 

 with what has been advanced in ' Climate and Time ' 

 in reference to the heating power of ocean currents, 

 and also as to their existence being due to the impulse 

 of the winds. But he differs widely from me in regard 

 to the heat conveyed by aerial currents. 



On the Heat conveyed by Aerial Currents, — I stated 

 that the quantity of heat conveyed by the air from equa- 

 torial to high temperate and polar regions is trifling in 

 comparison with that conveyed by ocean currents ; for 

 the heated air rising off the hot ground of the equator, 

 after ascending a few miles becomes exposed to the 

 intense cold of the upper regions, and having to travel 

 polewards for thousands of miles in those regions, it 

 loses nearly all the heat which it brought from the 

 equator before it can possibly reach high latitudes. 

 To this Professor Newcomb objects as follows : " He 

 (Mr Croll) speaks of the hot air rising from the earth 

 and becoming exposed to the intense cold of the 

 upper regions of the atmosphere But what can this 

 cold be but the coldness of the very air itself which has 

 been rising up? If the warm air rises up into the cold 

 air, and becomes cooled by contact with the latter, the 

 latter must become warm by the very heat which the 

 former loses ; and if there is a continuous rising current 

 the whole region must take the natural temperature 



* " Evaporation and Eccentricity as Co-factors in Glacial Periods," 

 " Geological Magazine" for November, 1881. 



