[ 223 J 



XXX. Examination of Dr. Croll's Hypotheses on Geological 

 Climates. By Dr. A. Woeikof *. 



THE hypotheses of Dr. Croll have attracted such general 

 attention during the last ten years, especially in Great 

 Britain, but also in the United States and other countries, 

 that a review of them by a meteorologist is, I think, desirable. 

 This work is just now particularly opportune, because Dr. 

 Croll has recently published a new book on these matters f in 

 which he further explains and extends his views, and replies 

 to his critics, whilst in the preface he mentions that he wishes 

 to devote the coming years to work in a wholly different 

 direction. Thus we have now before us a system as complete 

 as it is likely to be made by the author. 



I do not propose to review the whole work of Dr. Croll in 

 glacial geology and cosmology, but only to consider some 

 points which are within my line of study. 



In answer to Prof. Newcomb, Dr. Croll considers the 

 mean temperature of land and oceanj, and arrives at the 

 startling conclusion that " the ocean must stand at a higher 

 mean temperature than the land." Now, once the mean and 

 not surface temperature is mentioned, the meaning of the 

 author is, it seems, clear ; but the result is entirely opposed 

 to what we know. Not only have the oceans which receive 

 cold under-currents from polar seas a much lower mean tem- 

 perature § than the land, but even seas which receive no such 

 cold water, and are known as exceedingly warm, as the Medi- 

 terranean and Red Seas, have a temperature considerably 

 lower than the land. 



It might perhaps be remarked, that Dr. Croll's startling state- 

 ment is a simple " lapsus pennce" and that he does not consider 

 the mean temperature of the whole volume of ocean water, 

 but the mean annual temperature of the surface ; yet, especially 

 from p. 33, it is seen that this is not so. Dr. Croll considers 

 the difficulty of the sea "getting quit of its heat as rapidly as 

 the land;" and in this passage, as in the former, he seems 

 entirely to have forgotten the mobility of the particles of 

 water, which is so exceedingly important and so essentially 

 affects the thermal relations of water by the convection-cur- 

 rents which it causes. (A few pages before he mentions the 



* Communicated by the Author. 

 f ' Climate and Cosmology/" Edinburgh, 1885. 

 % P. 26 and following-. 



§ I understand the mean temperature as that of the whole column of 

 water from top to bottom. 



