APPENDIX. 401 



of climate have actually occurred. Notwithstanding recent 

 objections, it appears to me that Darwin's hypothesis as to the 

 subsidence of a great tract in the Southern Pacific is that which 

 best accounts for the existence of the countless coral islands in 

 that region ; nor is the probability of a nearly continuous barrier 

 of volcanic islands across the Atlantic to be completely dis- 

 missed. That such changes would have largely affected the 

 climate of the earth cannot, I think, be doubted. 



If I may venture to express my own view on this difficult 

 subject, I must say that, although it has not been overlooked by 

 the able men who have discussed it, the paramount importance 

 of aqueous vapour as an agent for modifying climate has not 

 yet been fully recognized. Mr. Croll has constantly discussed 

 the phenomena of ocean-currents, as if their chief function were 

 to affect climate by heating or cooling the surrounding air, 

 which is thence diffijsed over the land surfaces, and he has 

 devoted little attention to the effects of evaporation from the 

 sea, and the subsequent condensation in some other region of 

 the vapour produced. When we remember that as much heat 

 is consumed in the conversion of one cubic mile of water into 

 vapour as would raise the temperature of nearly ninety-seven 

 cubic miles of water by 10° Fahr., we get some measure of the 

 vast power of vapour as a vehicle of heat. Admitting, as I am 

 disposed to do, that 166,000 cubic miles of water are annually 

 conveyed northward by the Gulf-stream, and suffer an average 

 loss of 20° Fahr. before returning to the torrid zone, I must 

 point out that the entire heat requisite to maintain this great 

 volume of water at the higher temperature would be consumed 

 in the conversion of 3433 cubic miles of water into vapour. 

 In point of fact, I believe that more than one-half of the 

 quantity specified is expended in evaporation, and that the 

 cooling of the waters of the Gulf-stream is mainly due to this 

 agency. To follow the vapour thus produced, to ascertain 

 where it is condensed, and where the heat disengaged in the 

 act of condensation becomes available to raise the temperature 

 of the air, is a task which is beyond our present resources ; but 

 it is one which must be performed before we can reason with 

 any confidence as to the ultimate distribution of the heat carried 

 by the Gulf-stream or any other ocean-current. Whatever 

 part of the vapour produced by evaporation from the Gulf- 



2 D 



