720 THE POPULAR SCIENCE MONTHLY. 



mate. Nor is this a phenomenon of the present age only. Geology 

 proves from its records that, while a torrid zone may have existed 

 since climates began, yet polar and temperate regions have witnessed 

 changes both of climate and of life in wonderful succession, and in 

 periods of immense duration. The causes which have contributed to 

 these results have been sought and studied by some of the most emi- 

 nent scientists of our time. Humboldt, Sir Jolin Herschel, Sir Wil- 

 liam Tiiomson, Lieutenant Maury, Sir Charles Lyell, and more recent- 

 ly Dr. Carpenter and others, have investigated the subject in many 

 of its aspects, and their conclusions are before the world ; but no one, 

 we believe, has presented it from so many points of view, or attacked 

 its complex problems with greater vigor, than Mr. James Croll, whose 

 volume is now before us. His conclusion is, that not only great secu- 

 lar changes of climate, but the distribution of temperatures upon the 

 earth's surface at the present time, are due to causes which alter the 

 volume, intensity, and direction, of the trade-winds and other prevail- 

 ing winds of the globe. For the question at issue is not the amount 

 of heat received upon the earth's surface, but the means by which it is 

 distributed. It is not claimed that the great heat of the equatorial 

 regions is carried directly to the polar regions by winds. That such 

 a result is impossible is shown by the fact that the heated air rises 

 at the equator, and moves toward the poles at an elevation where the 

 temperature is at freezing, and its heat is lost or radiated into space. 



But the prevailing winds, and mainly the trades, give rise to great 

 surface-movements of the ocean, chief of Avhich is the Gulf Stream. 

 Here, however, the author enters on disputed ground. We have not 

 space to follow him in his criticisms on Lieutenant Maury and Dr. 

 Carpenter, each of whom attributes oceanic circulation to difference 

 of specific gravity of the water rather than to winds. 



It is evident, however, that the value of this question depends 

 upon a previous one, What influence have ocean-currents on climate, 

 and what is their capacity for the transference of heat? For clear- 

 ness and brevity we will confine our inquiry to the Noi-th Atlantic 

 and its great current, the Gulf Stream. By one of the lowest esti- 

 mates made of the volume and velocity of this stream, there is con- 

 veyed by it not less than 2,787,840,000,000 cubic feet of water every 

 hour. This is about 1,200 times as much as the average hourly dis- 

 charge of the Mississippi. "No droughts affect it;" the flow is in- 

 cessant, and the volume continues unchanged, save by causes which 

 we will presently mention. 



The mean temperature of this water as it emerges from the Gulf is 

 about (J5 Fahr. As the stream spreads over the North Atlantic its 

 heat is imparted to the atmosphere, and diffused by winds. It is cer- 

 tain that the entire volume of Gulf-Stream water loses in this way 25 of 

 its heat; and this represents its warming capacity. To this we refer 

 the mild climate of England, of Norway, and Iceland. The heat thus 



