8 12 THE POPULAR SCIENCE MONTHLY. 



affected by the Kuro Siwo ; though there the effect is less distinctly 

 marked, owing to the more unfavorable conformation of the coast. 

 So effective are ocean-currents in distributing the heat of tropical and 

 the cold of i^olar regions, that our author concludes that the globe 

 would not be habitable for existing orders of beings were their influ-- 

 ence to cease. 



The heat-conveying power of aerial currents is also discussed ; but 

 it seems possible that Dr. Croll has under-estimated their capacity, 

 many times increased as it is by the aqueous vapor with which they 

 are laden. Space will not, however, permit the discussion of this in- 

 teresting point. 



The combined effect of aerial and aqueous currents is estimated to 

 reduce the difference in temperature between equator and poles from 

 218° to 80°. This astonishing result may be substantially verified by 

 a simpler and probably more accurate method than that employed by 

 Dr. Croll. Meech shows * that the relative solar intensity at the 

 equator, at the poles, and over the whole earth, varies in the ratio of 

 81*50, 33'83, and 66*73, respectively ; and Dove, many years ago, de- 

 termined the mean annual temperature of the earth to be approxi- 

 mately 58°. Now, the solar energy elevates the temperature of the 

 earth from that of stellar space, or —239° (as determined by Herschel 

 and Pouillet), to -f 58°. 297° is, therefore, the mean elevation of the 

 earth's temperature by solar action. Eliminating the distributing 

 power of aerial and aqueous currents, and assuming the present mean 

 relation between absorption and radiation to remain constant, it is 

 quite probable that the temperature of the various parts of the earth's 

 surface would vary directly as the solar intensity. A simple propor- 

 tion, then, shows us that while the equator would be raised 363°, or to 

 + 124° of absolute temperature, the poles would be elevated only 

 150°, or to —88° ; making a difference of 212° between equator and 

 poles. If, as is extremely probable, the temperature of stellar space is 

 really below —239°, this difference would be still greater. Moreover, 

 it can be shown that any decrease in temperature tends to increase the 

 radiating capacity of the earth by rendering the surrounding atmos- 

 phere diathermous. Hence the poles would sink lower in tempera- 

 ture, proportionally, than the equator. 



Six chapters (VI.-XI.) are devoted to the "gravitation theory " 

 of oceanic circulation, with the object of completely refuting it. Two 

 reasons for the very full discussion of this branch of the subject are 

 given : 1. Because the gravitation theory " lies at the root of a great 

 deal of the confusion and misconception which have prevailed in re- 

 gard to the whole subject of ocean-currents " ; and, 2. Because, " if 

 the theory is correct, it militates strongly against the physical theory 

 of secular changes of climate advanced in this volume." The advo- 



* " Relative Intensity of the Sun's Light and Heat," " Smithsonian Contributions," 

 vol. ix. 



