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 polar regions, that our author concludes that the globe would not be habitable for existing orders of beings were their influence to cease.
The heat-conveying power of aërial 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 interesting point.
The combined effect of aërial 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[1] 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, determined the mean annual temperature of the earth to be approximately 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 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 proportion, 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 atmosphere diathermous. Hence the poles would sink lower in temperature, 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 regard to the whole subject of ocean-currents"; and, 2. Because, "if the theory is connect, 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.
