CROLUS ''CLIMATE AND TIME:' 811 



seek for the cause of secular climatal variations in the earth's astronom- 

 ical relations. In the same chapter, but more fully in the Appendix, 

 Dr. Croll goes on to show that the variable length of the seasons con- 

 sequent upon the ellipticity of the terrestrial orbit had begun to at- 

 tract attention before the close of the last century ; and that, as early 

 as 1830, Sir Charles Lyell had expressed the idea that the long winters 

 and short summers of the southern hemisphere might have some in- 

 fluence in lowering the temperature of that portion of the globe.* Sir 

 John Herschel and others, however, soon after demonstrated that the 

 light and heat received by any portion of the earth's surface during 

 any year is practically invariable, whatever the eccentricity of the 

 terrestrial orbit ; the greater proportionate length of winter in the 

 hemisphere whose winters occur in aphelion being exactly counter- 

 balanced by the greater proximity of the sun in summer. This is, 

 indeed, a legitimate deduction from Kepler's second law, and was long 

 ago demonstrated by D'Alembert. The hypothesis, therefore, fell into 

 disrepute. Over fifteen years ago, however, the author of the work 

 under consideration began to point out, in a series of papers (chiefly in 

 the " London and Edinburgh Philosophical Magazine "), the substance 

 of which is reproduced in " Climate and Time," that while the variable 

 length of the seasons resulting from this cause could never produce a 

 glacial epoch directly, yet the same cause might, especially when in- 

 tensified by a high degree of eccentricity, bring into operation a chain 

 of physical agencies which could not fail to very materially affect the 

 climate of the globe. 



As a further introduction to that portion of the work devoted to 

 the elucidation and application of the above-named astronomical and 

 physical principles, and as an illustration of the efiiciency of one of the 

 secondary agencies on the operation of which the theory is based, the 

 heat-conveying power of ocean-currents is discussed at length in the 

 second and third chapters. The importance of these currents is shown 

 to be immense. Thus, according to Professor Dove's " Temperature 

 Tables," the temperature of the British Isles, and of western Europe 

 generally, is 12° Fahr. above the normal — or, more properly, the 

 mean — for that latitude, while the temperature of corresponding por- 

 tions of eastern -North America is nearly as much below the normal. 

 Dr. Croll attributes this difference to the effect of the Gulf Stream in 

 warming western Europe, and of the cold counter-current in chilling 

 our American coasts. The same subject is recurred to frequently 

 throughout the volume, notably in Chapters XI. and XII., in the latter 

 of which Mr. Findlay's objections are answered by calculating from 

 his own data that the heat liberated from the Gulf Stream in the 

 North Atlantic is equal to more than one half of that received direct- 

 ly from the sun in the same latitude. An analogous condition of 

 things exists on the shores of the North Pacific, which are similarly 

 * "Principles," first edition, 1830, vol. i., p. 110. 



