Wellington Philosophical Society. 481 



Mr. Croll — a theory rendered more interesting because Darwin has used it to 

 explain facts in connection with, the distribution of plants and animals, and 

 because it involves the admission that a glacial period existed in the southern 

 hemisphere, but at a period of about 13,000 years, or some multiple of that 

 time, prior to the northern epoch. This, again, is in support of Captain 

 Hutton's views, who, with his usual caution, states that if he were to adopt 

 the opinion that a glacial epoch once existed in the southern hemisphere, it 

 would be with the distinct understanding that it was far anterior to the 

 pleistocene period — that is, anterior to the northern epoch. 



In giving you an epitome of Croll's views, I shall, with slight exceptions, 

 use his own words. 



Croll's theory is based on the assumption that during the glacial period 

 the eccentricity of the Earth's orbit was at least double what exists at present, 

 but not so great by far as the eccentricity of the orbit of the planet Mars. 

 This supposition does not involve any increase in the mean distance of the Earth 

 from the Sun, nor in the period of revolution — both of these elements of our 

 orbit are absolutely unchangeable. 



The first step in Croll's argument is that ocean currents are produced by 

 winds ; that the main ocean currents agree with the direction of the prevailing 

 winds, as may at once be seen by comparing the direction and paths of the 

 prevailing winds, shown in Messrs. Johnston's small Physical Atlas, with the 

 ocean currents as shown in the Current Chart published by the Admiralty. 

 Of course the conformation of sea and land partially interferes with this 

 agreement, but the principal currents of air and of water agree precisely. 

 There is nothing new or extravagant in this. The younger Herschell 

 vindicates to the winds their supremacy in the production of ocean currents. 

 If, says Herschell, there were no wind there would be no Gulf Stream, or any 

 other considerable oceanic current. 



Croll next shows that oceanic currents are the great distributors of heat 

 over the globe. Taking for instance the Gulf Stream, he calculates the 

 amount of heat conveyed as so enormous as to be equal to one-fourth of all the 

 heat received from the sun by the Atlantic Ocean from the Tropic of Cancer 

 up to the Arctic Circle ; and were it not for the Gulf Stream, and other ocean 

 currents, only a small portion of the globe would be suited to the present 

 order of sentient beings ; that London, instead of possessing a mean annual 

 temperature of nearly 50°, would have a mean temperature of not over 10°. 

 Of this enormous amount of heat one-half is derived from the southern 

 hemisphere by means of ocean currents. Without such transference of heat it 

 would be impossible to account for the North Atlantic being 5° warmer than 

 the South Atlantic. This current from the south is owing to the superior 

 strength of the S.E. trades. 



n2 



