Reviews — Dr. Croil's Life and Work. 73 



the most part embodied in that work, more especially one " On the 

 Path of the Ice-sheet in North- Western Europe and its Relations to 

 the Boulder-clay of Caithness." In the Geological Magazine for 

 September, 1878, he wrote an article criticizing "Cataclysmic 

 Theories of Geological Climate," and there have been a few short 

 communications of a subsequent date. 



Very different estimates have been formed from time to time of 

 the value of Croil's speculations, though none can doubt that they 

 offer problems of a deeply interesting nature. We may almost regard 

 " Climate and Tinie " as a poem ; though its author was perhaps 

 somewhat too controversial in his nature to be regarded as a genuine 

 vates. It is not for us, as geologists, to question the accuracy of the 

 calculations whereby the eccentricity of the Earth's orbit has been 

 estimated throughout three million years of past time and one 

 million years of future time. Levei'rier, it seems, was content to 

 give a table of the eccentricity for 100,000 years before and after 

 A.D. 1800. This was thought sufiScient for ordinary astronomical 

 purposes, but is clearly too limited for geological epochs. The 

 most curious feature in this connection is the statement by Croil's 

 biographer (p. 491) that " the keenness and directness of his 

 marvellous logical faculty is by no means least clearly demonstrated 

 in his marvellous calculations of millions of years and intricate 

 mathematical problems, which, as he knew little of mathematics, 

 he worked out by a process of figure-logic entirely his own." 

 We also learn incidentally from a statement made by Lord Kelvin 

 (p. 500) that Croll was not at all times very sound in arithmetic. 

 Nevertheless, the same authority observes that his " Essay on the 

 Physical Cause of the Changes of Climate during the Glacial Epoch" 

 deservedly gained the careful consideration both of geologists and 

 of astronomers. 



Although some may be disposed to question the value of Croil's 

 calculations, there can be little doubt that many of his arguments 

 on terrestrial physics carry considerable weight with them. There 

 is a certain fascination about his theory of the alternate glaciation 

 of the hemispheres ; whilst it is indisputable that at the present 

 time, notwithstanding the low value of the eccentricity, the hemi- 

 phere whose winter is in aphelion is much the colder, as required 

 by his hypothesis. How far Lyell's explanation of this fact is to 

 be relied upon it is difficult to say ; but one apparent cause, as 

 pointed out by Croll, may be sought in the northerly deflection of 

 the equatorial currents, whereby a portion of the warmth which 

 should belong to the southern hemisphere is transferred to the 

 northern. But what is the agency which brings this about ? 

 According to Croll, it is the excess of southern cold producing a 

 preponderance of the south-east trade- winds. We are thus forced 

 to fall back on the primary cause of this excess of cold, which 

 Croll no doubt would attribute to the fact of the southern winter 

 occurring in aphelion. Other physicists there are who contest this 

 conclusion. A better acquaintance with the Antarctic regions than 

 we at present possess might be of service at this stage of the inquiry. 



