58 E. P. Culvenvell — Theory of the Ice Age. 



than it is nearer the earth's surface. For in the first place, there is 

 probably a very rapid interchange of air between the poles and the 

 equator, and in the second place, the upper strata of the atmosphere 

 are warmed by the sun with far more constancy than the lower 

 strata ; even at the earth's surface twilight lasts till the sun is 18° 

 below the horizon, and as we ascend the horizon is depressed. If 

 it were not for this, we should expect the polar regions to go on 

 cooling down during all their long night of winter. 



From p. 48 we pass to p. 78, where the exposition of the theory 

 really begins. There is nothing in the well-known theorems here 

 set forth which calls for comment till p. 89, where I find the first 

 indication of a statement, repeated more than once later on, to the 

 effect that Dr. Croll was under the erroneous impression that the 

 winter sun-heat was equal to the summer sun-heat, the difference in 

 temperature arising only from the greater length of the winter. 

 It is due to Dr. Croll that this misconception should be removed, 

 and I am sure Sir Eobert will be the first to acknowledge the 

 justice of the correction. In "The Cause of an Ice Age," p. 89, 

 we read : " Do not read this as if it asserted that the heat received 

 during summer is equal to that received during winter. This is not 

 true, but it has been often asserted. It is, in fact, the fundamental 

 error which has vitiated the asti'onomical theory of the Ice Age as 

 previously presented, and which it is the chief object of this little 

 book to expose." 



Again, p. 115 : "Not only did I find that the true law had been 

 overlooked but it became apparent that sometimes another law had 

 been adopted which was absolutely incorrect. I found, furthermore, 

 that the astronomical theory of the Ice Age as ordinarily stated was 

 contaminated with absolute blunders in the simple mathematical 

 questions which are involved." 



Having then quoted what is certainly either a careless mistake 

 or a mere oversight of Hei'schel, who incidentally states that half 

 the heat of the year is I'eceived in summer and half in winter, 

 Sir Robert proceeds (p. 118): "Indeed, on such a matter it was 

 not unnatural that the words of Herschel should have been adopted 

 without any very careful scrutiny. I examined CroWs booh care- 

 fully on this poiyit So far as I can gather from the various 



allusions to the astronomical element of the problem in his writings, 

 it ivoidd seem as if he entertained the same view as that which is 

 incorrectly set down in the ' Outlines of Astronomy y Again, p. 135 : 

 " He [Dr. Croll] appears, in common with many others, to have 

 been influenced by Herschel's unfortunate inadvertence." 



I had read Croll's "Climate and Time" before I read "The Cause 

 of an Ice Age," and I certainly had not received the impression that 

 Croll was under the misconception supposed by Ball. Indeed, it 

 seemed to me a thing incredible that anyone should really suppose 

 the winter sun-heat at, say, the North Pole, was equal to the 

 summer sun-heat at the same place. Accordingly 1 examined 

 Croll, and I cannot find the slightest ground for the idea that Croll 

 entertained the erroneous view referred to ; but, on the contrary, he 



