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NATURE 



\yune 24, 1875 



result, and we must look to the indirect effects. While 

 agreeing in the existence of many of those pointed out by 

 Mr. CroU, we cannot think it quite so settled a matter as 

 he does, as they do not all act in the same way. In the 

 first place, though the total amounts of summer and 

 winter heat together are equal in the two hemispheres, 

 yet, since a larger proportion of the greater summer heat 

 is available than of the smaller winter heat, the more 

 unequal these are, it follows generally that more heat 

 must be obtained, and therefore the more uniformly 

 heated hemisphere will be coldest ; but secondly, as Mr. 

 CroU states, we must consider the formation of snow, i.e. 

 take into account the latent heat of water and other physi- 

 cal properties. Some of his arguments on this point are 

 rather circular, for whatever amount of heat is rendered 

 latent in the melting of ice, as much will be supplied to 

 radiation in the freezing ; and no increase of ice would 

 arise from this. There are, however, two points that 

 seem to be made out. First, that snow and ice are better 

 reflectors of light than almost any other substance, and 

 therefore less heat enters into them ; and, secondly, that 

 moist air is much less transparent to heat than dry, so 

 that the vapour raised by the sun in summer would be an 

 opposing influence, whereas the frozen vapour in winter 

 when fallen as snow would leave the air above freer for 

 radiation. This result would overbalance that spoken of 

 in the first place, and be a powerful influence in the pro- 

 duction of a glacial epoch. The vapour, too, that was 

 raised in summer would come in a large degree from the 

 warmer tropics, and therefore continue to add each 

 winter to the mass of the snow and ice in the more polar 

 regions. 



These seem to us to be among the most convincing of 

 Mr. Croll's arguments, and they are in agreement, as he 

 shows, with the condition of the earth at the present time 

 as regards the more glaciated condition of the southern 

 hemisphere, and they agree with what has been pointed 

 out by Prof. Tyndall, that heat, to bring the snow in form 

 of vapour, is just as necessary for a glacial epoch as cold 

 to freeze it when brought. It has been argued by Mr. 

 Murphy that under exactly the same circumstances it 

 would be the more equally heated hemisphere that would 

 be glaciated, as the cool summer would melt less snow ; 

 but according to the above theory the summer of the 

 other hemisphere, though naturally hotter, would also be 

 rendered cool at the earth's surface. We see that the 

 whole of this argument depends on the relation of the 

 atmosphere to heat rays, and what has been stated 

 above has been experimentally verified ; yet we are far 

 from being fully informed on this point, and the example 

 of the planet Mars, which is almost exactly under the 

 circumstances of great eccentricity and winter aphelion 

 supposed above, and yet has not much glaciation, teaches 

 us that this may depend on other combinations of cir- 

 cumstances beyond those we have considered above. 



The glaciation, Mr. CroU thinks, would be assisted by 

 the deflection of ocean currents, on which he accordingly 

 spends his strength ; but the vertical circulation of Dr. 

 Carpenter, no less proved than the influence of the Gulf 

 Stream, would be antagonistic to this, and we may safely 

 leave the unknown residuum out of consideration. 



Such is Mr. CroU's theory of the cause of the glacial epoch, 

 to the illustration of Which he brings forward many interest- 



ing facts. Among these are the proofs he gives of the occur- 

 rence of warm interglacial periods. Some of these proofs 

 are coUected from other writers, but many are from his 

 own observations, and consist of the intercalation of beds 

 of fossiliferoussand between two masses of boulder clay, the 

 fossils being often of a southern rather than of a northern 

 type. He also refers to the records of borings collected 

 by him and already published, which showed, in several 

 instances, three, four, or even five boulder clays in 

 succession, separated by stratified sands. These inter- 

 glacial periods are certainly more easily accounted for on 

 Mr. CroU's theory than on any other, as, owing to the 

 numerous terms on which it depends, the eccentricity of 

 the earth's orbit is hable to rapid changes. Many of the 

 instances, however, of interstratified fossiliferous sands 

 seem too insignificant to require so vast an apparatus as a 

 cosmical cause to account for them ; rather are they evi- 

 dences of the dependence of temperature on the atmo- 

 sphere, whose changes are much more comparable to 

 those of limited beds. Another set of facts adduced by 

 Mr. CroU in illustration of his theory is the evidences we 

 have of glacial conditions in former geological periods, 

 of which he gives a very useful summary, though it seems 

 to us he goes too far in taking proofs of a warm cUmate 

 to indicate glacial epochs preceding and succeeding it, on 

 the ground that all warm periods imist be interglacial — 

 this is Incus a non lucendo truly. Indeed, the warmth of 

 North Greenland in the Miocene period seems to us one 

 of those facts which are not satisfactorily accounted for 

 by the theory — for the eccentricity has seldom been much 

 less than now — and our northern winters are in peri- 

 helion. 



He thinks he can identify the glacial period proper, 

 and those of the Eocene and Miocene periods, with por- 

 tions of past time when the eccentricity has been great 

 and yet rapidly changing to small ; and attempts thus to 

 get a measure of the length of a geological period, and 

 hence with the aid of other theories and supposed mea- 

 surements to arrive at the total length of past geological 

 time. These speculations may be ingenious, but they 

 can give no assistance to the solution of a problem of 

 which we reaUy have not yet the data. The title of the 

 book leads us to believe that all the discussion about the 

 glacial epoch is engaged in only to lead up to this, but 

 we must regard that as a much more manageable and 

 therefore interesting problem, and turn now to examine 

 the other theories that have been broached to account 

 for it. 



The theory of the sun being a variable star is not in 

 such an advanced state as to warrant a complete discus- 

 sion from this point of view, and we have seen that mere 

 absence of heat can never cover the land with snow and 

 ice, and this theory therefore may be dismissed. 



The only remaining one is that which accounts for it 

 by increased obliquity of the ecUptic. This theory, which 

 has recently been broached in different lorms by Lieut.- 

 Col. Drayson and Mr, Thomas Belt, has been espoused 

 by Mr. Woodward in his address to the Geologists' Asso- 

 ciation, whose paper has been deemed worthy of insertion 

 in the " Arctic Manual." Col. Drayson's form of it, which 

 imagines that the whole mass of ice was formed every win- 

 ter and melted every summer, may be dismissed as ab- 

 surd. Not so Mr. Belt's. There can be no doubt that an 



