Change of Climate during Geological Epochs. 123 



been so far different from what it is at present, that by- 

 means of it the internal heat could have produced and main- 

 tained that high temperature of climate which is supposed by 

 some to have prevailed during the long Palaeozoic ages of the 

 earth's history. And besides, the important fact is overlooked 

 that any change in the condition of the atmosphere which 

 would prevent the dissipation of the earth's internal heat into 

 surrounding space, such as an increase in the quantity of 

 aqueous vapour contained in the atmosphere, would at the same 

 time tend to lower the temperature of the earth's surface by 

 diminishing the quantity of radiant heat reaching the surface 

 from without. Such a state of things would no doubt equalize, 

 to a certain extent, the extremes of summer and winter tem- 

 perature, but would not very sensibly increase the mean annual 

 temperature of the climate. In fact it would rather have an op- 

 posite tendency. 



Some have attempted to account for the change of climate 

 by assuming that the earth's axis of .rotation may have shifted 

 its position in consequence of the uprising of large mountain 

 masses on some part of the earth's surface between the equator 

 and the poles. But it has been shown by Professor Airy* and 

 others, that the earth's equatorial protuberance is such, that no 

 geological change on its surface could ever possibly alter the 

 position of the axis of rotation to an extent which could at all 

 sensibly affect the climate. 



Others, again, have tried to explain the change of climate by 

 supposing, with Poisson, that the earth during its past geological 

 history may have passed through hotter and colder parts of 

 space. This, to say the least of it, is not a very satisfactory hy- 

 pothesis. There is no doubt a difference in the quantity of 

 force in the form of heat passing through different parts of space ; 

 but space itself is not a substance which can possibly be either 

 cold or hot. If we adopt this hypothesis, we must therefore 

 assume that the earth during the hot periods must have been in 

 the vicinity of some other great source of heat and light besides 

 the sun. But the proximity of a mass of such magnitude as 

 would be sufficient to affect to any great extent the earth's cli- 

 mate would, by its gravity, seriously disarrange the mechanism 

 of our solar system. Consequently if our solar system had ever 

 during any former period of its history really come into the vi- 

 cinity of such a mass, the orbits of the planets ought at the 

 present day to afford some evidence of it. But again, in order 

 to account for a cold period, such as the glacial epoch, we have to 

 assume that the earth must have come into the vicinity of a 



* Athenaeum for September 22, 1860. 



