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regions of space. It appears certain, however, that we receive 

 scarcely any heat from the stars ; so that we can never have 

 passed through a colder region than that which we are passing 

 through at present ; and it does not seem at all probable that 

 our system, since its first formation, has ever been near enough 

 to any of the stars to be sensibly warmed by them. 



Three causes remain which have probably, indeed almost 

 certainly, had their influence in changing the climates of our 

 planet. These are — 



i. Changes in the relation of the earth to the sun in its 

 orbit. 



2. The secular cooling of the sun. 



3. Changes in the distribution of land and ocean. 



I maintain that glacial climates have been due to changes in 

 the relation of the earth to the sun ; — that the warm climates 

 of the early geological periods were due to the sun's heat being 

 greater than at present; — and that the effect on climate of 

 changes in the distribution of land and ocean, though probably 

 of sensible magnitude, has not been very great. 



Respecting the first of these three causes, I have little to add 

 to what was contained in my paper on " The Glacial Climate 

 and the Polar Ice-cap," which was read to this Society on the 

 1 st December 1875, and afterwards printed in our Proceedings. 

 I will briefly recapitulate what I then offered as an explanation 

 of the glacial climates. 



The chief cause of glaciation is neither a low mean tempera- 

 ture nor a cold winter, but a cool summer, which leaves the 

 snow of winter unmelted. This is shown by the fact that the 

 height of the snow-line on mountains is chiefly determined 

 by the summer temperature ; — the warmer the summer, the 

 greater is the height to which we must ascend in order to find 

 perpetual snow ; — winter temperature has no effect on it. 



Supposing the sun's heat to be constant, the total heat 

 received by the earth from tbe sun in the course of a year is 

 very nearly constant from year to year ; but the excentricity of 

 the earth's orbit is subject to considerable though very slow 



