THE PARALLEL ROADS OF GLEN ROY 229 



man maintained the fixed opinion that, at a certain stage 

 in the history of the solar system, the sun's radiation had 

 suffered diminution, the glacial epoch being a consequence 

 of this solar chill. The celebrated French mathematician, 

 Poisson, had another theory. Astronomers have shown 

 that the solar system moves through space, and "the tem- 

 perature of space" is a familiar expression with scientific 

 men. It was considered probable by Poisson that our sys- 

 tem, during its motion, had traversed portions of space 

 of different temperatures; and that, during its passage 

 through one of the colder regions of the universe, the 

 glacial epoch occurred. Notions such as these were more 

 or less current everywhere not many years ago, and I 

 therefore thought it worth while to show how incomplete 

 they were. Suppose the temperature of our planet to be 

 reduced, by the subsidence of solar heat, the cold of space, 

 or any other cause, say one hundred degrees. Four-and- 

 twenty hours of such a chill would bring down as snow 

 nearly all the moisture of our atmosphere. But this 

 would not produce a glacial epoch. Such an epoch would 

 require the long-continued generation of the material from 

 which the ice of glaciers is derived. Mountain snow, the 

 nutriment of glaciers, is derived from aqueous vapor raised 

 mainly from the tropical ocean by the sun. The solar fire 

 is as necessary a factor in the process as our lamp in the 

 experiment referred to a moment ago. Nothing is easier 

 than to calculate the exact amount of heat expended by 

 the sun in the production of a glacier. It would, as I 

 have elsewhere shown,* raise a quantity of cast iron five 



> **Heat a Mode of Motion/' fifth edition^ chap, vi.: Forms of Water, §§ 55 

 and 56. 



