RIVER-DRIFT PERIOD. 297 



:e by Professor Tyndall,* who argues that the ancient 

 glaciers indicate the action of heat as much as of cold. 

 " Cold," he says, " will not produce glaciers. You may have 

 the bitterest north-east winds here in London throughout the 

 winter, without a single flake of snow. Cold must have the 

 fitting object to operate upon, and this object the aqueous 

 vapour of the air is tlio direct product of heat. Let us put 

 this glacier question in another form : the latent heat of 

 aqueous vapour, at the temperature of its production in the 

 tropics, is about 1,000 Fahr., for the latent heat grows 

 larger as the temperature of evaporation descends. A pound 

 of water thus vapourised at the equator, has absorbed one 

 thousand times the quantity of heat which would raise a 



pound of the liquid one degree in temperature It is 



perfectly manifest that by weakening the sun's action, either 

 through a defect of emission, or by the steeping of the entire 

 solar system in space of a low temperature, we should be 

 cutting off the glaciers at their source." 



Secondly. Admitting the proper motion of the sun, it has 

 been suggested that we may have recently passed from a 

 colder into a warmer region of space. 



I must refer to Mr. Hopkins' memoir for his objections to 

 this suggestion ; they certainly appear to " render the theory 

 utterly inapplicable to the explanation of the changes of 

 temperature at the more recent geological epochs." 



This hypothesis, moreover, is liable to the same fatal ob- 

 jection a,s the first. To produce snow requires both heat and 

 cold ; the first to cause evaporation, the second to produce 

 condensation. In fact, what we require is a greater contrast 

 between the temperature of the tropics and that of our lati- 

 tudes ; so that, paradoxical as it may appear, the primary 

 cause of the " glacial " epoch may be, after all, an elevation 

 of temperature in the tropics, causing a greater amount of 

 * Heat Considered as a Mode of Motion, p. 192. 



