CROLL ON CLIMATE AND TIME. 723 



in the conditions referred to the winters would exceed the summers by 

 thirty-six days. 



It is not claimed by Mr. Croll that a cold or glacial epoch is di- 

 rectly caused by the increased distance of the earth from the sun, but 

 from physical agents thus brought into operation. Some of these we 

 will j^roceed to mention. 



As tlie winters increase in length, and the cold in intensity, the vol- 

 ume of snow-fall will become greater, and its area extended. The 

 limit at which the summer sun melts it will move slowly southward. 

 Behind it will be a gradual accumulation of snow forming into ice. 

 Mountain-slopes will be covered with it, until it flows down into val- 

 leys and onward, a vast sheet of glacial ice, equally on lowlands and 

 mountains. 



Out of this condition will arise several results which powerfully 

 react, increasing and intensifying the cold of the growing winter. A 

 volume of snow and ice covering the gi'ound chills the air by direct 

 radiation, and by contact lowers its general temperature, thus delay- 

 ing or arresting the process of melting by the summer's sun. It is a 

 familiar fact that in snow-covered regions the direct rays of the sun 

 may be intensely hot, melting pitch from timber, or heating rocks, 

 while the temperature of the air is that of the ice upon the ground. 

 The regions of Hudson Bay are sterile, not because the heat from the 

 summer sun is not intense, but because they are covered with ice all 

 the year. But for this the climate might be as genial as that of Eng- 

 land. Ice and snow maintain steadily a temperature ot 32, no mat- 

 ter how hot the sun's rays may be, and a rock or piece of earth will 

 become greatly heated, while a block of ice consumes the heat that 

 falls upon it. The solar heat is, therefore, expended in breaking 

 down the molecular structure of the ice, and must continue to do so 

 until it disappears. By as much heat as is used over a region in this 

 way is the heating effect of the sun's rays diminished, and a low at- 

 mospheric temperature is the result. 



But the sun's rays falling on snow are to a considerable extent re- 

 flected back into space from its innumerable surfaces, greatly decreas- 

 ing their heating effect. 



The snow-sheet exerts another important influence on temperature 

 by condensing the vapor of the air into fogs as the summers come on. 

 In this way the solar rays are arrested, and their heating power dissi- 

 pated. This occurs continually during the summers of arctic and ant- 

 arctic regions. Dr. Scoresby observes in regard to the arctic regions 

 that ' the sun, when near the northern tropic, gives scarcely any 

 sensible quantity of light from noon till midnight ; it is frequently 

 invisible for several successive days, and snow is so common that it 

 may be boldly stated that it falls nine days out of ten from April to 

 July." These are the conditions of climate in which glaciers grow 

 and throw abroad their chilling influence. We are now to consider 



