ibTre 



272 S. Neiocomh—CrolVs Climate and Time. 



The second reason is that the rays which fall on snow and 

 ice are to a great extent reflected back into space, the grounds 

 of this statement being apparently the transparency of the 

 atmosphere to the calorific rays of the sun on which he had 

 just been insisting, and which he must have had in mind as 

 explaining how the reflected rays can be got back into space. 

 How great the reflecting and transmitting power of the snow and 

 ice must be, to keep the snow unraelted all summer, may be 

 inferred from the fact that during this perihelion summer the 

 amount of heat received from the sun by every part of the 

 northern hemisphere would suffice to melt from four to six 

 inches of ice per day, over its entire surface, that is, it would 



ffice to melt the whole probable accumulation in three or four 

 . The reader can easily njake a computation of the incred- 

 reflecting power of the snow and of the unexampled trans- 

 parency of the air required to keep the snow unmelted for three 

 or four months. 



The third cause of non-melting of the snow during summer 

 is that snow and ice chill the air and condense the vapor into 

 thick fogs which "would eflFectually prevent the sun's rays 

 from reaching the earth, and the snow in consequence would 

 remain unmelted during the entire summer." Here again, he 

 says nothing about the latent heat set free by the condensation, 

 nor does he say where the heat goes to which the air must lose 

 in order to be chilled. The task of arguing with a disputant 

 who in one breath maintains that the transparency of the air 

 is such that the rays reflected from the snow pass freely into 

 space, and in the next breath that thick fogs effectually prevent 

 the rays ever reaching the snow at all, is not free from embar- 

 rassment. We can accept calmly any and every possible hy- 

 pothesis respecting the properties and nature of the atmosphere 

 which he choo.'^es'to propound, but must insist that the conclu- 

 sions be drawn in accordance with the first principles of the 

 theory of heat. We might therefore show that if the snow, air, 

 fog, or whatever throws back the rays of the sun into space is 

 so excellent a reflector of heat, it is a correspondingly poor 

 radiator, and the same fog which will not be dissipated by the 

 summer heat will not be affected by the winter s cold, and will 

 therefore serve as a screen to prevent the radiation of heat from 

 the earth during the winter. 



Perhaps the shortest way of meeting the case is to refer to 

 the best known facts of our own climate. Every winter a 

 large portion of the northern hemisphere i,« covered with snow 

 precisely as Mr. Croll supposes was the case during the Glacial 

 epoch. According to his theory this accumulation of snow 

 ought to offer a great resistance to the melting power of the 

 sun's rays. How much resistance it does offer everyone knows. 

 What effect the difference of astronomical conditions during 



