Professor Newcomb's "Rejoinder." 281 



if he admits this, then he must also admit the soundness of 

 my third reason, for this is the principle on which it is based. 

 The aqueous vapour of the air absorbs a considerable amount 

 of the heat which is being constantly radiated by the ocean ; 

 a portion of this heat thus absorbed is thrown back upon the 

 ocean, the tendency of which is to keep the surface of the 

 ocean at a much higher temperature than it would otherwise 

 have. Prof. Langley has concluded, from observations made 

 at Mount Whitney, that were it not for the heat thrown back 

 by the atmosphere, or " trapped" as it is popularly called, 

 mercury would remain solid under a vertical sun. 



In his Review of ' Climate and Time,' Prof. Newcomb ad- 

 vocated, as a fatal objection to my theory, that the quantity 

 of heat received from the sun during summer would be suffi- 

 cient to melt in a few days the entire amount of snow and ice 

 accumulated during winter*. This objection, I pointed out in 

 my reply (Phil. Mag. Oct. 1883, p. 256), is based on the 

 erroneous assumption that the quantity of snow and ice melted 

 must be proportionate to the amount of heat received from the 

 sun. In proof of the erroneous nature of this assumption, I 

 refer to the fact that on the lofty summits of the Himalayas 

 and Andes, for example, the quantity of heat received from 

 the sun would be sufficient to melt at least fifty feet of ice per 

 annum, and that is no doubt more than ten times the quantity 

 actually required to be melted ; yet notwithstanding the snow 

 remains permanent. The cause of this non-melting I showed 

 to be due to the fact that at these elevations, owing to the dry- 

 ness of the air (want of aqueous vapour), the loss of heat from 

 radiation into stellar space is so excessive that the rays of the 

 sun, intense as they undoubtedly are, are unable to raise the 

 temperature of the snow to the melting-point ; consequently, 

 no matter what may be the amount of heat received, the snow 

 can never melt. It may evaporate, but it cannot melt. I 

 further pointed out that, were the aqueous vapour possessed 

 by the atmosphere sufficiently diminished, the snow-line would 

 descend to the sea-level even at the equator, and perpetual 

 snow would cover our globe down to the sea-shore. 



I was much pleased to find that Prof. Newcomb has not 

 only adopted these views regarding the effects of an absence 

 of aqueous vapour, but suggested that they may yet afford 

 an explanation of the cause of the Glacial Epoch. Every 

 one familiar with the subject, however, knows that that epoch 

 was not due to a dryness of the air, but the reverse. 



* Were this objection correct it would prove that there could have 

 been no Glacial Epoch ; for it is obvious that had not the sun's heat failed 

 to melt the winter's snow, not during the course of a few days merely but 

 during the entire summer, there could not possibly have been permanent ice. 



Phil Mag. S. 5. Vol. 17. No. 106. April 1884. U 



