146 Mr. S. Newcomb on some 



to be ultimately reached. I must repeat that I know not the 

 slightest authority for the statement in the last sentence 

 quoted, and can gain no clear idea from what Mr. Croll says 

 on the subject. 



In considering the third reason, which I need not quote, but 

 which is found in Mr. CrolPs reply, I suggested in my former 

 paper what I supposed to be a reductio ad absurdum of Mr. 

 C roll's method of reasoning, by pointing out the apparent con- 

 clusion that two bodies could heat each other up by their 

 mutual radiation. T supposed he would disclaim this conclu- 

 sion and try to show that I had misunderstood his premises in 

 drawing it, but he apparently accepts its possibility as a logical 

 result of Provost's well-known theory of exchanges. 



The fourth reason may be summarily disposed of in the 

 same way as the preceding ones. Let the reader take it up as 

 presented by the author ; let him substitute quantitative state- 

 ments at pleasure for the words u more freely/'' " greater," 

 " greater difficulty," "more rapidly," u of higher mean tem- 

 perature/' &c, and let him also bear in mind that it is stationary 

 temperatures and not quantities of heat with which we are 

 ultimately concerned, and the inconclusive character of the 

 reasoning will be at once apparent. 



I shall next pass to the question of the non-melting of snow 

 during a short perihelion summer, in which, as I stated in my 

 former review, calculating temperatures by Mr. CrolPs formula, 

 we should have a mean temperature ranging from 100° to 

 150° Fahr. I had to acknowledge some embarrassment from 

 Mr. Croll's causes producing their effects through the two 

 diametrically opposite modes of operation, to wit : — 



1st. By making the air exceedingly transparent, and thus 

 permitting radiation into space. 



2ndly. X\y filling the air with fogs, and thus preventing the 

 solar heat from reaching the ground. 



His reply to this is that he did not suppose the fogs and the 

 clear atmosphere to exist at the same place and at the same 

 time, but that in either case an inability on the part of the sun's 

 rays to melt the few inches of snow which could have fallen 

 during winter would have resulted. 



I see no use in arguing this point, for the simple reason 

 that 1 do not know enough about the relations of temperature 

 to the aqueous vapour in She atmosphere to admit of my saying 

 any thing of value on the subject. I would merely remark 

 that 1 cannot see in Mr. CrolPs reasoning the slightest ground 

 for admitting that the perihelion summer radiation would 

 produce any other effect than it does now. 



1 am surprised that Mr. Croll should have been willing to 



