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LXVI. Note on Mr. C roll's Paper on the Influence of the Obli- 

 quity of the Ecliptic on Climate. By John Carrick Moore, 

 F.R.S., V.P.G.S., £<?.* 



IN an elaborate memoir in the last Number of the Philoso- 

 phical Magazine, Mr. Croll estimates the annual heat at the 

 poles received from the sun, when the obliquity of the ecliptic 

 was at its maximum (24° 50' 34"), to be greater than at present 

 by j 1 ^ of the whole quantity ; and as this fraction -^g applies 

 to all the subsequent computations, any inaccuracy in it will 

 affect all the numerical results. 



Mr. Croll deduces this quantity from a paper by Mr. Meech 

 (Smithsonian Contributions to Knowledge, vol. ix.), where Mr. 

 Meech proves the annual amount of heat at the equator, with a 

 maximum of obliquity, to be to that at the poles as 365*51 to 

 160*04. Mr. Meech, in his very meritorious paper, takes into 

 consideration the variations of heat due to change of obliquity, of 

 latitude, of the length of the day, and of the height of the sun 

 above the horizon at each instant ; but he makes no allowance 

 for the effects of atmosphere ; and he expressly warns his readers 

 (pages 21 and 44) that his results apply only to the outer limits 

 of the atmosphere, and will not give the resulting terrestrial 

 temperatures. The same warning may be found in Sir Charles 

 LyelFs last edition of .his c Principles of Geology/ 



The effects of the increased length of ray and of the increasing 

 density of the lower strata of the atmosphere are considered by 

 the authors of the article " Climate " in the Encyclopaedia Bri- 

 tannica ; and their result is that the heat received at the equator 

 is to that at the poles as more than 8 to 1, instead of less than 

 2J to 1, as Mr. Croll computes. Mr. Croll does indeed allude 

 to the article " Climate/' but dismisses it rather summarily by 

 saying that the results are " wholly erroneous." " The researches 

 of Melloni and Tyndall/' he says, "show that when a ray passes 

 through any substance, the absorption is rapid at first, but the 

 ray is soon sifted, as it is called, and it then passes onwards with 

 but little further obstruction." 



But the experiments in the place cited are quite beside this 

 question. They show indeed that if a ray is made to pass through 

 a number of transparent plates of the same substance and thick- 

 ness, the proportion of the rays transmitted through the first 

 plate is greater than the proportion of those which, having 

 emerged through the first plate, can pass through the second. 

 But they do not show that, if the successive plates had increased 

 in thickness or in density, the ray (or rather all that remains of 



* Communicated by the Author. 



