Mr. J. Croll on the Change in the Obliquity of the Ecliptic. 127 



exciting light. If I understand correctly Professor Stokes's 

 statements concerning solution of guaiacum*, they already es- 

 tablish the existence of negative fluorescence ; for the fluorescent 

 light is said to be violet; it must hence contain rays of great 

 refrangibility, while the fluorescence is described as beginning 

 between Fraunhofer's lines D and b. 



Aschaffenburg, February 1867. 



XV. Remains on the Change in the Obliquity of the Ecliptic, 

 and its Influence on Climate. By James Croll f. 



N the Supplementary Number of the Philosophical Magazine 

 for last month Mr. John Carrick Moore objects to the result 

 stated in my paper on the Obliquity of the Ecliptic, that when 

 the obliquity was at its maximum the annual amount of solar 

 heat received by the poles was ^ greater than at present. He 

 questions the correctness of this fraction, on the ground that I 

 have underestimated the amount of heat absorbed by the atmo- 

 sphere in polar regions. But assuming that I have done so, 

 how can this affect the result in question ? Whether we suppose 

 the quantity absorbed to be equal to 22 per cent, or to 99 per 

 cent., or to nothing at all, the result in each case must be the 

 same. For it holds true that when the obliquity was at its 

 maximum, the poles would be receiving nineteen rays for every 

 eighteen that they are presently receiving, however much those 

 rays may have been weakened by absorption. And if so, then the 

 quantity reaching the poles would be T ^ greater than at present. 



But it is incorrect to say that I did not take into account the 

 fact that the quantity of heat absorbed at the poles is greater 

 than at the equator. (See page 434.) In page 442 I have shown 

 that, supposing 75 per cent, of the sun's heat were cut off by 

 the atmosphere, still the extra heat reaching the poles when the 

 obliquity was at its maximum would be sufficient to melt a sheet 

 of ice 470 feet thick in about 640 years. 



Mr. Meech does not give the relative quantities of heat at the 

 equator and poles when the obliquity is at its maximum, but, 

 what is better, he gives us formulas from which those quantities 

 may be calculated for any amount of obliquity. 



Mr. Moore says that the researches of Melloni and Tyndall 

 show that " if a ray is made to pass through a number of trans- 

 parent plates of the same substance and thickness, the propor- 

 tion of the rays transmitted through the first plate is greater 



* I have here consulted the Fortschritte der Physik for 1852, p. 263. 

 t Communicated by the Author. 



