Cn. XIII.] 



AND THE STARS. 



279 



of the atmosphere at different heights reached m balloons, 

 Sir John Herschel infers that the heat of space if there 

 were no snn wonld be 271° Fahr. below the freezing point of 

 water, or 



numbe 



aaaea xo the number indicated at any moment by the ther- 

 mometer, gives ns the total absolute heat received at that 

 moment from the sun ; and, according to him, we mnst have 

 reference to this temperature of space in measuring the 

 variations of heat and cold which must result from the 



reater or less remoteness of the earth from the sun during 

 the varying ellipticity of the earth's orbit. The difficulty may 

 be understood by supposing ourselves in a large hall 180 feet 

 long, partly lighted by hundreds of candles so placed as that 

 their light should be everywhere equally diffused, and partly 

 by a central lamp. Then let this lamp be J 



and a half from the centre towards one end of the hall. 



This would increase the light received by an observer at that 





moved 



foot 



diminish 



proportion of one-thirtieth, or in that of its altered distance, 

 but in proportion of one -fifteenth, the variation of the light 

 being in an inverse ratio of the squares of the distance. But 

 in order to estimate the difference between the absolute 

 quantity of light received by the two observers under this 

 change of circumstances, we ought to know what proportion 

 the light of the candles bears to that of the lamp, which, if 

 we were not permitted to extinguish the latter, would not 

 be easily arrived at. For what we are in^ search of is ^the 

 relation of the solar to the stellar heat. TT ~ n "" *~~~ 



it as proved that the power of the stars to heat space is very 

 inferior to that of the sun. * "" ' * J1 "" — 



Herschel 



Whatever 



temperatur 



same 



all parts of the orbit of our planet, being 



wholly unaffected by its greater or less distance from the sun. 



If the difference of the heat received by the earth be m 



the ratio assumed by Herschel, it is not easy to reconcile 



geological phenomena with the exaggerated climates which 



•_j.i™„ ~? ^^nfvi^i+.v wonld cause, unless, indeed, the 



me 



* Trans. Geol. Soc. 2nd Series, vol. ii. p. 297. 



