1882.] fluctuations in a variable. 193 



Various formulae have been given for the capacity for vapour 

 of air at various temperatures (to which we have supposed evapora- 

 tion proportional). For our present purpose a rough approximation 

 suffices. The formula 



A"(l+ 0-014 5 , 



where K is capacity at zero centigrade (an adaptation of Young's 

 formula) will be found to give at ordinary temperatures results 

 within about 1 per cent, of the truth. From this we obtain for the 

 first term of the bracket 



•056 



1 + 0-014*' 



Thus for all ordinary temperatures -jX is positive. Similarly 



-rh is negative. 

 dh" ° 



Hence then fluctuations in heat-supply about a mean increase 

 mean evaporation and decrease mean radiation. 



It is obvious that an increase in the amplitudes of such oscilla- 

 tions will still further increase or decrease mean values. Now an 

 increase in the eccentricity of the earth's orbit will make the earth 

 nearer the sun at Perihelion, and further off at Aphelion, than it is 

 at present. Thus the difference between the temperatures of 

 summer and winter will be intensified for a hemisphere whose 

 winter is in Perihelion, but mitigated for the other. It appears 

 from these investigations that these changes tend in the one case 

 to lower the mean annual temperature, in the other case to raise it : 

 to increase the mean annual evaporation of the one, and diminish 

 that of the other. From increased evaporation follows increased 

 precipitation both of rain and snow. The probable effects of this 

 in accumulating snow, and its bearing on the problem of glacial 

 epochs I have pointed out in a paper read before the British 

 Association at York, and published in the Geological Magazine for 

 November, 1881. As I mentioned there, some rough calculations 

 seem to shew that this cause is capable of producing a considerable 

 effect. I hope before long to renew these calculations by a more 

 satisfactory method, and perhaps make them the subject of a 

 further communication to the Society. 



