THE 



GEOLOGICAL MAGAZINE. 



NEW SERIES. DECADE II. VOL. VIII. 



No. XI.— NOVEMBER, 1881. 



c>:Eixc3rXi<rj^Xj .a.i^tioxj:es. 



I. — On Evaporation and Eccentricity, as Co-Factors in 

 Glacial Periods.' 



By Eev. E. Hill, M.A., F.G.S., Tutor of St. John's College, Cambridge, 



niHE effects of Evaporatiou on Temperature have often been con- 

 I sidered, yet much remains unknown. I am not aware that 

 any one has called attention to the rapidity of its growth as tem- 

 perature increases, and the consequences that follow. 



Dry air coming into contact with water absorbs moisture. It 

 becomes saturated, unable to take up more, when it has absorbed a 

 certain quantity dependent on its temperature. A cubic metre of 

 air at 00. can hold 4*88 grammes of moisture, at 50. it can hold 

 6-81, at 100. 9-38, at 150. 12-77, and at 200. 17-19. Thus the 

 amount of vapour increases much faster than in a simple proportion 

 to the temperature. One well-known consequence of this is that if 

 two currents of saturated air meet at different temperatures, their 

 mixture must be attended by rain. Cubic metres of air at 100. and 

 200. as above together contain 26-57 grammes of moisture. When 

 mixed they take the temperature 150., but can only hold 25-54: 

 grammes, and the surplus quantity must be precipitated. 



Conversely, the capacity for vapour of two cubic metres of dry air 

 at different temperatures is greater than the capacity of two metres at 

 the mean of those temperatures. Now the rate of evaporation from 

 the surface of water is dependent on the capacity for vapour of the 

 air above the water. When other things are equal, the amount of 

 evaporation in a given time will be proportional to this capacity. 

 The amount will therefore depend on the temperature ; increasing as 

 the temperature increases, but at a faster rate. Thus the quantity 

 evaporated in an hour of 100. followed by an hour of 20 0. will 

 exceed that evaporated in two hours of 15 0. The excess as above 

 will be about 4 per cent. So the evaporation during a year at 25 0. 

 will fall short of that resulting from two half years at 20 0. and 

 30 0. Thus fluctuations in temperature must increase the amount 

 of evaporation. The wider these fluctuations, the greater that 

 increase. The cubic metre of air at temperatures — 30 0., 0., and 

 300. can hold respectively 0-41, 4-88, 30-85 grammes of moisture. 

 The mean of the first and third is 15-63, which is more than three 

 times the second. Thus the evaporation at a steady freezing tem- 



1 Communicatecl to the British Association at the York Meeting of 1881. 



DECADE II. — VOL. VIII. — NO. XI. 31 



