LAWS OF HEAT. WATER. 03 



8. Another office of water which it discharges 

 by means of its relations to heat, is that of sup- 

 plying our springs. There can be no doubt that 

 the old hypotheses which represent springs as 

 drawing their supplies from large subterranean 

 reservoirs of water, or from the sea by a process 

 of subterraneous filtration, are erroneous and un- 

 tenable. The quantity of evaporation from water 

 and from wet ground is found to be amply suffi- 

 cient to supply the requisite drain. Mr. Dal ton 

 calculated* that the quantity of rain which falls 

 in England is thirty-six inches a year. Of this 

 he reckoned that thirteen inches flow off to the 

 sea by the rivers, and that the remaining twenty- 

 three inches are raised again from the ground by 

 evaporation. The thirteen inches of water are of 

 course supplied by evaporation from the sea, and 

 are carried back to the land through the atmos- 

 phere. Vapour is perpetually rising from the 

 ocean, and is condensed in the hills and high 

 lands, and through their pores and crevices de- 

 scends, till it is deflected, collected, and con- 

 ducted out to the day, by some stratum or 

 channel which is watertight. The condensation 

 which takes place in the higher parts of a 

 country, may easily be recognised in the mists 

 and rains which are the frequent occupants of 

 such regions. The coldness of the atmosphere 



* Manchester Memoirs, v. 357. 



