Mr Hen wood on the temperature of Mines* 243 



portant principle seems to affect the preservation of vegetable 

 life, by protecting the roots of plants from the rigour of the 

 winter's cold^ and in summer preventing their being parched 

 by the intensity of solar heat. That evaporation obtains at all 

 times, even in the severity of winter, may be readily ascertain- 

 ed by the inversion of a glass over a spot from which the fro- 

 zen earth has been removed. The conversion of this vapour 

 (which, by parting with its caloric, must mitigate the severity 

 of the cold to plants,) must also materially operate on the at- 

 mosphere. Indeed, many meteorological phenomena seem inti- 

 mately connected with this subject. The condensation of va- 

 pour in hills and elevated stations must be the principal, if noj; 

 the entire, cause of the formation of springs. 



The source of this moisture is another object well deserving 

 attention. It does not seem probable that the atmosphere can 

 be the only one. Some of Mr Fox's discoveries seem to af- 

 ford us light in the inquiry. He observes that the relative pu- 

 rity of the water seems to have no reference to the tempera- 

 ture or depth of the mines. The deposit from the water from 

 Huel Abraham and Dolcoath, the two deepest mines in Corn- 

 wall, did not in either case exceed two grains from a pint, 

 whilst that from the consolidated mines yielded, from a like 

 quantity of liquid, ten grains ; from Huel Unity, 16 grains ; 

 from one shaft in Poldice, 19 grains ; and from another, 

 92 grains. The salts most abundantly afforded by evapo- 

 ration are the chlorides, especially that of calcium, although 

 Mr Fox has frequently detected the presence of chloride of 

 sodium, particularly in the water from the united and consoli- 

 dated mines, Huel Unity and Poldice. Of the 92 grains from 

 the latter, 52 grains were of the chlorides of calcium and mag- 

 nium, 24 grains of the chloride of sodium, the remainder be- 

 ing muriatic acid, with iron and sulphate of lime. The water 

 from another part of the same mine afforded, by the evapora- 

 tion of the same measure of water, 5.5 grains of the chloride 

 of calcium. " All these mines are in killas, or primitive clay- 

 slate, and are several miles from the sea."" From such facts 

 may it not with propriety be inferred, " that the sea- water pe- 

 netrates into the fissures of the earth, and may, in a greater or 

 less degree, assist in supplying the loss of moisture by evapora- 



