On Underground Temperatures. 



29 



No further particulars and no greater depths were given, but 

 Mr. Fox's later observations in rock give different results. 



The Effects of the Percolation of Water. — But while the effects of 

 ventilation are not so general and disturbing in Metallic Mines as in 

 Coal Mines, the effects produced by the underground waters are of 

 much greater importance. The alternation of impermeable with per- 

 meable strata, and the multiplicity of faults in the Coal-measures, so 

 impede the descent of the surf ace- waters, that there are mines so 

 dry as to necessitate the introduction of water to keep down the dust. 

 The Metallic Mines being, on the contrary, commonly in crystal- 

 line, schistose, and slaty rocks, have more uniformity of structure ; 

 and, being also generally hard and compact, they are more or 

 less impervious. When, however, they have been disturbed and 

 fissured, they give freer passage to water ; and when, further, they are 

 traversed by veins and faults, these frequently serve as channels or 

 conduits, more or less free, for the surface-waters, and considerable 

 quantities of water pass through them. Consequently water is one 

 of the great obstacles to deep mining with which the workmen have 

 to contend. Water finds its way to all depths, and with more or less 

 rapidity. Mr. Henwood states that in some mines a great increase 

 follows soon on heavy autumnal rains, and that in others, months 

 intervene before the effects are felt. 



In districts formed of the usual alternations of sedimentary strata, 

 it is estimated that on an average about one-third of the rainfall passes 

 underground ; while in Cornwall, where granites and slates exclu- 

 sively prevail, Mr. Hen wood estimated in his survey of the Gwennap 

 district — which consists chiefly of slates — that about a fourth of the 

 rainfall is absorbed, the mean annual rainfall there being 46 inches, 

 or equal to 166,834 cubic feet per acre. The local percolation is, how- 

 ever, extremely variable, as in some mines the quantity pumped up 

 does not exceed 5 gallons, while in others it amounts to 186 gallons 

 per minute. 



The same observer found that water passes more freely through 

 slate than through granite, the quantity yielded by mines in slato 

 being about four times as much as that in granite. In a period of five 

 years (1833-7) the water pumped per minute from forty mines, 

 amounted on a mean to the following proportions : — 



Granite. 



V ; " ^ 



Maximum. Minimum. 



30 .... 16 



Mr. Henwood's account of the quantity of water that passes under- 



Slate. 



A 



Maximum. Minimum. 



cubic feet 

 per minute. 



122 .... 63 | 



