240 Mr Hen wood on the temperature cf Mines, 



57 °5 to 70°. The surface of this mine is about 62 fathoms 

 above the level of the sea, the deepest workings being in gra- 

 nite, and those nearer the surface in clay-slate. The Tres- 

 kerby mine is worked under circumstances of strata and eleva- 

 tion very similar to Dolcoath. In December 1819, the tem- 

 perature at the surface being 50°, those of two streams pro- 

 ceeding from the opposite extremities of the deepest gallery, 

 149 fathoms below the surface, were 72° and 76°. The tempera- 

 tures of these streams were precisely the same in January 1 820, 

 that at the surface then being 30°. In September 1820, the tem- 

 peratures of the streams were respectively 73° and 76°, the air 

 at the surface being at 67° The galleries nearest to the sur- 

 face are almost universallv more extended than those at ffreat 

 depths, consequently affording space for the employment of a 

 greater number of labourers ; and this being invariably the 

 case, were the high temperatures prevailing in mines due to 

 their presence, the shallower levels would be much warmer 

 than those at considerable distances beneath. Moreover, the 

 tin, copper, and other veins, as well as the arrangement of 

 stratification in Cornwall, approach nearly to a perpendicular 

 direction. As a consequence, the water from the surface and 

 superior parts of mines descends to the inferior excavations. 

 Thus every mine drains the neighbouring district to a consider- 

 able extent, not unfrequently to a distance of two, three, or 

 even more fathoms in length for every fathom in depth. Hence 

 it would seem that the temperature of streams, when gushing 

 from the rock, is probably below that of the earth at that 

 depth. In opposition to the facts and arguments in favour of 

 an internal terrestrial heat, considerable stress has been laid by 

 Mr Moyle on the comparatively low temperature of the water 

 accumulated in abandoned deep mines. The substance of the 

 greater number of observations on this division of the subject 

 is included in the following table : — 



