24 REMARKS ON TEJIPEEATURE OF THE EARTH IN JHNES. 



invariable plane is taken by Henwood at lC-6 fathoms (about 

 100 feet) down, and the annual mean temperature at the surface 

 is taken as the mean temperature at that place. This, however, 

 IS likely to be very variable according to the latitude, the height 

 above the sea, and other local pecularities, in respect to which 

 no general rule has yet been formulated, although that plane is 

 probably much nearer the surface than the point mentioned. 



Taking now a case from the coal pits it appears that even in 

 the same shaft the irregularity is apparent. 



A number of observations which were taken at Duckenfield 

 colliery show that the mean rate of increase from observations, 

 between the mean depths of 25o'4 fathoms and 281-4, was 9-0 

 fathoms to a degree, but between 281-4 fathoms and 309-3, the 

 mean ratio was 46-5 fathoms to a degree ; and in the two shafts 

 the temperature, and more especially the ratio of increase, 

 varied considerably from each other at corresponding depths. 

 The increase, however, w^as at the rate of 14-8 fathoms and lG-8 

 fathoms to one degree of heat, or about 89 and 101 feet 

 respectively. And at the Rosebridge colliery, near Wigan, 

 similar observations show 1 degree to each 11 fathoms of descent. 

 This mme was at the time probably the deepest mine in Britain, 

 being 403 fathoms in depth, but here again the increase was very 

 irregular, being 4 fathoms to a degree in one portion of the shaft, 

 i.e,, from 331-5 to 339-5 fathoms the temperature rose from 85 

 degs. to 87 degs., but from 339-5 to 367 fathoms the temperature 

 increased 1-5 degrees, that is, at the rate of 1 to 18*3 fathoms, 

 again showing the probability of local influences. The crystal- 

 line rocks are not common in the coal measures, but a consider- 

 able stratum of sandstone or hmestone might so modify the 

 pressure on the weaker strata below, and so reduce the relative 

 amount of chemical decomposition taking place as to account 

 for the variations of the accession of temperature at these 

 different depths. 



At the Agoa Quenta mine in Brazil, situated 3,400 feet 

 above the sea, where a large stream of water issued from a 

 depth of 30 fathoms, the temperature of that water increased 1 

 degree for every 2'12 fathoms. But curiously enough the average 

 ratio per degree of heat in Brazil is given as 106-6 fathoms in 



