60 POPULAR LECTURES AND ADDRESSES. 



29. All sound naturalists agree that we cannot 

 derive accurate knowledge of underground tem- 

 perature from mines. But every bore that is made 

 for the purpose of testing minerals gives an op- 

 portunity of observation. If a bore is made, and 

 is left for two or three days, it will take the tem- 

 perature of the surrounding strata. Let down a 

 thermometer into it, take proper means for ascer- 

 taining its indications, draw it up, and you have 

 the measure of the temperature at each depth. 

 There are most abundant opportunities for geo- 

 thermic surveys in this locality by the numerous 

 bores made with a view to testing minerals, and 

 which have been left either for a time or perman- 

 ently without being made the centre of a shaft. 

 Through the kindness of Mr. Campbell, of Blyths- 



thc foot of the vertical shaft be used, the down current of fresh air 

 will he warmed to the amount of nearly lC. for every fifty fathoms 

 (.f <lt.-su.-nt, by the natural compression of the air through its own 

 weight or more exactly 18 cent, per 1000 fathoms ; being -3^ of a 

 ilrgree centigrade per foot, according to an investigation which I 

 have given in the Proceedings of the Manchester Literary and 

 Philosophical Society, January 1862, "On the Convective Equili- 

 brium of Temperature in the Atmosphere." (Mathematical and 

 Physical Papers^ Vol. iii., p. 255.) 



