24 



Prof. J. Prestwich. 



collieries, in consequence of the deflections of the isothermal lines by 

 irregularities of the surface. 



The particulars of the Norley Colliery, Wigan, (No. Ill) are 

 insufficient, though the result there obtained seems to confirm the 

 observations at the Rosebridge Colliery. 



The observations in the Mons Collieries (Nos. 215, 216) were made 

 at a distance from the shaft, and in dry galleries without ventilation. 



At Liege (No. 141) the unusual precaution was taken of making 

 the holes 5 metres deep, while the temperature of the air in the 

 galleries was only 1° lower than that of the coal. 



I have not taken any of the earlier observations, owing to the un- 

 certainty attaching to the instruments, and, after eliminating others 

 for the reasons named, we are reduced to the following small number 

 of available observations : — 



Original 







Tempera- 



Eate 



number 



Place. 



Depth of 



ture of 



of increase 



in 





pit. 



rock 



for 



Table I. 





or coal. 



1° Fahr. 







feet. 





feet. 



15© 





1514 



79° 



47 



136 



South Hetton, Durham — the last 













1736 



77 



52 



126 





2445 



94 



53 



211 





620 



61 



45 



120 



Wakefield 



1455 



78 



50 



141 





1656 



87 



46 



216 





1680 



82 



54 



215 





1456 



81 



49 







Mean rate of increase. . 



.. 49-5 



This seems a very small selection to make, but I believe that the 

 disturbing causes I have mentioned operate so constantly, that an 

 element of uncertainty is introduced at the very opening of a pit, and 

 that, without further precautions than hitherto taken, the observa- 

 tions are not reliable for the purpose of those exact values necessary 

 to determine the true thermometric gradient. 



Instead of making the observations in a newly-opened and well- 

 ventilated part, with fresh faces of the coal emitting pent-up gases, it 

 might be desirable to try an isolated abandoned unventilated gallery, 

 and there place thermometers in the coal and the rocks above and 

 below the coal. The holes should be 5 or 6 or more feet deep, and the 

 gallery then closed at distances of 200 to 300 feet by a series of 

 diaphragms to stop ventilation and convection currents, and the place 

 not opened for several months, by which time the end station would 



