88 



The depth stated is, in each case, that of the deepest observation that 

 has been utilised. 



F. In deducing a mean from these very various results, it is better to 

 operate not npon the number of feet per degree, but upon its reciprocal 

 — the increase of temperature per foot. Assigning to the results in the 

 foregoing list weights proportional to the depths, the mean increase of 

 temperature per foot is found to be -01563, or about J^ of a degree per 

 foot— that is, 1° F. in 64 feet. 



It would be more just to assign greater weight to those single results 

 which represent a large district or an extensive group of mines, especially 

 where the data are known to be very accurate. Doubling the weights 

 above assigned to Przibram, St. Gothard, Mont Cenis, Schemnitz, Kentish 

 Town, Rosebridge, and Seraing, and quadrupling that assigned to 

 Sperenberg, no material difference is made in the result. The mean still 

 comes out 1° F. in 64 feet, or more exactly -01566 of a degree per foot. 



This is a slower rate than has been generally assumed, but it has been 

 fairly deduced from the evidence contained in the Committee's Reports ; 

 and there is no reason to throw doubt on the results in the upper portion 

 of the above list more than on those in its lower portion. Any error that 

 can reasonably be attributed to the data used in the calculations for the 



