18 REPORT 1871. 



The uumbers in the last column are the quotients of those in the two pre- 

 ceding, and denote the average number of feet of descent for 1° F. of in- 

 crease, as deduced from comparing the temperature at each depth of obser- 

 vation with the temperature at the lowest depth. The earlier numbers in 

 this column of course carry more weight than the later ones. The amount 

 of steadiness in the increase of temperature of the water is best seen by 

 inspecting the third column, which shows that the freest interchange of heat 

 occurs at about the depth of 600 feet. This must be due to springs. The 

 soil, from the depth of 569 to that of 702 feet, is described as " light-grey 

 chalk, with a few thin beds of chalk-marl subordinate." The soil consists 

 in general of chalk and marl, from 325 to 910 feet, and below this of sandy 

 marl, sand, and clay (see list of strata in last year's Report, p. 41). The 

 mean rate of increase in the former is a degree in 56 feet, and in the latter 

 a degree in 49 feet. The mean rate of increase from the surface of the 

 ground to the lowest depth reached is certainly very nearly 1° F. in 54 feet. 



Mr. David Burns, of H.M. Geological Survey, has furnished observations 

 taken in the W. B. lead-mines, at and near AUenheads, Northumberland, by 

 the kind permission of Thomas Sopwith, Esq., F.ll.S., and with the valuable 

 assistance of Mr. Ridley, Underground Surveyor, who continued the obser- 

 vations after Mr. Burns had Isft. 



The mineral for which these mines are worked is galena. There are very 

 extensive old workings at a lower level than the present workings, and filled 

 with water, which is kept down by pumping ; but the quantity daily pumped 

 out is very small in comparison with the whole, so that the change of water 

 is slow. 



From the offices of the lead-mines a small windlass with a supply of fine 

 brass wire was obtained, which enabled the thermometer to be lowered 

 steadily and quickly. 



The first observations were taken in Gin-Hill shaft, 3rd June, 1871. The 

 observers proceeded as far down in the works as they were able, and took 

 their station in a level leading from the shaft, 290 feet from the surface of 

 the ground, and 38 feet above the surface of the water in the shaft. 



The following observations were then made : — 



Depth under Depth in Temperature, 



ground. water. Fahr. 



ft. ft. 



340 12 49-3 



340 12 49-2 



390 62 51- 



390 62 51- 



440 112 51-3' 



440 112 51- 



} 



The mean temperature at the shaft mouth for the year ending 31 st May 

 1871, was 44°-3, as derived from daily obsei'vations of maximum and mini- 

 mum thermometers, without applying a correction for diiirnal range. Add- 

 ing 1° to this, to obtain the probable mean temperature of the surface of 

 the ground, and taking the temperature at 400 feet of depth as 51''-3, Mr. 

 Burns computes that the rate of increase downwards is 6° in 400 feet, or 1° 

 in 66'6 feet. The data for this calculation are obviously in many respects 

 very uncertain. 



On the 21st June Mr. Ridley took observations in another shaft in the 

 same workings, called the High Underground Engine Shaft. It is sunk 



