Part L.]| EVIDENCE OF EARTHA’S INTERNAL HEAT. 47 
has calculated that any considerable area of the earth’s surface 
covered for several thousand years by snow or ice, and retaining, 
after the disappearance of that frozen covering, an average surface 
temperature of 13° C., “ would during 900 years show a decreasing 
temperature for some depth down from the surface, and 5600 years 
after the clearing away of the ice would still show residual effect of 
the ancient cold, in a half rate of augmentation of temperature 
downwards in the upper strata, gradually increasing to the whole 
normal rate, which would be sensibly reached at a depth of 600 
metres.” But beneath the limit to which the influence of the 
changes of the seasons extends, observations in most parts of the 
globe show that the temperature invariably rises as we penetrate 
towards the interior of the earth. According to present knowledge, 
the average rate of increase amounts to 1° Fahr. for every 50 or 60 
feet of descent, and this rise is found whether the boring be made at 
the sea-level or on elevated ground. The subjoined table gives the 
results of temperature observations at widely separated localities :’— 
Feet. 
Dukinfield, near Manchester (2040 ft., Coal measures) 1° Fahr., for every 83°2 
Rose Bridge, Wigan (2445 ft., Coal measures) . : 5 hs 54°3 
South Balgray, Glasgow (525 ft., Coal measures) : 3 5 4] 
Kentish Town, London (1100 ft., London clay, Chalk, 
Gault, &.) . : : ; : ; : we 5s 54°6 
La Chapelle, Paris (660 metres, Chalk, &c.) : % de 84 
Grenelle Well, Paris (1795°6 ft. do.) ¢ ‘ : 33 5 56°9 
St. André, do. (263 metres, do.) : : : a. 55 56°4 
Neu Salzwerk boring, Westphalia (2281 ft.) . 5 Ss es 54°68 
Mendorff bore, near Luxembourg (2394 ft.) : é ee . 57'0 
Bore near Geneva . z : : ‘ : ; - 3 Hoe 
Mont Cenis tunnel (5280 ft. below summit of Mount 
Frejus, metamorphic rocks) . , ,  . » @) 81 
Yakutsk, Siberia (656 ft,, limestone, &c, and granite), Md : 60 
Irregularities inthe Downward Increment of Heat, 
—While these examples prove a progressive increase of temperature, 
they show also that this rate of increase is not strictly uniform. 
The more detailed observations which have been made in recent 
years have brought to light the important fact that considerable 
variations in the rate of increase take place even in the same bore. 
If, for instance, we examine the temperatures obtained at different 
depths in the Rose Bridge colliery shaft cited in the foregoing list, 
we find them to read as in the following columns :— 
Depth in Temperature Depth in Temperature 
Yards. (Fahr.). Yards. (Fahr.). 
ERM oes hea SLO De OE i ee cult ae Fee 
LS Ata Se da age i) Mok eh ee ee oe 
Remind fiiwcty iis) 9) SB Tieashee WO 2 C4 TAY NS rs 
Pimms pot Wt hcs, ise OO oars Dei iet & Pals some queen 
eee £1 40% 8 Foe hOB BD Gi mista pe macn 5 lea’ Sapte 
ye es wi BU Bil Guile Ae rae en ees cre 
Res MRE oman «ok ou) gor, BB a Pa eee rae ai: 
1 See “ Reports of Committee on Underground Temperature,” Brit. Assoc, Rep. from 
1868 to 1879. 
