206 



REVOKT — 1876. 



])rpth in Teitiptraturo 



illioTiish B'^aiiiiiur. 



feet. o 



700 :. 17-270 



900 18-780 



nOO 21-147 



1300 21-510 



1500 23-277 



1700 24-741 



1900 26-504 



2100 28-668 



3390 37-238 



Assuming, with Herr Dunker, the mean temperature of the surface to be 

 7°-18, which is the mean annual temperature of the air at Berlin, we have 

 the following increments of temperature with depth : — 



The mean rate of increase found by comparing the temperatures at the 

 surface and 3390 feet is exactly 1° Fahr. for 50 Ehenish or 51*5 English 

 feet. 



The numbers in the last two columns exhibit upon the whole a diminu- 

 tion with increase of depth ; in other words, the temperature increases less 

 rapidly as we go deeper down. As regards the first 700 feet, which exhibit 

 a decidedly more rapid rate than the rest, it must be remembered that nearly 

 half of this distance was in a different material from the rest of the bore, 

 being in gypsum with some anhydrite, while all the rest was in rock-salt. 

 Prof. Herschel has found, in recent experiments not yet published, that the 

 conductivity of rock-salt is exceedingly high ; and theory shows that the 

 rates of increase, in superimposed strata, should be inversely as their conduc- 

 tivities. We may therefore fairly attribute the rajnd increase in the first 

 700 feet to the relatively small conductivity of the portion (283 feet) which is 

 not rock-salt. The slow rate of increase observed in the long interval between 

 the depths of 2100 and 3390 feet is not so easily accounted for ; we can only 

 conjecture that this and the other inequalities which the above Table pre- 

 sents, for depths exceeding 700 feet, are due to fissures or other inequalities 

 in the rock which have not been put in evidence. 



With the view of summing up his results in small compass, Herr Dunker 

 has assumed the empirical formula — 



t = 7-18 + ax + hx", 



