Xlviii PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



face of the moon, and during the passage of this himinaiy over the 

 stars watch in vain for the signs of an atmosphere. Yet the moon 

 has been partly covered by water in ancient times, as the undu- 

 lated plains, the dry deltas, the dry seas on her surface appear to 

 prove ; she has had an oxygenous atmosphere, if we may judge by 

 the lava-streams and monticules, with small craters, as well as the 

 greater mountains, which seem to tell of explosive vapours, and the 

 oxidation of minerals and rocks. If her atmospheric life be ended, 

 and she now fluctuates between great heat and violent cold, what is 

 to assure us that the earth, whose physical history is bound up in 

 so many ways with that of her brilliant companion, has not expe- 

 rienced in a lesser degree the effects of similar vicissitudes ? * 



The flow of heat and cold in the crust of the earth is accomplished 

 partly by conduction from particle to particle in a given rock, and 

 from one mass of rock to another, across the divisional surfaces of joints 

 and beds ; and partly by convection through the agency of water and 

 aii\ An experimental investigation lately undertaken by Mr. 

 Hopkins t shows that the conducting power of the rocky crust of 

 the earth varies much, while the rate of increase of heat as we go 

 downwards is more nearly uniform. The conducting powers of dif- 

 ferent substances tried, as dry powder, dry solid, and moist solid, 

 were thus found : — 



Dry powder. Dry solid. Moist solid. 



Chalk -056 -170 -300 



Clay -070 -230 -370 



Sand -150 (New Eed) -250 (New Eed) -600 



Freestone — -330 -450 



Sand and clay . . -110 — — 



Ancaster oolite . . — -30 '40 



Hard compact limestone '50 '55 



Millstone -grit .. — -51 -76 



Hard Palaeozoic sedimentary rock . . -50 -61 



Igneous rock .... — -53 -100 



Now, as artesian wells sunk in chalk, triassic, and carboniferous 

 strata yield water whose temperature exceeds that of the surface 

 by about 1° from 45 to 60 X feet, and as these are about the limits of 

 the ratio of increase given by experiments in the solid rock — it ap- 

 pears that the range of conductivity of the rocks is twice and a half 

 as great as the range of the observed ratio of increase of tempera- 



* G-eologists may be reminded that the figure of the moon is not Uke that of 

 the earth. She has been conjectured to be a slightly prolate spheroid, with the 

 polar axis, which is directed to the earth, 186 feet (see MacLaurin's 'JSTewton') 

 longer than the equatorial diameter. Nor is her centre of gravity coincident 

 with her centre of figure (according to Hansen), but excentrical, 33^ miles fur- 

 ther from the earth, on the polar axis. 



t Eoy. Soc. Proc. 1857, and Trans. 1857, p. 805. 



I Mr. Hopkins selects six examples to show the comparative uniformity of the 

 law of increase of temperature and depth, varying from 54 feet to 65 feet for 1 ° 

 Falir. But the hmits are really much wider, from less than 30 feet to more than 

 60 feet in the mines of Cornwall alone. 



