290 ANNUAL OF SCIENTIFIC DISCOVERY. 



which the whole terrestrial mass was originally subjected. It might, 

 at first sight, appear that this enormous mass of molten matter, inclosed 

 in so thin a shell, could scarcely be consistent with the general exter- 

 nal condition and temperature of our globe ; but it is quite certain 

 that the real external temperature and this supposed internal tempera- 

 ture of the earth are not inconsistent with each other, and that no 

 valid argument of this kind can be urged against the above hypothesis. 



The above estimate, however, of the thickness of the earth's solid 

 crust, entirely neglects the possible effects of the enormous pressure to 

 which the terrestrial mass at any considerable depth is subjected. 

 Now, this pressure may produce effects of two kinds bearing directly 

 on the question before us. In the above calculation, terrestrial matter 

 placed at the depth of 40 or 50 miles, with a pressure of more than 

 200,000 pounds on the square inch, is assumed to be fusible at the same 

 temperature as if it were subjected merely to the ordinary atmospheric 

 pressure ; whereas the temperature of fusion may possibly be very 

 much increased by such immense pressure as that which I have men- 

 tioned. In such case, the terrestrial matter may be retained in a solid 

 state at much greater depths than it otherwise would be : ?'. e. the 

 solid crust may be much thicker than the above estimate of 40 or 50 

 miles. Again, in this estimate, it is assumed that heat will pass as 

 easily through the most superficial portion of the earth's mass as 

 through the compressed portions at considerable depths. Now, in 

 this assumption there is, I think, a great a priori improbability, and 

 especially with reference to those superficial rocks in which observa- 

 tions on the increase of terrestrial temperature in descending have 

 generally been made ; for these rocks are, for the most part, sedimen- 

 tary strata, which in general, independently of the effect of pressure, 

 arc doubtless worse conductors than the older, more compact, and 

 more crystalline rocks. But if the heat passes through the lower por- 

 tions of this terrestrial mass with more rapidity than through its 

 uppermost portion --i. e. if the conductive power he greater at greater 

 depths the temperature at considerable depths must increase more 

 slowly as we descend than it is observed to increase at the smaller 

 depths to which we can penetrate, and consequently it would be 

 necessary, in such case, to descend to a- greater depth before we should 

 reach the temperature necessary to produce fusion. On this account 

 also, as well as from the increased temperature of fusion, the thickness 

 of the earth's crust may be much greater than the previous estimate 

 would make it. 



It has been for the purpose of ascertaining the effects of great press- 

 ure that Mr. Fairbairn, Mr. Joule, and myself, have undertaken the 

 experiments in which we have been for some time engaged at Man- 

 chester. The first object in these experiments is, the determination of 

 the effect of pressure on the temperature of fusion of as many substan- 

 ces as we may be enabled to experiment upon. We expected to meet 

 with many difficulties in the use of the enormous pressures which we 

 contemplated, and these expectations have certainly been fully veri- 

 fied ; but we were also satisfied that those difficulties might be over- 



