Royal Society. 59 



a demand for labour, might perhaps be of some interest, but it would 

 lead me from the object which I have in view, which is simply to 

 point out the grounds upon which I have ever felt a strong con- 

 viction, that whatever means were placed at the disposal of the Royal 

 Society, no lasting difficulty could occur in turning them to useful 

 account. 



During the last year considerable progress has been made by Mr. 

 Hopkins in the important experiments which he has been carrying 

 on in conjunction with Mr. Fairbairn and Mr. Joule. You no 

 doubt are aware, that, as we descend below the surface of the earth, 

 it has been found that the temperature increases : numerous experi- 

 ments made in different places with all the necessary precautions to 

 guard against fallacy, seem clearly to have established the fact. The 

 increase is about one degree of Fahrenheit for a depth of from 50 to 

 60 feet, if therefore the conducting ])Ower of the materials of the 

 globe was the same at all depths, we should have a series, which 

 would give us the depth proportional to every required temperature. 

 Reasoning in this way, we conclude that a temperature higher than 

 that of melting iron exists at a depth of thirty miles, and that at 

 double that depth the materials of the surface of the globe, combined as 

 we find them in nature, would enter into fusion. It has therefore been 

 supposed hj many that the solid crust beneath our feet is not more 

 than forty or fifty miles thick. It has however been assumed that the 

 increasing pressure at increasing depths does not alter either the 

 conducting power of materials, or the temperature at which they 

 melt. This no doubt is to a certain extent incorrect, and it is highly 

 probable that the conducting power of the different strata increases 

 considerably with the depth, the materials becoming more compact 

 under augmented pressure. It is not improbable also that pressure 

 may raise the temperature of fluidity. In either case the solid crust 

 of the globe would be thicker than it had been supposed to be on 

 the assumed data. With the view of throwing light upon this 

 question so interesting to all geologists, Mr. Hopkins undertook, 

 with the assistance of Mr. Fairbairn and Mr. Joule, to subject 

 various substances under diflferent temperatures to enormous pres- 

 sure. Considerable time was required, even with Mr. Fairbairn's 

 unlimited mechanical means, to construct the necessary apparatus ; 

 however, recently it has been completed, and in the few substances 

 examined it has been found that the temperature of fusion has 

 increased with the pressure : in the case of wax, by a pressure of 

 13,000 pounds to the square inch, the fusing-j)oint was raised 30°. 

 Whatever may be the influence of these experiments asaffecting the 

 great questions of Terrestrial Physics, we may predict with certainty 

 that data will be obtained most valuable in philosophical research. 



Mr. Joule, I find, has been actively engaged, in conjunction with 

 Professor Thomson, in his experiments on the thermal effects of 

 fluids in motion, and has determined with considerable accuracy, 

 oj)erating on a great scale, the dej)ression of temperature when com- 

 pressed air escapes into the atmosphere througii a porous plug. The 



