ADDRESS. U 



perature and this supposed internal temperature of the earth are not incon- 

 sistent 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, witli 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 ; Avhereas the temperature of fusion may 

 possibly be very much increased by such immense pressure as that I have 

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

 state at much greater depths than it otherwise would be — i. 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 observations on the increase of terrestrial temperature in descend- 

 ing have generally been made ; for these rocks are, for the most part, sedi- 

 mentary strata, which in general, independently of the effect of pressure, are 

 doubtless worse conductors than the older, more compact, and more crystal- 

 line rocks. But if heat passes through the lower portions of this terrestrial 

 mass with more rapidity than through its uppermost portion — i. e. if the cow- 

 ductive power be greater at greater depths — the temperature at considerable 

 depths must increase more slowly as we descend, than it is observed to in- 

 crease 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 

 therefore, 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 pressure, 

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

 in which we have for some time been engaged at Manchester. The first 

 object in these experiments is the determination of the effect of pressure on 

 the temperature of fusion of as many substances 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 verified ; but we wei-e also satisfied that those diflSculties 

 might be overcome by perseverance and patience, and in this also we have 

 not been disappointed ; for I may now venture to assert that our ultimate 

 success, with respect to a number of substances, is beyond doubt. M^ithout 

 the engineering resources, however, at Mr. Fairbairn's command, success 

 would have been hopeless. 



At present our experiments have been restricted to a few substances, and 

 those of easy fusibility ; but I believe our apparatus to be now so complete 

 for a considerable range of temperature, that we shall have no difficulty in 

 obtaining further results. Those already obtained indicate an increase in 

 the temperature of fusion proportioiial to the pressure to which the fused tnass 

 is subjected. In employing a pressure of about 13,000 lbs. to the square 

 inch on bleached wax, the increase in the temperature of fusion was not less 



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