ORIGIN OF ERUPTIVE AND PRIMARY ROCKS, 309 



snext 792 feet, 123 feet j which figures plainly shew an increase 

 with the depth of the distance corresponding to 1°C, Bis- 

 -chof's experiments on the cooling of large masses of melted ba- 

 salt also furnish a very convincing proof that the increase of 

 (temperature takes place less rapidly at greater depths* 48 hours 

 after easting a globe of melted basalt, having a diameter of 27J 

 inches, he found it to possess the following temperatures : — 



In the centre .. 153.5° R. 



45 inehes from do. 136.0° » 



6.75 " " " .' 124.9° " 



9. « " ji,,; „.' 109.8°* " 



These observations also shew with increasing depth a diminution 

 of the rapidity with which the increase of temperature takes 

 place. They by no means furnish us however with secure data 

 upon which to found a calculation as to the thickne-sof the earth's 

 ■crust. Like the careful experiments in the mines of Prussia and 

 Saxony, M a general law with regard to the increase of tempera- 

 41 ture cannot be dedueed from them." They are useful in so far 

 as they prove the inaccuracy of all estimates of the earth's thick- 

 ness founded u)©n the arithmetical progression of the increase of 

 'temperature, and justify the supposition of Naumann, that the 

 •Crust of the earth may have a thickness of upwards of 120 English 

 •geographical miles. 



There is however yet another estimate of the thickness of the 

 •earth's crust, the consideration of which will lead us to refer to the 

 various views entertained as to the constitution of the interior of 

 the earth. This estimate is thus referred to by Naumann : " W* 

 u Hopkins has adopted a peculiar method for the solution of 

 4i this problem. By very acute observation and reasoning on the 

 ■"nutation of the earth's axis, and the preeession of the equinoxes, 

 " he finds that these two phenomena must come out with different 

 i l values according as the earth is solid throughout, or fluid 

 •" throughout, or solid externally and fluid internally ; in which 

 ■" latter case different thicknesses of the solid crust will produce 

 il different results. It is certainly the case that in order to a cor- 

 ■" rect estimate, the values of two important elements are necessary, 

 u which are as yet unknown, viz., the condensing action of pres- 

 u sure, and the expanding action of such high temperatures. 

 u Nevertheless, Hopkins has attempted to answer the question ap- 



* Naumann, Geognosie, I, 63. 



