Royal Society. 505 



of equal pressure as its extreme limits ; and of these the author pro- 

 ceeds to determine the position. 



The forms of the isothermal surfaces within a spheroid have never 

 been completely determined. The determination given by the au- 

 thor is an extremely approximate one when the ellipticity is small, 

 and the time during which the process of cooling has been going on 

 is very great, as it is presumed to be in the case of the earth. 



The author then enters into the analytical investigation of this 

 problem ; and deduces the conclusion that we must descend to a 

 depth greater than about one-fifth of the earth's radius before we 

 arrive at a surface of equal fluidity (or solidity) having an ellipticity 

 of the requisite value : that is, the effective thickness of the crust 

 must be at least equal to one-fourth or one-fifth of the earth's ra- 

 dius, in order that the precession may have its observed value : a 

 conclusion, the author observes, which entirely removes the founda- 

 tion of certain vague and somewhat fanciful speculations in geology, 

 proceeding on the hypothesis of the thickness of the earth's crust not 

 being greater than twenty or thirty miles. It has been imagined 

 that in active volcanos, the volcanic vent may communicate directly 

 with the central fluid nucleus, whence the ejected fluid mass has 

 been supposed to be derived. This notion, the author conceives, is 

 rendered totally inadmissible, when it is proved that the thickness' of 

 the solid portion of the globe cannot be less than 800 or 1000 miles. 

 It is also remarked, that it follows from the great thickness of the 

 crust, that the present interior temperature of the earth cannot be 

 due to its original heat unless pressure be effective in promoting so- 

 lidification, a fact not yet established by experiment : for, if the 

 present temperature be due to that cause, it is certain that it must 

 be sufficient at the depth of probably less than fifty miles to reduce 

 the matter composing the crust of the globe to a state of fusion un- 

 der the atmospheric pressure ; whereas it has been proved that the 

 earth is solid to a very much greater depth ; which can be account- 

 ed for, therefore, only by supposing its solidity to be preserved by 

 the enormous pressure to which, at considerable depths, the mass is 

 subjected. The author then offers an explanation of the phfenomena 

 of volcanos on the supposition that a portion of matter more fusible 

 than the general mass of the globe exists in a state of fusion in sub- 

 terranean reservoirs, forming so many subterranean lakes of deter- 

 minate extent ; in some cases originally distinct ; in others, commu- 

 nicating with adjoining lakes, by more or less obstructed chaimels ; 

 a theory which will also account for all the obscure geological ele- 

 vations, except perhaps the earliest, as being produced by a sinud- 

 taneous action of a fluid pressure on every portion of the lower j)art 

 <jf a solid mass of definite extent. The author considers this har- 

 mony in Ids general views witli the results of analytical investigation 

 as constituting for them a strong claim to the attention ol*^ geo- 

 logists. 



Another important conclusion which the author deduced from his 

 researches is, tiiat if tin: interior temperature of tiie eartli be due to 

 its ])rimitive heat, pressure must be ettective in promoting solidifica- 

 tion of nia.sses at high temperatures. 



