260 Afclideacou Pratt on the Thickness of 



physical property or condition into the problem by which the 

 solid and fluid parts may be distinguished. Mr. Hopkins intro- 

 duces the condition that the fluid part presses according to the 

 laws of fluidity against the solid parts and produces a preces- 

 sional motion in them, which is an object for measurement. 

 Mr. Hennessy, in his investigation, introduces physical conditions 

 which serve the same purpose according to his method of treat- 

 ing the pi'oblem. But this Professor Haughton does not do. 

 His investigation, therefore, is not one which aff"ects the problem 

 of the thickness of the earth's crust one way or the other. 



3. It is against the assumptions which are made in attempting 

 to solve this problem that Professor Haughton protests ; and not 

 least against "Laplace's arbitrary law of density." He will 

 find others who agree with him in this, at any rate to a consi- 

 derable extent. Had this been his only apparent objection, I 

 should not have written my former paper. I3ut as he had taken 

 up the subject as a physico-mathematical one, and had brought 

 to bear upon it the only machinery which is of any value and 

 power in such problems — the analysis of Laplace — his investiga- 

 tion seemed to call for special attention. 



4. And now one word with regard to "Laplace's arbitrary 

 law of density." If we are to make no hypotheses, there is an 

 end to all philosophizing, and to attaining results such as have 

 been reached in the first instance only by such guides and helps. 

 Of course an hypothesis, to be worth anything, must be reason- 

 able. In this respect Laplace's law cannot be said to be alto- 

 gether ariz'/y'flr?/. \t\s a priori not at all an improbable law ; 

 and it brings out physical results in the problem of the figui'e of 

 the earth which well accord with fact. Of course, a philosopher, 

 remembering on what basis results deduced from this law rest, 

 will receive theui with confidence proportionate to his belief in 

 the jDrobability that the law correctly represents the facts of 

 nature. But I hardly think that "calculations" thence pro- 

 ceeding " must be only regarded as so much useless and learned 

 labour in vain." 



5. In commenting upon my argument for a large thickness, 

 drawn from the circumstances of the Himalaya Mountains and 

 of the vast ocean in these eastern parts. Professor Haughton 

 points out what is very likely to be a mis-statement — and indeed 

 is certainly so, if the law of density of cooling masses, such as 

 the earth consists of, is the same as that of metals and some 

 kinds of lava at the earth's surface. He quotes the following 

 words from me, the italics being his own : — " It has been sug- 

 gested that the crust may project downwards into the lava so as 

 to be supported by buoyancy. But this will not produce the 

 desired eflfect ; for the crust being formed from the liquid, will 





