the Crust of the Earth. 361 



have pretty nearly the same density as the parts of the lava from 

 which it was formed; if anything, it will be soinetrhat more dense." 

 And then he adds an expression of surprise that I should base a 

 theory " on a ])hysical assumption contrary to all our experience." 



It was for a long time thought that the law of water becoming- 

 heavier as it cooled to a certain point, and then lighter before 

 solidifying, was unique. But jNIr. Nasmyth has shown that this 

 is common to many bodies, though the interval through which 

 the law changes is generally so small as heretofore to have 

 escaped observation. This new law juay, then, be true for the 

 earth's mass ; in which case the words in italics should be struck 

 out. (See 'Year Book of Facts,' 1858, p. 128.) 



But although Professor Haughton has given special emphasis 

 to these words, it is obviously not upon them that my argument 

 turns, but upon the words before, viz. " the crust being formed 

 from the liquid, will have pretty nearly the same density as the 

 parts of the lava from which it was formed." The deviation im- 

 plied in this " pretty nearly," even if it be the wrong way, can- 

 not make difference enough to meet the downward force of the 

 preponderating mountain mass, aided by the deficiency of coun- 

 teracting pressure from the ocean, as explained in my paper. 



6. I need not say that it was entirely unintentional on my 

 part to misrepresent Professor Haughton's meaning, as he thinks 

 I have done in one place. He says, " The answer to this pro- 

 blem comes out to be 768 miles, which Archdeacon Pratt, by 

 some strange misconception of my meaning, takes to be my de- 

 liberate conclusion as to the thickness of the crust of the earth." 

 (Phil. Mag. p. 397.) 



What I stated was this : — " Mr. Haughton makes the thick- 

 ness less than 768 miles, but also adds that, in fact, ' the subject 

 would appear to be excluded from the domain of positive science, 

 and to possess an interest for the mathematician only.' " 



The ground of my saying this was the following passages in his 

 original paper : — " This [768 miles] is the thickness of the earth's 

 crust, on the [improbable (p. 267)] hypothesis that both crust and 

 nucleus are homogeneous." " I will now prove that this thick- 

 ness of crust is a major limit to the depth to which the density of 

 the rocks at the surface can extend into the interior, the density 

 being supposed heterogeneous." (P. 268.) " The results which 

 have just been obtained are to be regarded merely as examples 

 of the manner in which the equations should be used, if we were 

 acquainted with the laws of density and ellipticity of the solid 

 and fluid parts of the earth." (P. 269.) ''The subject would 

 appear to be excluded from the domain of positive science, and 

 to possess an interest for the mathematician only." (P. 266.) 

 • 7. The question at issue regarding the thickness of the earth's 



