398 Mr. G. Gore on an Apparatus for examining the Electrical 



to the contrary, might have any value from 10 milea to 4000 

 miles ; and I regret extremely that I should have expressed my- 

 self so carelessly, as to leave the impression on the mind of 

 so good a mathematician as Archdeacon Pratt, that I had seriously 

 attempted to calculate the thickness of the earth's crust, if it 

 has a crust at all : I would rather that he should class me with 

 those mathematicians who periodically square the circle and dis- 

 cover perpetual motion. 



To show how little is known of the simplest elements of this 

 problem, which Archdeacon Pratt thinks Mv. Hopkins has solved, 

 I may quote the following passage, with which he introduces his 

 own theorem respecting the crust, derived from the consideration 

 of the Himalaya Mountains : — 



" It has been suggested that the crust may project downwards 

 into the lava so as to be supported by buoyancy. But this will 

 not produce the desired effect ; for 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 ; if anything, it will be 

 somewhat more dense." 



I have never before met a student of nature who held this 

 opinion — which is highly improbable when we consider that all 

 metals float in their respective liquids at the melting-point, and 

 that lava will float on liquid lava. 



How can the Archdeacon suppose there is any value in Mr. 

 Hopkins's determination of the thickness of the earth's crust — 

 based as it is on the impossible absence of friction, and on 

 Laplace's arbitrary law of density — when we find him at the 

 same time basing a theory of his own on a physical assumption 

 contrary to all our experience ? 



In my humble judgment, if thei'e be a liquid molten mass 

 beneath our feet, the Himalaya Mountains float on its surface, 

 as an iron bar would float on a bath of mercury, an iceberg on 

 the surface of the sea, or a penny bun in a basin of milk. 

 I am, yours faithfully. 



Trinity College, Dublin, Samuel Haughton. 



May 16, 1859. 



LXV. Description of an Apparatus for examining the Electrical 

 relations of unequally heated Mercury and Fluid Alloys in con- 

 ducting Liquids. By G. Gore, Esq.* 

 [With a Plate.] 



IN a former Number of the Philosophical Magazine (January 

 1857), I described an apparatus for examining the electrical 

 relations of unequally-heated metals in liquids ; but that appa- 

 • Communicated by the Author. 



