582 Correspondence — Professor J. Young. 



and in summing up says, " that the most probable hypothesis in the 

 present state of our knowledge of the earth, is that it consists of an 

 immense solid nucleus, a hardened outer crust, and an intermediate 

 region, of comparatively slight depth, in an imperfect state of 

 igneous fusion." 



This last is precisely the state of the earth as imagined by Hal ley 

 when endeavouring to account for the phenomena of the magnetic 

 needle. To account for these phenomena he assumes the existence of 

 four mngnetic poles, two in each hemisphere, the relative positions of 

 which undergo a constant change ; to effect this he makes one pole in 

 each hemispliere to be situated on an external crust, and the two 

 other poles on an interior mass, separated from the crust by a fluid 

 medium ; this interior mass he supposes to revolve more slowly by 

 an extremely small quantity than the outer crust. Subsequently 

 Hanstein examined the subject, and came to the same conclusion 

 with Halley as to the existence of four poles ; these he made to be 

 all of unequal magnetic force, and to revolve round the terrestrial 

 poles at unequal periods; the periods being as near as possible, allow- 

 ing for errors of observation, all multiples of that, mystic number 

 432, the weakest north pole revolving in 432 X 2 = 864 years, and 

 the strongest in 432x4 = 1728 years, the weakest south pole in 

 432x3 = 1296 years, and the strongest in 432 x 10 = 4320 years. 

 While, curious enough, the least common denominator of these four 

 periods is 432 x 60 = 25,920, which is the period of the revolution 

 of the precession of the equinoxes ; therefore the shortest time for 

 the four magnetic poles to complete a cycle is equal to the precessional 

 period of revolution.^ 



Whether such a thing gives any real clue to the present state of 

 the interior of our globe is a question which I would leave others to 

 determine for themselves ; at the present time I would merely draw 

 attention to the similarity that exists between the supposed internal 

 condition to account for magnetic phenomena by Halley and Hanstein, 

 and that condition as put forward, to account for the " Elevation of 

 Mountain Chains," by Messrs. Fisher and Shaler in their recent articles 

 in this Magazine. 



M. Delaunay shows that a slowly-revolving crust would take 

 with it a contained fluid. It would be an interesting thing to know 

 whether, supposing a solid occupied the centre of the fluid, it also 

 would revolve with the said fluid at the same or a slower rate. 



J. Clifton Ward. 

 York, Oct. 19^, 1868. 



EETEEOFHYLLIA MIRABILIS AND H. LYELLL 

 Sir, — Mr. De Wilde's letter is quite satisfactory. Had the ap- 

 pearances referred to by Mr. Young (Geol. Mag. Oct. p. 448, etc.) 

 been present in Dr. Duncan's specimens, neither he nor Mr. De Wilde 

 would have failed to notice them. It is therefore to be regretted 

 that you did not submit Mr. Young's specimens to that artist, who 

 has an interest in the matter, rather than to Mr. Fielding, who has 

 ^ See Chap. viii. of " lludimentary Magnetism," by Sir Wm. Snow Harris. 



I 



