182 
NATURE 
[Fuly 6, 1871 

It has already appeared to me that the object to be attained 
was not the construction of a lifeship, but rather the fitting of 
lifeboats with steam machinery, thereby improving their efficiency 
and diminishing the risk of life, as a boat so constructed could 
be worked, and that more efficiently, by at most three men, 
instead of the large number now required to man them. The 
only means of propulsion which can be applied is, in my opinion, 
the hydraulic propeller, as, the turbine being enclosed, all risk of 
fouling pieces of wreck, weed, &c., is thereby avoided. To 
atrempt to use a lifeboat fitted with a screw or paddle would only 
be courting danger and disaster. Such being the case, the boat 
designed by me consists of three tubes, the outer ones being 
circular, and the centre one in which the propeller works being 
semicircular, and placed underneath the platform grating connect- 
ing the two circular tubes. The three tubes would be turned up 
and unite at the ends, and would somewhat resemble a whaleboat. 
The peculiar advantage of the hydraulic propeller when applied 
in this manner is, that the boat could be tumed round on its own 
centre, and sent ahead or astern by the man in charge by simply 
turning a handle, without issuing an order to any one, an advan- 
tage which I need hardly say is of the very greatest moment 
under such circumstances as those in which lifeboats are usually 
employed. 
The system of towing lifeboats by means of steam tugs to some 
point as near as possible to the site of the wreck, is one attended 
with danger, and the lifeboat, when cast off, is deprived of its 
means of propulsion at the very time when engine power would be 
most effective in enabling it to contend with the broken water 
round a wreck. Iremember a case at Bombay, when a lifeboat 
proceeding to a wreck was towed right under, and the Chinese 
crew swept out of the boat and nearly all drowned. 
Tubular lifeboats, I need hardly say, are no novelty, and the 
addition of a centre tube to carry the propeller and the steam 
engine and boiler will certainly not diminish their efficiency. 
JOHN FELLOWES 
Naval and Military Club, Piccadilly, July 3 
The Internal Stucture of the Earth 
ARCHDEACON PRATT’s letter in NaTURE for June 22 calls for 
some remarks on my part. He communicates a few marginal 
notes written by Mr. Hopkins on a copy of the second part of 
my ‘Researches in Terrestrial Physics,” which appeared in the 
‘Philosophical Transactions,” and seems seriously to regard these 
curt expressions as judicial utterances beyond which there can be 
no appeal. ) 
In the first place, I am accused of incorrectly stating the nature 
of Mr. Hopkins’s hypothesis as to the non-existence of friction 
between the fluid nucleus and solid shell of the earth. The 
words quoted from my paper as incorrect immediately follow a 
symbolical expression presented by Mr, Hopkins as the final 
result of his analysis, and my remark distinctly refers to this 
mathematical expression, and to nothin: else. Remembering that 
the whole of Mr. Hopkins’s mathematical investigations on the 
internal structure of the earth culminated in the deduction of this 
very expression, it is well to examine what are the words he uses 
in the course of his investigations which refer to the existence of 
friction between the shell and nucleus. 
In his first memoir, ‘‘ Philosophical Transactions,” 1839, he 
says, ‘and since there will be no friction with the assumed perfect 
fluidity of the interior matter,” p. 386. In his second memoir, 
I do not recollect thar anything about friction is mentioned ; 
but in his third, which summarises the whole of his pre- 
ceding labours, after presenting the formula already alluded 
to, he states that it was established on the suppostion of 
‘the transition being immediate from the entire solidity of the 
shell to the ferfect fluidity of the mass.” He afterwards gives 
reasons for believing that a stratum of imperfect fluid probably 
exists between the shell and the perfect fluid, and he further uses 
the words, “ Consequently ¢he assumption made in our investiga- 
tions of the absence of all tangential action between the shell and 
fluid will not be accurately true,” p. 43. As my remark relers 
to these investigations and their immediate result, it is unnecessary 
to say to whom the charge of inaccuracy may just y apply. In 
affirming the existence of friction between the shell and nucleus 
to such an extent as to cause both to rotate as one solid mass, 
friction between the particles of the fluid is clearly implied ; for 
if no such friction existed, the film of liquid touching the shell 
and moving with it might slip over the remainder of the nucleus. 
? 

I have, therefore, been all along at issue with Mr. Hopkins on 
this point, when I concluded that the rotation of the shell and 
nucleus must take place as if the whole were solid. Mr. Hopkins 
declares this conclusion to be ‘‘a mechanical impossibility.” It 
isthis “impossibili y”’ which has been reaffirmed by M. Delaunay 
in stronger terms than those I used. It has been shown to be 
not merely possible, but rigorously true, in a particular case, by 
an experiment of M. Champagneur, which I have myself recently 
verified, and it has been further so clearly illustrated in these 
pages by two correspondents A. J. M. and A. H. Green (May 
18, p. 45) as to require no further observation. The coincidence 
of the axes of instantaneous rotation of the shell and nucleus 
necess irily follows if the whole moves asa solid mass ; and to 
charge me with implying the coincidence in one of my formulz 
is equivalent to charging me with being strictly consistent On 
this point Mr. Hopkins is of course at issue with M. Delaunay 
as well as with myself. The next imp »rtant question referred to 
on which I totally differ from Mr. Hopkins is that of the form of 
the inner surface of the shell. If the shell has been gradually 
formed by solidification from a fluid mass, it is evident that the 
rate of progressive solidification at the interior of the shell must 
depend on the rate of refrigeration of the surface of the nucleus. 
This takes place, and has probably taken place for ages, at an 
almost insensible rate of slowness, and therefore also the succes- 
sive additions of matter to the shell’s inner surface. Between 
the perf: ctly solidified and comparatively rigid part of the shell 
and the fluid nucleus, the matter on the point of becoming 
solidified is probably in a pasty or imperfectly fluid state (as Mr. 
Hopkins has admitted), and it is this matter which is subjected 
to a moulding action by the changes of shape of the nucleus, as 
I pointed out in the publication already alluded to. This 
pasty matter becoming slowly impressed with the shape of 
the nucleus, and freely yielding to the impression as it passes 
to the solid state, the more rigid part of the shell, precisely 
as the outer case of a mould, is saved from strain, and cannot 
undergo a corresponding change of figure. In the discus- 
sion which followed the reading of my commun cation to 
the French Academy of Sciences on March 6, it appears 
from the Comptes Rendus that M. Elie de Beaumont made 
some remarks which illustrate and support this view of the 
process of formation of the shell. The conclusion to which I 
was thus led, that the inner surface of the shell could not be less 
elliptical than its outer surface, was reaffirmed soon after the 
publication of my researches by an eminent mathematician, the 
late Baron Plana, of Turin. All this Mr. Hopkins considers as 
quite inadmissible, and very reasonably, too, in the opinion of 
Archdeacon Pratt, and all the results deuced therefrom are judi- 
cially pronounced to be ‘‘ valueless.” But my conclusion as to 
the interior ellipticity of the shell is only a necessary deduction 
flowing trom the fundamental principles from which my inquiries 
start, a principle upon which I am as much at issue with Mr, 
Hopkins as upon anything referred to in his marginal notes. As 
this is the really viral divergence between us, a few words of ex- 
planation are desirable 
The hypothesis of the entirely fluid state of the earth anterior 
to its present state forms the groundwork of mathematical in- 
quiries as to the earth’s figure. The problem, as hitherto 
treated, always involved an additional hypothesis either openly 
or tacitly implied, namely, that the distribution of the 
particles composing the earth underwent no change by 
the earth’s transition from a completely fluid condition 
to its present state, While Mr. Hopkins tacitly assumed this 
second hypo'hesis throughout his investizations, [ have reason to 
believe that it was for the first time rejected in my paper on the 
‘*Figureand Primitive Formation of the Ear‘h,” which forms the 
first of my ‘‘ Researches in Terrestrial Piysics.” By this step 
we are at liberty to investigate, with the aid of mechanical 
and physical laws and the known properties of the earth’s 
materials, the probable arrangement and laws of density of the 
interior strata of the shelland nucleus. In attempting to doso, I 
was led to conclusions as to the earth’s internal structure widely 
differing from those of Mr. Hopkins. I have great difficulty in 
believing that the crude comments on my researches communi- 
cated to Archdeacon Pratt, could have been intended to meet 
the public eye. Long before Mr. Hopkins sent these remarks to 
Archdeacon Pratt, he wrote to me promising to comment pub- 
licly upon my conclusions ; and since then an opportunity oc- 
curred for poi-ting out in his presence at a meering ot the 
British Association what I conceived to be the inconclusive charac- 
ter ofhis results, Mr, Hopkins promised to reply, but neither this 
