
¢ 
= 
- under discussion there. In the latter of these notices M. Delaunay 
“observes that he had been anticipated in his objections by Mr. 
Sune 22, 1871 | 
NATURE 
I4I 


a good means of determining the existence of Hyperme- 
tropia, Myopia, Astigmatism, and other affections of the 
retina of the eye, and seem to us to be well adapted for 
the purpose for which they are intended. EL. 


ELELTTERS TO THE EDEGOR 
[Zhe Editor does not hold himself responsible for opinions expressed 
by his Correspondents. No notice is taken of anonymous 
communications. | 
Thickness of the Earth’s Crust—Mr. Hopkins and 
M. Delaunay 
IN your numbers for March 16 and 23, 1871.4(pp. 400, 420) 
yeu give brief notices of the proceedings of the Academy of 
Sciences in Paris on the 6th and 13th of that month, from 
which it appears that Mr. Hopkins’s method of determining 
whether the crust of the earth is thick or thin has been again 

Hennessy. 
It so happens that Mr. Hopkins sent to me in Calcutta in 
a copy of Mr. Hennessy’s paper (which was published in 
ie Philosophical Transactions of 1851) with his remarks in 
wriling in the margin ; and I think it will be interesting to your 
readers if I give Mr. Hopkins’s opinion of the paper. 
Mr. Hennessy remarks (p. 546) that Mr. Hopkins’s ‘‘ result 
was founded on the hypothesis of the non-existence of friction and 
pressure from molecular causes at the surface of contact of the 
shell and nucleus.” On which Mr. Hopkins writes :—‘* This is 
not correct. My hypothesis is the absence of friction between 
the fluid particles themselves.” 
Again, Mr. Hennessy considers, as a result of his calculation, 
“*that we are entitled to assume that the motion of rotation of 
both shell and nucleus takes place nearly as if the mass were 
entirely solid.” On which Mr. Hopkins observes: ‘‘ Nothing 
conl justify the assumption of a mechanical impossibility.” 
And he traces this erroneous conclusion to the fact that Mr. 
Hennessy has made two assumptions in the course of his calcu- 
lations which iate it throughout, viz., (1) that the axes of in- 
stantaneous rotation of the shell and nucleus would coincide 
(which is implied in the last formula in par. 2. p. 514), ‘‘ which,” 
wrote Mr. Hopkins, ‘‘ they certainly would not,” and (2) that 
“the shell (or crust) is rigid” (p. 519, 525) so as to resist, with- 
out change of form, the internal pressure which arises from the 
inner surface, that is, the surface of the nucleus, ceasing tobe a 
surface of equilibrium, which Mr. Hopkins very reasonably con- 
siders to be quite inadmissible, and that accordingly the results 
deduced from these assumptions are ‘‘ valueless.” . 
2. I will take this opportunity of reverting to my letter to you 
of Aprilro, 1871. I there point out that what Mr. Hopkins 
did consists of two parts—(1) his conception of the idea that 
as the crust is not solidly connected withthe fluid nucleus the 
amount of precession must depend in some measure upon the 
thickness of the) crust ; and (2) his ca/ew/ation of the amount 
of precession this idea would lead to, so that by comparison with 
observation the thickness might be approximately found. In 
this way he discovered that if the crust and nucleus were homo- 
geneous, and of the same density (which they are far from being), 
the inner and outer surfaces of the crust being similar and simi- 
larly situated spheroids, the internal pressure of the fluid would 
act so as to leave terms in the precession depending on the thick- 
ness of the crust, only of the second order of small quantities ; 
whereas, in the case of the earth where the mass is heterogeneous, 
the density being double of the superficial density, 
the ness is involved in terms of the first order in the ex- 
pression for the precession, and by a comparison with observa- 
tion leads to Mr. H 
very great, something 
Calcutta, May 24 
» 
kins’s result, viz., that the thickness is 
<e 800 or 1,000 miles at least. 
JouNn H. Pratr 
The Duties of Local Societies 
It is undoubtedly the work proper to local natural history 
societies to study well the productions of their own immediate 
neighbourhood, to catalogue all the fossils, plants, and animals, 
and to note any peculiarities regarding them. In the settlement 
of the great questions still under discussion, much will depend | 
x. 

upon their faithfully performing this duty. All naturalists will 
cordially endorse whatever has been said in regarding such 
societies from this point of view, and will agree in declaring that 
it is far better they should be occupied in such labour than in 
the discussion of theories and abstruse general questions, which 
are better left to larger and more influential bodies. It is their 
office to collect facts upon which individual minds may generalise. 
This of course applies to such bodies in their collective capacity, 
and not to the members as individuals ; it is very probable that 
such individuals may make use of the facts collected by the 
society. 
But it should also be remembered that local societies have 
another duty to perform, and one, too, of hardly secondary im- 
portance, and that is the inculcation of the love of natural history 
in other minds. Indeed, it will often be found that for a time 
at least this must take precedence of the work already mentioned. 
It is well known to all who have taken any active part in field 
clubs and the like, that the real work devolves upon two or three 
members, sometimes fewer still, and it is totally beyond the 
power of these two or three to work up the whole natural history 
of the district by themselves ; they may have the will and the 
ability, but neither the time nor the means. We have known 
societies numbering above a hundred members where this was the 
case ; the great majority could not be called working members 
at all; they joined for various reasons, some merely because it 
was rather “the thing” to do in the town or village ; the greater 
number probably because they were interested in seeing speci- 
mens, hearing pleasantly-written papers at meetings, highly 
delighted at microscopical conversations, &c., &c., but did not 
care about working in the subjects for themselves. It is very 
evident that in such cases the first and foremost aim should be to 
induce as many as possible to become students of nature, at least 
to enable them to make intelligent notes of what they observe. 
Hence we find that the reports of provincial societies occasionally 
appear with little addition to what is already known, very little 
local information in them, and though this is to be greatly re- 
gretted (and in reality is so by the editors), it shou!ld not be 
regarded, as it often is, merely as matter of reproach by others 
more advanced. Local magazines and reports are issued mainly 
for the perusal of the members and others living in the locality, 
and such persons naturally wish to see the best of the papers that 
have been read ; and the matters that interest them most are not 
always new discoveries, wherever they may be made. 
But besides the ordinary method of reading papers at meetings 
principally, if not solely, attended by members, there are others 
to be followed by way of inducing outsiders to join the society. 
One is that of ‘* Penny Rambles,” which has been very successful 
in some localities. These should be conducted by some one not 
only well versed in natural history, but gifted with the not 
common ability to impart his knowledge in an attractive and 
popular way. There should be a change of conductors as often 
as possible, but care ought to be taken always to secure good 
ones, as one disappointment lasts a long time ; this implies that 
some competent person or committee should have the control 
over the arrangements. Sometimes it will be found that one man 
makes himself so well understood, and consequently so attractive, 
that he is preferred to others, and here the vox fopulz, within 
due limits, should have weight. As for subjects, no naturalist 
will ever be at aloss: the geology of the neighbourhood, some 
quarry, sand pit, or sea beach; the botany, some of the rarities 
and their peculiarities ; entomology also, and an occasional chat 
about some interesting antiquity in the vicinity. No abstruse 
theories should be taken up, they may be intensely interesting to 
the real naturalist, but are in a general way unknown, and if 
known, unappreciated by the multitude. 
The science lectures, jately so admirably carried out at Man- 
chester, and already referred to in these pages, will occur at once 
to every one; they might well be taken up in all large towns. 
But could not Museum Lectures also be started? The duty of 
every local society to establisha museum has been ably argued by 
another pen—a museum not for its own sake merely, but with 
this secondary idea before them, the good of the town or village 
in which they may be located ; one in which the labouring man 
shall find displayed specimens of the wealth of his own neighbour- 
hood, as well as typical forms from far off lands. Prof, Huxley 
has well said in one of his ‘‘ Lay Sermons” that there is a general 
impression among people that every event of importance hap, 
pened a long time ago; it is equally true that they fancy any 
natural object worth looking at must be sought fora long way off ; 
it is our duty to eradicate both impressions. If some plan could 
be adopted of giving penny lectures in the Museum, it would 
