Nov. 30, 1 87 1 



NATURE 



On the Constitution of the Solid Crust of the Earth. By 



Archdeacon Pratt, F.R.S. (Phil. Trans., 1871.) 

 Another contribution to a subject on which the author 

 his laboured for many years — never perhaps very brilli- 

 antly, but always in the main soundly. Such unmitigated 

 nonsense has been talked on the subject of the thickness 

 of the solid ciust of the earth, even by scientific men of 

 real power — generally mere mathematicians, sometimes 

 geologists, rarely indeed physicists— and such extravagant 

 views on the subject are still propounded and defended 

 by men like Delaunay, whohavedone good work in closely 

 allied questions, that it is really refreshing to read Arch- 

 deacon Pratt's paper. Yet its tone is somewhat hesitating, 

 almost apologetic, and he finally arrives at the conclusion 

 that what seems to us to be at least a natural assumption 

 to make at stariing (viz., that a level surface may be drawn, 

 not very many miles under the surface of the earth, such 

 that in spite of hills and ocean beds the amount of matter 

 shall be the same in every vertical line between these two 

 surfaces) le.ads to results not after all very inconsis- 

 tent with those derived from actual pendulum observa- 

 tions made over the Indian Continent. Sir VV. Thomson's 

 bold investigation of the tides in the solid earth, due to 

 elastic yielding, furnishes us with by far the most power- 

 ful mode of attacking the general question which has been 

 devised since Hopkins's celebrated suggestion of the in- 

 formation to be derived from precession and nutation ; 

 and it is to be hoped that the labours of the Tidal Com- 

 mittee of the British Association will soon furnish, from 

 observation, the data required for its numerical application. 



LETTERS TO THE EDITOR 



[ Tlie Editor Joes not hold himself responsible for opinions expressed 

 hv his correspondents. No notice is taken of anonymous 

 communications. ] 



Instruction in Science for Women 



In ihankirg you for the useful account given in your last 

 number of the various attempts being made m 'different parts of 

 Enyland to improve the scientific education of women, may I 

 give you a few more details of the effort now beirg made at 

 Cambridge to assist the training of those ladies who live too far 

 from any educational centre to be able to get oral instruction ? 



Correspondence classes have been formed in some of the sub- 

 jects selected for the University Examination of Women, arid the 

 teachers (chiefly resident fellows of colleges) are attempting to 

 assist the leading of their correspondents by means of advice, 

 examinalion papers at fi.xed intervals, and free criticism. 



Of course this scheme cannot offer the advfniages which the 

 lecture svstems of London, Edinburgh, and Cambiidge itself 

 afford ; but that it does meet a real want in what I may venture 

 to call the " rural districts " is shown by the fact that more than 

 seventy women have joined the scheme within a month. Among 

 the subjects of which you take notice in your article, Mr. Stuart 

 of Trinity has ra derlaken the higher mathematics, Mr. Hudson 

 of St. John's the arithmetic (how woefully ill-taught in the average 

 girls' school no one but the examiner can appreciate), Mr. Bonney 

 of St. John's the geology, and myself the botany. I should add 

 that it is not at all the wish of the promoters to limit the scheme 

 to possible candidates for the Cambridge examinations, but as 

 f-ir as possible to assist any woman who may be struggling with 

 the difficulty of reading a new subject by herself 



All women who wish to avail themselves of this scheme are 

 requested to communicate with the Hon. Sec, Mrs. Peile, of 

 Tninipington, near Cambridge. F. E. KnCHENER 



Rugby, Nov. 25 



True and Spurious Metaphysics 

 Df. Ingleby is evidently a strategist of no mean order. The 

 appalling suddenness of his totally unexpected personal attack, 

 and the skill with which he has almost made it impossible for me to 

 reply without laying myself open to the charge of Egotism (second 

 only in gravity to a charge of Immorality), shows that he is a 

 good deal more than a mere metaphysician. Of metaphysics 

 anon — meanwhUe about mathematicians. 

 1 altogether repudiate the Trichotomy, as Dr. Ingleby gives 



it. The man is either a Mathematician or a Non-Mathematician. 

 There is no intermediate class. Merely to be able to integrate, to 

 solve differential equations, to work the hardest of Senate-Houes 

 Problems, &c., &c., is not to be a Mathematician. To deserve 

 the name a man must have some of the creative faculty ; must be 

 the noir)T?(s, if ever so little. And to be a Creator in this sense 

 it is not necessary that one should have devised a new Calculus, 

 Are .Stokes, Thomson, Clerk-Maxwell on the one hand, or Cayley, 

 S) Ivester, Clifford on the other mere Experts ? Vet there can 

 be no doubt that, in Dr. Ingleby's classification, tliis is their 

 rank. 



As regards Hamilton's having placed Metaphysics higher than 

 Mathematics, I may avail myself of the remark, which I heard 

 not long ago in conversation, that " what Hamilton thus exalted 

 was the Rletaphysics of the great thinker (and Mathematician) 

 Kant, not the common Cant of Metaphysicians." The distinc- 

 tion imphed in this poor pun is one of enormous importance, for 

 there are Metaphysicians and Metaphysicians. Here I am 

 happy so far to agree with Dr. Ingleby, and I shall dichotomise, 

 but not quite as he proposes. 



Metaphysicians A. The genuine article. To this class al I 

 men worthy of the name of Mathematicians necessarily belong, 

 as do the higher Physicists, &c., &c., such as Faraday. Hence, 

 of couise, Archimedes, Descartes (Cartesius, not Cartes!) 

 D'Alembert, Hamilton, &c., &c., appear in the list. Leibnitz 

 was, I fear, simply a thief as regards Mathematics, and in Physics 

 he did not allow the truth of Newton's discoveries ; so he does 

 not belong to this class. 



Metaphysicians B. The spurious article, which has somehow 

 managed to arrogate to itself the title belonging of right to the 

 genuine one. Test this class by what it has to show "even in 

 the present advanced state of metaphysics" (as Dr. Ingleby has 

 it) : what have we but stagnation, ropes of sand, bitter quarrels 

 as to the meaning of unintelligible words, and, above all, com- 

 placent pride in being "not as other men" but dwellers in a 

 sublimer sphere ? Even Longfellow's idiotic " Youth," who 

 ascends the Matterhorn when "the shades of night aie falling 

 fast," carrying a pompous " banner with a strange device," does 

 not so ridiculously contrast with the practical Whymper and 

 Tyndall carrying their ropes and ice-axes, as do the Metaphysi- 

 cians B with the Metaphysicians A : — the Drones with the 

 Working-Bees. 



When I asked for the name of a Metaphysician who was also 

 a Mathematician, it was of course of Class B that I spoke, the 

 class containing Hegel and Sir William Homiiton, Bait, (the 

 former of whom proved that Newton did not undei stand 

 Fluxions nor even the Law of Gravitation, while the latter 

 asserted that the pursuits of the Malhematic'an reduce him 

 either to passive Credulity or to absolute Unbelief!), the class 

 which is popularly, and (almost therefort) erroneously, known by 

 the name. P. G. Tait 



" Wormell's Mechanics" 

 I REQUEST to make a few tbservations upon Mr. Wcrmell's 

 letter in your last number. I shall refer to the paragiaphs he 

 has numbered. 



1. It is true that, by a collation of two passages, a really intel- 

 ligent student might be able to eliminate the eiror from the first 

 statement in Mr. Wormell's book to which we have taken excep- 

 tion. I consider that such collation should be unnecessary in 

 a text-book. 



2. A mathematician would, of course, understand what Mr. 

 Wormell means, however he might disapprove of its logic ; but 

 Mr. Wormed writes for beginner,*, and should state his demon- 

 strations without ambiguity. 



4. " Curious" is not the adjective we are templed to apply to 

 such a blunder as that on p. 112. This has not been coirecied 

 in even the second edition of the bcok, notwithstanding the 

 " schoolboy's " aid. 



5. We had read Sec. 71, and consequently made the lemaik 

 about the block and tackle to which Mr. Wormell objects. We 

 now re-assert that the effect of friction upon the mechanical 

 powers is too important to h.ive been omitted in a book profess- 

 ing to treat of Theoretical and Applied Mechanics. 



Nov. 25 The Reviewer 



Solar Halo 



The following description and drawing of a solar halo and 

 mock suns seen on the morning of the 13th inst., by the Rev. J. 



