IQ4 



NATURE 



[April 5, 1917 



with the Continental ones. Where single orders for 

 thousands were received by Continental firms from all 

 sources, British firms received orders for tens. Any- 

 one who is familiar with industry knows how difficult 

 it is to comf>ete with large firms under such conditions. 



During the war, however, the British optical firms 

 have received large and continuous orders, and as a 

 result the increase of output has been very great. To 

 have rushed a small industry in war-time and in so 

 short a period up to its present size is a marvellous 

 performance, probably unsurpassed in any other in- 

 dustry demanding exceptional skill. 



But the jmportant points to observe are that, so far 

 as their knowledge was concerned, the principal British 

 firms were competent to undertake the work, and that, 

 having the orders, they were able to erect new pre- 

 mises, install plant, provide special jigs and tools, 

 train unskilled labour, find suitable materials, and 

 in these extraordinary circumstances to produce 

 instruments that satisfy the requirements of the 

 Services. 



If in peace-time military orders of reasonable size 

 had been placed in this country, the optical industry 

 would have dealt with them as it has during the war. 

 It is largely a question of orders of reasonable size 

 and, above all, continuity of orders. 



Having pilloried the optical manufacturer. Sir Joseph 

 Larmor proceeds to praise the British optical writers. 

 It is suggested that t.ieir works contain information 

 that the manufacturers lack. 



In Germany and in Britain it is not the type of 

 book cited by Sir Joseph Larmor that is used by the 

 optical manufacture»s. Not one of the books cited 

 deals with the method of optical computation actually 

 adopted in the German workshops, and, indeed, there 

 are extremely few books in Germany that do divulge 

 the whole system. The British books are, no doubt, 

 well adapted to enabie students to pass examinations 

 on general optics. For example, one of the best of 

 those cited has a large index, which includes the rain- 

 bow and the principle of relativity — questions no doubt 

 that will find a place of honour in examination papers 

 — but which does not refer to so vital a question as 

 coma, to which alone a whole book should be devoted. 



There is a great similarity in the present optical 

 books. They all contain the Si.me stereotyped material. 

 Some deal with it ir a non-mathematical way, while 

 others attack the propositions with heavy algebraic 

 artillery. ^ Generally the sign convention changes with- 

 out warning from page to page, foi the simple reason 

 that the matter is mostly copied from previous writers 

 who used no standard system. 



No doubt these bo iks' are the unfortunate result of 

 circumstances. A book devored to, say, coma would 

 have a very limited market, whereas a book on optics, 

 if made sufficiently general, can be made to aopeal to 

 students and school teachers and thus find a profitable 

 market. 



In the early days of the optical industrv in this 

 country our pure mathematicians were also real crafts- 

 men. They were not content with the evolution of 

 general equations. Today "the science of the best 

 optical instrument makers is far ahead of the science 

 of the text-books." That is the opinion of the late 

 Prof. Silyanus P. Thompson, who" also said: "But 

 the teaching of the colleg^es and the university teachino- 

 at Cambridge — well, what is it in optics? Thev call 

 it optics, but it is really jjurelv mathematical gymnas- 

 tics applied to the optical problems of a hundred years 

 ago. I do not think there is reallv what one can truh' 

 call optical work goine jn at Cambridge. . . . Optical 

 teaching, I am sorry to sa's, is very largely at its 

 lowest conceivable ebb." 



If our present-d^y mathematicians wish to help the 

 industrv (and \W\r beln is desired), thev must enter 

 NO. 2475, VOL. 99] 



the workshops first as learners, not teachers. They 

 may find the work laborious and monotonous from the 

 point of view of the ma hematician to whom a pretty 

 solution is an object of importance, but once they have 

 experienced the pleasure of testing a system that 

 accords with their calculations, they will never again 

 be satisfied with the publif^ation of untried formulae. 



James Weir French. 

 Anniesland, Glasgow, March 28. 



Floating Earths. 



In reference to the inquiry of Dr. Walter Leaf in 

 Nature of March 15 as to the interpretation of a 

 passage of Strabo, the fact may possibly be of some 

 interest that in the island of Mors, in Denmark, bricks 

 are made from a local sandy clay which, after burn- 

 ing, float in water. These bricks are used, I under- 

 stand, both as a refractory material and for ordinary 

 building purposes, their lightness and porosity giving 

 them certain advantages for the latter purpose. Their 

 mechanical strength is said to be considerable. The 

 porosity is not obtained by the addition of combustible 

 or volatile matter during moulding. 



If the expression irriypvufvas, used by Posidonius, 

 be consistent with a process of burning the clay into, 

 bricks, and if clays of somewhat similar physical 

 character to that of Mors, although of different geo- 

 logical origin, occur in Asia Minor and Spain, an 

 explanation of the passage might perhaps be found 

 in this direction. Cecil H. Desch. 



Metallurgical Laboratory, 



L^niversity of Glasgow, 

 March 24. 



Gravitation and Thermodynamics. 



If Dr. P. E. Shaw's contention (Nature, March 

 29) for a perpetual motion consequence of gravita- 

 tional heat were justified, it would be an argument 

 against the supposed effect on which such a con- 

 clusion could be based ; but it does not seem to me 

 that the contention is justified. For the line joining 

 maximum to minimum temperature is vertical, and, 

 unless the rate of heating differs from the rate of 

 cooling, every horizontal chord will be an isothermal ; 

 so there is nothing to keep a vertical disc rotating. 



Oliver Lodge. 



The suggestion in Nature of March i that thermo- 

 dynamics might throw light on the question of the 

 temperature variation of gravitation has not been 

 unkindly received. The criticisms have not been 

 directed so much against this suggested application 

 of thermodynamics as against the expression deduced 

 for the attraction between two bodies. 



It has been pointed out to me that dQ is not a 

 perfect differential, and therefore it is not valid to 

 equate 



dr.de dB.dr 



The correct expression for the attraction, assuming 

 that the specific heat is independent of r, is 



F = m.d./(r) + ylf(r), ' 



where tn is the mass of the body the temperature of 

 which is 6. This expression has none of the objections 

 which the previous incorrect expression had, for at the 

 absolute zero the temperature coefficient vanishes and 

 yp^(r) is probably GMnir-^. 



The assumption that dsldr=o is, of course, only a 

 special case, for 5 may depend on r or on the gravita- 

 tional field in which the mass m is situated. Since 



