624 



NATURE 



[Oct 30, 1 8; 9 



easy to a beginner ; that, in fact, there are no real diffi- 

 culties which must be fairly faced by every student. We 

 are surprised to find that this opinion can be held by any 

 sound and successful teacher. Our own experience has 

 always been dead against it. Dr. Lodge says of the 

 elementary works of Thomson and Tait, Clerk- Maxwell, 

 and Clifford, that they are " far too difficult for beginners." 

 We do not think that his process of dilution makes the 

 matter a whit less difficult. It has rather a tendency to 

 conceal from the reader the place where the real difficulty 

 lies, and a necessary difficulty avoided is certainly not 

 overcome. Second, the avoidance of difficulties is 

 managed by loose and sometimes even metaphysical 

 language (see, for instance, pp. 83-5); evidently em- 

 bodying some of the speculations in which the author has 

 indulged while excogitating his work. 

 As an instance of loose writing take this (p. 16) 

 " 5. The effects of force on matter are : 



A. Change of motion, which is called acceleration. 



B. Change of size or shape, which is called strain 



or deformation. 



If only one force acts on a body, it must produce the 

 effect A. If two or more forces act in different directions 

 on a body, they must produce B, and they may produce 

 A also." Now, at first sight, this looks well enough, and 

 certainly Dr. Lodge knows the facts thoroughly. But 

 how is change of motioti called acceleration ? Accelera- 

 tion is correctly defined (p. 19) as Rate of change of 

 velocity. But (p. 18) velocity is defined as "the rate of 

 motion of a body." Put these extracts along with A 

 above, and we find "change of that whose rate is called 

 velocity is rate of change of relocity ; " a very remarkable 

 proposition, indeed one of high metaphysical interest. 

 Again, if only one force act on a body, it tnust produce B 

 unless the body be perfectly rigid. And two or more 

 forces do not necessarily produce B, even on the most 

 plastic body. Take the case of two different sets of 

 parallel forces, for instance, each proportional to the mass 

 of the element on which it acts. 



In conclusion we may say that for the facts of ele- 

 mentary mechanics, for excellent e.xamples of application 

 of the formulas, and such like matters, the student may 

 use this work with profit :— but he ought to be warned 

 that where the text appears most simple it is generally 

 loose, often metaphysical, and here and there unintelli- 

 gible. 



Le couchiglie Pompeianc. Descritte dal Dott. Nicola 

 Tiberi. 4to, 12 pp. (NapoU, 1879.) 



This rernarkable and well written memoir was published 

 before the recent celebration at Pompeii of the eighteenth 

 centenary of its destruction by a volcanic eruption of 

 Vesuvius. It is the work of an excellent naturalist, who 

 lives at Resina, close to the site of the ruined city, and 

 who is especially conversant with the shells of the 

 Mediterranean. The point of view to which he directs 

 our attention is very different from that which has been 

 taken by the geologist, antiquary, artist, or architect. 

 He treats of the shells found in the ruins, and which had 

 served for food, or been used by the Pompeians for 

 ornament and other purposes. Indeed we know from 

 Athena;us and other ancient authors that moUusca were 

 then relished quite as much as they are at present by the 

 inhabit mts of Italy. I have been unable to discover in 

 the loose and incorrect twaddle of the younger Pliny, who 

 lost his life in the eruption, any mention of shells having 

 been collected by his countrymen for the study of natural 

 history It is a pursuit or amusement of comparatively 

 modern times. Dr. Tiberi gives a list of all the shells 

 which be has noticed as Pompeian, belonging to no less 

 than 44 species, with particulars of their relative abund- 

 ance .Tt 'Pbrnpeii, as well as of their distribution and 

 economy. ScJiiie were of eatable kinds, as the common 

 oy%KRx\\'iAtciyx%%A,'Pectcn jacoba:74S, Venus cjiione, Tapes 



decussatus, and several species of Helix. Others 

 adorned fountains, as Haliotis tuberculata, Murex trtin- 

 culus, and M. brandaris. The oriental pearl-shell 

 {Meleagrina inargaritifcrd) was represented only by a 

 single valve. But the ladies of Pompeii seem to have 

 attached considerable value to the Cypraa or Cowry, as 

 amulets or charms to prevent sterility ; and among these 

 shells were some of species from the Red Sea and 

 Persian Gulf. A single specimen of another exotic shell 

 (Conus textilis) must have been kept for its great beauty 

 as an object of curiosity. All the shells used in the orna- 

 mentation of fountains, five in the city and one in the 

 suburbs, are of species which still are common in the 

 Gulf of Naples ; these shells are separately distinguished 

 and named. 



The memoir forms a short but interesting chapter of 

 Roman history, and it tells us more than is generally 

 known about the habits of the former masters of the 



world. J. GWYN JEFFREY.S 



Banka und Biliton. Von Dr. E. Reyer. (Vienna, 1879). 



In this pamphlet, originally published as an article in the 

 Oesterreichischen Zeitschrtft fiir Berg - und Hiittenweseu, 

 the author has brought together a vast amount of useful 

 information on these two important tin-yielding localities. 

 At the present time, when the trade in this important but 

 sparingly-distributed metal has been almost entirely di- 

 verted from its ancient centres in Cornwall and Saxony 

 by the development of the sources of supply in the East 

 Indies and Australia, the valuable details contained in 

 this pamphlet cannot fail to be read with much interest. 

 By far the largest and most reliable part of the informa- 

 tion on these districts is inaccessible to most readers, 

 from the fact of its being written in the Dutch language, 

 and Dr. Reyer has done good service in bringing together 

 so much material in a compendious and available form. 

 The geological structure of the districts, the distribution of 

 the ore in them, the methods of working, and the mineral 

 statistics of the two areas, are very fully described, and 

 the monograph concludes with an interesting sketch of 

 the life of the Chinese immigrants who are engaged in 

 working these tin ores in the Malay Archipelago. 



J.W.J. 



LETTERS TO THE EDITOR 



[ The Editor does not hold himself responsible for opinions expressed 

 by his correspondents. Neither can he undertake to return, or 

 to correspond with the writers of, rejected manuscripts. No 

 notice is taken of anonymous communications. 



[ The Editor urgently requests correspondents to keep their letters as 

 short as possible. The pressure on his space is so great thai it 

 is impossible otherwise to ensure the appearance even of com- 

 munications containing interesting and novel factsi\ 



Greenwich Meteorological Observations 



Mr. Buchan (Nature, vol. xx. p. 602) now admits that 

 fundamental mean temperatures are to be found in Table 77. 

 But his original unqualified remark (p. 526) was that mean tem- 

 peratures for Greenwich "remain still to be calculated"; he 

 even endeavoured to infer the mean annual temperature from 

 the observations of the earth-thennometers, as though Table 77 

 (containing a value of this element with which no hitherto 

 determined value for Greenwich can compete) had no existence. 

 All this was likely to convey to an uninformed reader a very 

 erroneous impression. 



Table 52 contains simply a collection of the mean monthly 

 results given in the twelve tables (38 to 49) referring to diurnal 

 inequality, and as these numbers api)eared to sufficiently well 

 represent the varying temperatures of individual months, no 

 account was taken of omitted days. But we can without diffi- 

 culty determine their influence, usually small, in the months 

 affected, and, in consequence of the now expressed want, shall 

 probably take an opportunity of doing so. The question w as of 

 much greater importance as regards the fundamental values of 

 Table 77, in forming which, as before mentioned, and as is 





