364 



NATURE 



[February 20, 1896 



The fourth lecture is largely a reprint from Carri^re's 

 paper on bud-variation, and from Focke's work on the 

 hybridisation of plants ; whilst the fifth contains practical 

 instructions as to the methods of crossing employed by 

 experimenters. A glossary and an index terminate a 

 book which, if it contains little that is not familiar to 

 experts, will be extremely serviceable to beginners, and 

 will furnish the naturalist, who wishes to gain a general 



urvey of the matter, with just the information he requires. 

 For this latter purpose, a fuller bibliography would be an 



mprovement in a new edition. 



Maxwell T, Masters. 



OUR BOOK SHELF. 



Die Haustiere und ihre BezieJiungen zur Wirtschaft des 

 Mcnschen. Eine geographische Studie von Eduard 

 Hahn. Large 8vo, pp. 581, and a map. (Leipzig : 

 Duncker and Humblot, 1896.) 

 This volume is an important contribution to our know- 

 ledge of the relations existing between domestic animals 

 and human economy. It forms a large octavo volume 

 of nearly 600 pages, compiled from all sources, and of its 

 interest the reader may form some idea from the follow- 

 ing summary of its contents. After a brief introduction, 

 we have our domestic animals considered from a zoological 

 standpoint ; here the interesting questions of hybridisa- 

 tion and of the returning of once-domesticated animals 

 to a feral state, are investigated. Next the subject of the 

 profitableness of such animals is considered, the author 

 alluding but casually to the fact that some animals were 

 decidedly domesticated, without an eye to profit ; he cites 

 the case of some South American aborigines keeping a 

 " grylla " in their houses for the perfume, but has ap- 

 parently overlooked the case of the Greeks domesticating 

 the cigale. He on purpose omits the subject of animals 

 in connection with "worship," quoting Tylor, that "it is 

 a subject not wanting in interest, but is one abounding in 

 difficulties." 



The list of domesticated animals (using this term in the 

 widest sense of animals kept for the use or service of 

 man) given is a long one, com.prising not only such 

 familiar forms as the dog, horse, ass, horned cattle, sheep, 

 goats, reindeer, &c., but also the yak, gaur, llama, guinea- 

 pigs, and ferrets. The list of birds is extensive ; reptiles 

 are not mentioned, save the axolotl in an appendix ; and 

 among the fishes we find the carp, the gold-fish, and 

 the paradise-fish. Bees and several silk-spinners are 

 mentioned among the insects. 



The concluding portion of the work is devoted to 

 economic geography, and is illustrated by a map, in which 

 an attempt is made to mark out the world into areas 

 characterised by aboriginal industries. Certain regions are 

 coloured as being those of the hunters and fishers, then of 

 the several divisions of mankind living upon tubers and 

 cereals, or further advancing to the culture of such plants 

 as sugar-cane, tobacco, and the like ; still greater progress 

 is indicated by the type of gardening as practised, for ex 

 ample, by the Chinese. Of the various regions of the world 

 alluded to, that of Australia is the one most unsatisfac- 

 torily treated ; there is scarcely an allusion to the wonderful 

 culture of vegetables by the Maoris, for a very good know- 

 ledge of which we are long since indebted to the labours 

 of Colenso and others. The subject treated of in this 

 volume is of the widest range — in space covering the 

 known world, in time going back to days of indistinct 

 tradition, and for its complete investigation requiring 

 some knowledge of an immense range of literature, this 

 work is a contribution towards this history, and as such is 

 a most useful one. 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opinions ex- 

 pressed by his co7-respondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of NATURE. 

 No notice is taken of anonymous communications.'^ 



Velocity of Propagation of Electrostatic Force. 



Lord Kelvin, in his letter published in Nature, February 

 6, says that "it is an abuse of words to speak of the 'elastic 

 solid theory of electricity and magnetism' when no one has 

 hitherto shown how to find in an elastic solid anything analogous 

 to the attraction between rubbed sealing-wax and a little frag- 

 ment of paper ... or between two wires conveying electric 

 currents." 



It has, no doubt, escaped Lord Kelvin's notice that in 1884 a 

 paper was published in the Cambridge Philosophical Transactions 

 (vol. xiv. p. 45), in which I showed that two spheres pulsating 

 in an elastic medium will, if pulsating in opposite phases, attract 

 each other as sealing-wax attracts paper, and if in the same 

 phase will repel just as like electrified bodies do, the action being 

 propagated in the medium with a finite velocity. 



In a subsequent paper, published in 1885 {Camb. Phil: 

 Trans., vol. xiv. p. 188), I have shown that two straight oscil- 

 latory twists placed in an elastic medium will, if in the same 

 phase, attract each other as like electric currents do, and if in 

 opposite phases will be repelled. In fact, if the vibration of 

 the surface of a solid placed in a vibrating elastic medium is re- 

 solved into normal and tangential vibrations, the normal vibra- 

 tions will, as pointed out in these papers, cause the solid to 

 exhibit several of the phenomena of statically electrified bodies, 

 while the tangential vibrations will cause it to behave as if 

 carrying a current and acted on by a magnetic field. 



I only venture to mention these results because it appears 

 from Lord Kelvin's letter that they are not generally known. 

 They should, I believe, assist in forming a conception of a pos- 

 sible explanation of electric action, based on the supposition of 

 an elastic medium which resists changes of volume and shape. 



Firth College, Sheffield, February 11. A. H. Leahy. 



NO. 1373, VOL. 53] 



In making the statement quoted, I had fully taken into account 

 all such considerations as those referred to in Prof. Leahy's letter. 

 The rigidity of the solid absolutely prevents any phenomenon, 

 analogous to the attractions by rubbed amber or lodestone, from 

 being exhibited in an elastic solid. No such barrier exists if 

 the elastic medium be fluid ; and § § 733-740 of article 1 xli. of 

 my " Electrostatics and Magnetism " contain conclusions of 

 hydrokinetic theory regarding mutual forces between movable 

 tubes or rings with cyclic motion of an incompressible liquid 

 through them, showing magnitudes identical with, but directions 

 exactly opposite to, those of the forces in electro-magnetic 

 analogues consisting of movable conductors conveying electric 

 currents. The remainder of that article contains remarks on 

 Guthrie's interesting paper ^ " On Approach caused by Vibra- 

 tion," and " On the Attractions and Repulsions due to Vibration, 

 observed by Guthrie and Shellbach," from which the follow- 

 ing (§ 744)> being an extract from a report, in the North 

 British Daily Mail, of an address by myself to the Philosophical 

 Society of Glasgow on December 15, 1870, may possibly be 

 read with interest in connection with Prof Leahy's letter : — 



" The speaker began by stating that interesting papers had 

 recently appeared in the Proceedings of the Royal Society and 

 the Philosophical Magazine, by Prof. Guthrie, in which some 

 very curious hydrokinetic phenomena were described. From, 

 hints and suggestions in his paper, it seems that Prof. Guthrie 

 connected, in his own mind, these phenomena with possibilities 

 of explaining some of the more recondite actions in nature ; and 

 he (the speaker) believed that what gave the great charm to 

 these investigations for Prof Guthrie himself, and no doubt also 

 for many of those who heard his expositions and saw his experi- 

 ments, was, that the results belong to a class of phenomena to 

 which we may hopefully look for discovering the mechanism of 

 magnetic force, and possibly also the mechanism by which the 

 forces of electricity and of gravity are transmitted. The speaker, 

 however, did not lay any stress at present upon the possibility of 

 applying these results directly to explain magnetism. He 

 believed, on the contrary, that the true kinetic theory of mag- 



1 First published in the P?-oc. Roy. Soc Edinb. for Feb^ 1870. 



2 Proc. Roy. Soc, August 26, 1869. 



