August 20, 1896] 



NA rURE 



365 



It is becoming more generally recognised that the really 

 " practical " man is the one who combines practical 

 knowledge and experience with intelligent appreciation 

 of underlying principles. 



The aim of the book is to lay before sailors, in an 

 easily comprehended manner, the principles on which 

 the various mechanical devices employed by them are 

 founded. 



A large amount of useful information has therefore 

 been gathered together in the small compass of this 

 book, and rules and principles whose general applica- 

 tion is explained in various texl-books on practical and 

 applied mechanics, are here specially adapted to the 

 requirements of the sailor. To mention a few instances : 

 we find explanations of the mechanical advantage gained 

 by the various tackles and purchases ; the construction 

 of derricks and shears, and the weight they will support ; 

 the relative strengths of ropes ; the breaking strains of 

 spars ; the floating power of spars and casks, and the 

 weight which a raft, constructed of given materials, will 

 bear ; the effect of the wind on the sails in driving the 

 ship ahead and in causing leeway ; the effect of the 

 water on the rudder ; the extra strain thrown on slings 

 when a ship is rolling, &c. 



The principles of the composition and resolution of 

 forces, and of the mechanical powers are somewhat fully 

 explained in the opening chapters, and the idea of 

 applying the traverse table, so familiar to all sailors, to 

 the solution of the problems, is an excellent one ; but 

 more explanation of some of the rules given later, which 

 have to be taken for granted, could be desired, as it is 

 very difficult to retain bare rules in the memory. 



The size of the book no doubt imposed limits on the 

 amount of space to be devoted to explanation, but it 

 provides, nevertheless, an excellent book of reference : 

 and though it may not be necessary to make some of the 

 calculations referred to, it is always useful to know how 

 things are worked, and on what principles, and that the 

 principles are in accordance with well-known physical 

 laws. " A sailor is so often thrown on his own resources, 

 and the more exact his knowledge is of natural forces, 

 the more readily can he avail himself of the forces at 

 hand." 



The book contains several valuable tables, and a useful 

 collection of rules in mensuration. 



F. C. Stebbing. 



Power Locomotion on the Hig/iway. By Rhys Jenkins, 

 M.I.Mech.E. Pp.64. (London': William Gate, Ltd., 

 1896.) 



The sub-title of this publication sulificiently expresses the 

 character of the contents ; it is " a guide to the litera- 

 ture relating to traction engines and steam rollers and 

 to the propulsion of common road carriages and veloci- 

 pedes by steam and other mechanical power, with a brief 

 historical sketch." The historical sketch is a concise 

 statement of the lines along which progress in power 

 locomotion on common roads has proceeded. Following 

 it is a bibliography of works on mechanical carriages and 

 traction engines, a catalogue of papers read before, or 

 appearing in the Transactions of, scientific and technical 

 societies, indexed under names of authors, a list of 

 journals devoted to the mechanical carriage movement, 

 and an index to articles on the subject in periodical 

 literature up to the end of 1895. The periodicals indexed 

 inckule those of the LInited States, France, and Germany, 

 as well as of Great Britain. The author has evidently 

 been at considerable pains to prepare his descriptive 

 index, and his efforts deserve encouragement. It would be 

 an immense boon if indexes of the same description were 

 available for other branches of technology. The recep- 

 tion afforded to this little book will show whether the 

 demand is sufficient to justify the puljlication of others of 

 a like kind. 



NO. 1399, VOL. 54] 



LETTERS TO THE EDITOR. 

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

 pressed by his correspondents. 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 coinmtinications.'\ 



The Utility of Specific Characters. 

 I REtiRET very much that I did not correctly remember, 

 when writing to Nature a month ago, what my friend Mr. 

 Thiselton-Dyer had said at the Linnean Society's meeting. I 

 suppose that in consequence I must not greatly complain that 

 whilst telling us what he really did say, my friend has taken 

 the opportunity to present a version of my views which is far 

 from accurate. He has less excuse than I had in attempting to 

 cite his remarks, since he has before him my printed letter of 

 July 16. This fact also renders it easy to show wherein he is 

 inaccurate. 



I suppose that we are all agreed that it is in the highest 

 degree interesting to know what Mr. Darwin himself thought 

 and said on questions of the kind now under discussion. At 

 the same time, we are none of us, I imagine, likely to attribute 

 to Mr. Dyer a special knowledge either of Darwin's writings or 

 of their interpretation which we do not share. Naturalists are, 

 I believe, not prepared to accept any individual as the authori- 

 tative exponent of Mr. Darwin's teaching. Under these cir- 

 cumstances it is to me a matter for regret that a plain discussion 

 of the question whether specific characters are invariably useful 

 should be turned into a dispute as to whether the person who 

 suggests some special application of Mr. Darwin's doctrines, or 

 advances some subordinate hypothesis in relation to their ap- 

 plication, can or can not be solemnly regarded as an Orthodo.x 

 Darwinian. Mr. Dyer says that the Darwinian theory seems 

 hardly to have a convinced supporter left except Mr. Wallace. 

 He denounces my quotations from Mr. Darwin's own books as 

 to correlation of variation as a "difficulty" brought up by 

 me " against " the Darwinian theory : then, without more ado, 

 assumes the role of apostle of the Darwinian theory (a part to 

 which I cannot allow him any exclusive claim), and proceeds 

 to tell us what "will" be found in the twenty-fifth chapter of 

 Mr. Darwin's " Animals and Plants under Domestication," 

 viz. that Darwin has said pretty much all that can (as yet) be 

 said about the facts of correlation of variation. The attempt 

 on Mr. Dyer's part to represent my citation of Mr. Darwin's 

 own conclusions in regard to correlation of variation as un- 

 orthodox, is a little beside the mark. No one is ignorant that 

 it "will" be found that in the chapter cited Mr. Darwin 

 discusses "correlation"; not only that, but it has been so 

 found long ago and repeatedly by many other readers besides 

 Mr. Dyer. I expressly stated, both at the Linnean Society and 

 in my letter printed in Nature on July 16, that I was quoting 

 from Mr. Darwin both as to Wells and as to other instances. 



Perhaps the most unsatisfactory feature in Mr. Dyer's claim 

 to classify his friends as heterodox and orthodox in regard to 

 Darwinism, is that it leads him to undertake to give away the 

 Darwinian theory. " I frankly admit," he says, " that such a 

 case [a case of correlated variation such as that hypothetically 

 stated by me] if completely established would give the utility of 

 specific characters, and with it the Darwinian theory, a serious 

 blow," I do not value this frank admission. If Mr. Dyer feels 

 constrained to admit to some one that such a case would give a 

 serious blow to the Darwinian theory, he must not come to me 

 with his " admission" of a " point scored"; for I neither admit 

 that any such blow is given, nor can I accept Mr. Dyer's good- 

 natured offer to act as representative of the Darwinian theory. 

 -Ml that my friend can represent in this matter is the Dyerian 

 theory of Mr. Darwin's theory. Mr. Darwin never asserted in 

 so many words that specific characters are invariably "useful," 

 and in my judgment he did not hold that opinion. I?ut whether 

 he did or did not, that opinion can not, I think, be shown to be 

 a necessary outcome of the theory of natural selection, pro- 

 vided that we take into consideration important ascertained 

 properties of living matter. My impression is that Mr. Wallace 

 — whom Mr. Dyer has declared to be the only convinced sup- 

 porter of the Darwinian theory now left — stated at the Linnean 

 Society that he agreed with me as to " correlated variation " 

 .sometimes accounting for a specific character which accordingly 

 could not be regardecl as due to utility. Such also I gathered 

 was the view of Mr. Meldola. Yet neither of these gentlemen 

 regarded this conclusion as a serious blow to the Darwinian 



