Oct. 26, 1876] 



NATURE 



569 



on Mechanics always used the expressions "accelerating force" 

 and "moving force" (to the great confusion of learners), with 

 the noteworthy exception of Sandeman's " Motion of a Particle," 

 where "effect" was used for "force." So troublesome and 

 misleading was this terminology found for students that one 

 well-known Cambridge writer in a little work on Dynamics, 

 introduced it in a way which reminds one of the trembling and 

 caution with which Sidney Smith brought the word "meta- 

 physics " before his audience at the Royal Institution. But these 

 authors could claim the venerable authority of Newton for those 

 terms ; and if they had taken care to introduce them in the 

 exact 2C'ay in which he does, no difficulty would have ensued. 

 Unfortunately, until Mr. P. T. Main edited Newton's "Sec- 

 tions," our e itions of that work began with Lemma I., and 

 ignored his "Definitions" and "Laws of Motion." In the 

 "Definitions" Sir I. Newton tells us that the term " accele- 

 rative force " is used as an abbreviation for "the accelerative 

 quantity of a force, " or the velocity generated by it in a given 

 time ; and the term " moving " or " motive force " as an abbre- 

 viation for "the motive quantity of a force," or the momentum 

 generated by it in a given time ; and if these expressions had always 

 been explained in this way, i.e. as signifying what may be called 

 the velocity-effect and the momentum-effect of a force, there would 

 have been no room for misconception and no need of cautioning 

 the learner against the notion that there were two different kinds 

 of force. Perhaps with regard to Leibnitz it may be questioned 

 whether his physical ideas were so incorrect, and whether he 

 may not have used the terms referred to in the same way that 

 Newton did, viz., as abbreviations, and so as to embody the notions 

 of the different effects of a pov/er or influence on the motion of a 

 body, viz., its work-effect, its momentum-effect, its velocity- 

 eSect, &c. It must, however, be allowed that the term " con- 

 servation of force " (originally it seems due to Helmholtz) is very 

 misleading, for a meaning of "force" is therein required which 

 is not included in the original dynamical ideas ; and the 

 notion intended to be conveyed could only be given by a new 

 term, " energy," or work-power, with its attributives actual and 

 potential. But, after all, the whole controversy on the word 

 "force " is as to the method of measuring a pressure or tension ; 

 if we regard the time of the action, the effect is represented by 

 the momentum ; and if the space through which exertion is 

 made, the effect is represented by the work.^ Either of these 

 would then measure " force," and there would be no inaccuracy 

 if careful explanation were given as to the method used and the 

 sense of words. W. P. O. 



Arnesby, Rugby 



P.S. — In Prof. Tait's view of "force" is there not a con- 

 fusion between being a mere rate and being measured by a rate ? 



[Our correspondent refers merely to the short abstract given 

 by the Timts of Prof. Tait's Lecture. Some of his remarks will 

 be found inapplicable to the fuller report in our own pages. 

 —Ed.] 



Mr. Wallace and his Reviewer 



In Nature, vol. xiv. pp. 188, 189, in a review of Mr. Wallace's 

 recent work on " The Geographical Distribution of Animals," 

 occurs the following paragraph : ' ' Mr. Wallace admits the validity 

 of Elasmognathus of Gill as a genus of Tapirs, and adopts Dr. 

 Gray's multitudinous division of the well-defined and eminently 

 natural group of the Eared Seals (Otarid]. Many naturalists 

 would hesitate before following Mr. Gill or Dr. Gray as authori- 

 ties on these (or perhaps we may add many other) subjects." 



I freely admit the truth of the proposition that there are 

 "many subjects" on which I am not authority, if I am on any ; 

 there are none, I presume, who are authority on all things. 

 I will not even contest the allegation as to wrong-doings in 

 regard to the generic differentiation of Baird's tapir ; I beg, 

 however, to be allowed to excuse myself by "authority" for such 

 wrong-doing. 



The animal in question is distinguished from all others (I have 

 seen skins and skeletons of every known species, and about 100 

 skulls), and especially from the typical American tapirs by the 

 want of basal apophyses to the nasal bones, the extension of the 

 supramaxillaries behind, into their lowellas, and their extension 

 upwards into swollen portions, which tightly embrace the meseth- 

 moid, the complete ossification of the latter in the adult ; with 

 these features are co-ordinated others less marked, e.g., abbre- 

 viation of the cranial box, comparatively small size of the cere- 

 ' " Walton's Mechanical Problems," chap. x. 



bral cavity, &c. The genus has been accepted by Prof. Verrill, 

 Dr. von. Frantzius, Dr. Murie (see his article in j»urn. of Anat. 

 and Phys. vol. vi., pp. 131-169), as well as Dr. Gray, and every 

 trained mammalogist and anatomist to whom I have shown the 

 skulls {^e.g. the late Prof. Agassiz, Prof. Baird, Prof. Cope, E. 

 Coues, Dr. H. Allen, Mr. J. A. Allen) have concurred with me 

 that the type is entitled to generic distinction. 



As to the eared seals, the critic is wrong as to a matter of fact. 

 Mr. Wallace has not followed Dr. Gray in his arrangement 

 of the constituents of that family, but, as he expressly states, has 

 followed Mr. J. A. Allen's elaborate monograph of the Otariids 

 of Western America. Two more different arrangements of the 

 same group could scarcely be. For the generic features of the 

 arrangement adopted, I am quite willing to assume the responsi- 

 bility which Mr. Allen has devolved upon me,^ notwithstanding 

 the critic's emphatic condemnation. Beside Dr. Gray and 

 myself, F. Cuvier and many of the other older naturalists, as 

 well as Allen, Scammony Elliott, &c., have recognised generic 

 differences between the Otariids. 



But over and above all these I can plead in extenuation of my 

 wrong-doing the example of a very eminent and accomplished 

 naturalist, Mr. P. L. Sclater ; I feel assured that I am not mis- 

 taken in supposing he will be regarded as the best possible 

 authority on such subjects. That zoologist has differentiated the 

 deer into genera distinguished solely by the palmation or non- 

 palmation of the horns and many genera of birds on equally 

 slight ground which your limits forbid me to mention. I think 

 no rational naturalist familiar with the details of structure of the 

 deer and tapirs and the variations of horns in the former, will 

 contend that the d-fferences between the tapirs is of less syste- 

 matic importance than those used to differentiate Cervus and 

 Dama. Hence I think I have the best precedents for my action, 

 and if I am subject to censure, the eminent Englishman whom I 

 have cited is still more so. 



But far be it from me to deny that my critic is not at all correct 

 in his statement (shortly preceding the passage first quoted) that 

 " it would be easy to point out many passages in which Mr. 

 Wallace has not, in our opinion, made the most judicious choice 

 of authorities." One passage {Op. cit., vol. ii. p. 120) I beg to 

 reproduce in corroboration, but, in justice to Mr. Wallace, I 

 must add that although there are many other en-ors, the passage 

 thus quoted is an exceptional one in a valuable work. 



" Fresh-water Fishes.^ — The Nearctic region possesses no less 

 than (i) five peculiar family types, and (2) twenty-four pecuhar 

 genera of this class. The families are Aphredoderidce, consisting 

 of a single species found in the (3) Eastern States ; Peixopsida:, 

 founded on a species (4) peculiar to Lake Superior ; Hettropygii, 

 containing (5) two genera peculiar to the Eastern States ; Hyo- 

 dontidce and AmiidcB, each consisting of a single species. The 

 genera are as foUov/s : (6) Paralabrax, found in California ; 

 (7) Huro, peculiar to Lake Huron ; (8) Pilcoma, Bolcosoma, 

 (9) Brytlus and (10) Pomotis'va.\}a.t Eastern States — all belonging 

 to the Perch family. (11) Hypodilus and Noturus, belonging to 

 the Siluridm. (12) Thaleichthys, one of the Salmonidce peculiar 

 to the Columbia River. (13) Moxostoma, (14) Pimephales, (15) 

 Hyborhynchus, (16) Rhinichthys, in the Eastern States ; (17) 

 Ertcymba, (18) Exoglossum, (19) Leucosomus, and (20) Car- 

 piodes, more widely distributed ; Cochlognathus, in Texas ; 

 (21) Mylaphorodon axid. Orthodon, in CsLlilorms.; Meda,ia\.]xc 

 River Gila ; and Acrochilus, in the Columbia River — all belong- 

 ing to the Cyprirtidce. Scaphirhynchus, found only in the Mis- 

 sissippi and its tributaries, belongs to the sturgeon family {^Ac- 

 cipenseridce)." 



Whatever may be the " authority " followed, the following 

 are the facts almost all familiar to every American ichthyologist, 

 and matters of record respecting the forms enumerated, 

 (i) Five families are mentioned in one place (just quoted), 

 and six in others [op. cit., vol. ii., pp. 115, 143); but 

 the sixth (Lepidosteridce) is not peculiar ; (2) Twenty-four 

 genera are said to be peculiar, but ivrtr\\j-nine are enumerated, 

 as is indeed recognised in the next paragraph of the work. 



(3) The family Aphredoderida is represented by two species 

 found in the Western and Southern as well as Eastern States ; 



(4) The Percopsidce, far from being confined to Lake Superior, 

 are foimd at lease as far as Lake Champlain to the east, the 



» " These [genera recognised by Gill] appear to be natural groups of true 

 generic rank, and properly restricted ; and, after a careful examination of 

 the subject, and specimens of four of these five types, they appear to me to 

 include all the natural genera of the family." — Allen, " On the Eared Seals 

 {Otariidie)," p. 38. 



* The punctuation of the original is reproduced. 



