5l2 



NA TURE 



[A f arch 31, 1887 



normal, and again in otiiers the profile may remain straight in 

 liitin" (as in Fig. z). As far as I could observe in Taranlola, the 

 upper jaw was always raised in opening the mouth, and the 

 profile of the head straightened from its normal curve when at 

 rest, but on closing the mouth in biting the movement of the 

 upper jaw depended upon the relative position of the animal 

 to the object which it was biting. This depression of 

 the upper jaw may be also often witnessed when the 

 mouth is closed, and it may be produced by applying slight 

 pressure to the head. The animal seems to make the most of its 

 ]iowers of expression, for on provocation it opens its relatively 

 huge mouth with the greatest readiness, and will keep it open for 

 a considerable time, during which its appearance is sufficiently 

 awe-inspiring. The fact that the anterior part of the skull is not 

 co-ossified with the posterior part is well known. Thus in 

 Huxley's " Anatomy of Vertebrata " (1871, p. 225) the following 

 statement is made concerning the geckos: "Neither the upper 

 nor the lower temporal arcades are ossified, the post-frontal being 

 connected with the squamosal and the maxilla with the quadrate by 

 ligament ; " and Mr. Boulenger informs me that had he been asked 

 whether the upper jaw of such lizards is moved in biting, he 

 would have been inclined to answer in the affirmative, reasoning 

 from the well-known condition of the skull. But I believe it has 

 not been hitherto actually observed that such movable articu- 

 lations possess a functional value in the living animal, and that 

 the geckos must be added to the well-known instance of the 

 parrots as Vertebiata which move the upper jaw in biting. It is 

 extremely probable that the same observations will be found to 

 hold for other families of lizards. Edw.^rd B. Poulton 



Wykeham House, Oxford, March I 



Weight and Mass 



Till some quite new facts are discovered, such as shall force 

 us to reconsider our convictions (which have not been lightly 

 formed), I do not think it profitable to accept a quasi-meta- 

 physical challenge from my friend Prof. GreenhiU. He has at 

 heart, as strongly as I have, the cause of dcfinitcness and accu- 

 racy: — and if he, as is natural for one in his position, feels 

 inclined to sympathise with the "vernacular" of Engineers 

 rather than object to it as I do, there is nothing for it but to 

 agree to differ. My remarks on this aspect of the subject were 

 of the most cursory and general character ; and I went so far as 

 to say that, as the book in question was written by a practical 

 man for practical men, " perhaps we ought not to complain." 



I cannot, however, go so tntich fiiiiher as to allow, with Prof. 

 GreenhiU, that it is " perfectly correct " to use the words po;:nd 

 or ton " side by side in two senses." As regards this practice 

 I, in turn, must quote from an unpublished letter of Clerk- 

 Maxwell's. [The passage purports to be part of a (reported) 

 .speech by a well-known Evolutionist.] 



"He regretted that so many .... were in the habit of 

 employing the word in a sense too definite and limited to be of 



any use in a complete theory He had himself always 



been careful to preserve that largeness of meaning which was 

 too often lost sight of in elementary works. This was best clone 

 by using the ivord sometimes in one sense and sometimes in 

 another ; and in this way he trusted he had made the word 

 occupy a sufficiently large field of thought." 



I have three other remarks to make upon Prof. Greenhill's 

 letter :— 



(1) He shows the absurdity of defining the weight of a body 

 as "the force with which it is attracted by the earth." Of 

 course such a definition must necessarily be absurd provided it 

 comes after an explanation (given by Prof. GreenhiU) that 

 " weight " is to be understood in the sense of "mass." But 

 from this explanation itself it would unfortunately follow that 

 a body has lueight even tuhcn it is no longer heavy : as, for 

 instance, when it is in a (supposed) cavity at the centre of the 

 earth ! Prof. GreenhiU says that " weight " is " used in ordinary 

 language in most cases " in the same sense as " mass." .Surely 

 the great majority of men regard weight from the point of view 

 of the sublime Porthos : — 



" Ma vaisselle d'argent .... qui doit peser de mille a 

 douze cents livres, CAR je pouvais a grande peine soulever le 

 coffre qui la renferme, et ne faisais que six fois le tour de ma 

 chambre en le portant." 



(2) He also speaks of certain difficulties imposed by the 

 "rules of language." I do not ascribe to them so lofty an 

 origin. They are the offspring of the dogmatic ignorance which 



has peopled the realms of science with Centrifugal Force and 

 its fellow monsters. 



(3) He has commented solely on a passing remark in my 

 article, and says nothing as to its main purpose. I hope, how- 

 ever, that he will eschew " statical " measures offeree, and give 

 his hearty aid as well as his good wishes in the war of exter- 

 mination which must perpetually be waged against the too 

 luxuriant undergrowth of the scientific garden : — the circle- 

 squarers, the perpetual-motionists, and (in the case before us) 

 the tneasurcrs of potential energy in terms of horse-power. 



V. G. T. 



An Error in Maxwell's "Electricity and Magnetism" 



It may be allowed to me to remark that the error mentioned 

 on pp. 172 and 223 of Nature has its origin really in Helm- 

 holtz's renowned paper (" Ueber die Erhaltungder Kraft," 1847, 

 p. 67), and that it thence found its way into most of the text- 

 books on electricity. It has sometimes been detected and hinted 

 at ; for the first time, I believe, in C. Neumann's paper published 

 in the Bcr. d. k. sdchs. Ges. d. Wiss, at Leipzig (1871), 

 " Elektrodynamische Untersuchungen mit besonderer Riicksicht 

 auf das Princip der Energie." There we find (p. 436) the 

 formula — 



rv -I- riVi = T-'cu 4- 7;=OTi - rrfJ^"' ■ ''^ 



dt 



dl 



which is identical with the equation of energy given in Nature, 

 p. 223, if we put — 



V, = .-/i, ^i\ = A, - TT-,Vn = T„„ F = T„ &c. 



The formula is followed by the remark that it agrees entirely 



with the formula given by Helmholtz, the only difierence being 



the last term — , which in the latter is wanting ; thus we may 



say that by Helmholtz the potential energy of the system in- 

 variably is expressed by zero. 



Being formerly unaware of Neumann's researches, I, by 

 another way, came to the same results, see the paper "Das 

 Princip der Energie in seiner Anwendung auf die ponderomo- 

 torischen und elektromotorischen Wirkungen de- eleklrischen 

 Stromes," published in the Sitober. d. k. biihm. Ges. d. Wiss, 

 {vide Nature, vol. xxxii. p. 308). In this paper I have hinted at 

 one probable cause of this and similar mistakes and their rela- 

 tively difficult discovery, of which the repetition of the error in 

 the best text-books is a decisive proof. This cause I believe to 

 be the trivial circumstance that there is no consistent and 

 generally accepted notation of the different forms of potential 

 and energy. This renders the comparison of different writings 

 on this subject sometimes quite perplexing. When, for example, 

 two authors denote the same thing, the one by V, the other by 

 - V, and when the first writes the expression d{T^T^V) in 

 the form — 



TiT^V + T^VdTi -i- T.^VdT.,, 

 the other the 7i/t-H/;Vfl/ expression —d(TiT^V) in the form — 



T^T„dV - T.J{T^V) -T-^d{,T„V), 

 it may happen that they themselves and also other readers over- 

 look the difference of sign, and that they continue to reason as 

 if their V were identical. Such mistakes occur oftener than is 

 supposed. See the interesting note at the end of Sir William 

 Thomson's paper on "Capillary Attraction," Nature, vol. 

 xxxiv. p. 369. 



Could there not be found a means of avoiding the incon- 

 veniences caused by such mistakes and the loss of labour spent 

 in detecting them ? I dare not hint at the possibility of an 

 international system of notations of the most important physi- 

 cal quantities ; thoroughly consistent, and recommended by the 

 highest scientific authorities ; for the realisation of such a system 

 would probably meet difficulties quite insvirmountable. 



Prague University, March 12 A. Seydler 



Tabasheer 

 Mr. Dyer in his article on "Tabasheer" in N.\ture of 

 February 24 (p. 396), throws out the suggestion that the silica 

 deposited in the joints of bamboo may have undergone a process 

 of dialysis. It maybe of some interest to him, and to your readers 

 generally, to learn that plates of transparent compact silica, SiOo, 

 may be formed by dialysing the basic soda silicate. Four or 



