242 



NATURE 



{Jan. 1 6, 1879 



There is another paint with recrard to testing witli Wheat- 

 stone's Bridge, which is not noticed in the review, but to which 

 I may be allowed to direct attention ; — that is, the position of the 

 galvanometer. It is not indifferent in which diagonal of the 

 bridge the battery and galvanometer are placed when the 

 branches are unequal. In such a case the method is much more 

 delicate when the galvanometer is placed in the diagonal joining 

 the junction of the two largest resistances with the junction of 

 the two smallest. As, I believe, we have in this laboratory the 

 only Wheatstone's bridge yet constructed after Mr. Schwendler's 

 design, by which the position of the galvanometer and battery 

 can be altered by the shifting of four plugs, I have made a few 

 tests which will show the advantage of this arrangement. 



The diagonal joining the junction of the branches with the 

 junction of the comparison coil and the resistance measured is 

 called m n ; the other diagonal being / q. 



It will thus be seen that when the branch resistances are equal 

 it is indifferent in which diagonals the galvanometer and battery 

 are placed ; but this is not the case when branch a is greater 

 than branch b. It is hardly necessary to observe that in a prac- 

 tical test more than one cell would be used when the branches 

 are unequal, in order to obtain much larger deflections, and more 

 accurate measurements. Herbert McLeod 



Royal Indian Engineering College, Cooper's Hill, January 6 



The Unseen Universe — Paradoxical Philosophy 



The principle of continuity forbids us to imagine that the 

 collocation called the atom has existed as it is from all eternity. 

 This the authors of the " Unseen Universe" have insisted upon, 

 and I need not go further than their title-page to remind Mr. 

 Ilallowes that in like manner they do not contemplate a future 

 eternal existence for the atom. 



But this principle cannot tell us what was the exact nature of 

 the thinkable antecedent of the present universe, nor can it tell 

 us the exact nature of that state which will follow the disappear- 

 ance of the present system. There are, however strong scientific 

 analogies which lead us to believe that the thinkable antecedent 

 of the present system was a spiritual unseen, which not only 

 developed but which now sustains the present order. 



Is it therefore necessary that I myself should in like manner 

 nelp to sustain some inferior universe ? I repudiate any such 

 obligation. I am not fit for it. 



Because a little boy has a father, is it logically essential that 

 he should likewise have a son ? 



Hermann Stoffkraft 



Schloss Ehrenberg, Baden, January 11 



Molecular Vibrations 

 In Nature, vol. xix. p. 15S, col. 2, is the following :— 

 " It has been suggested tliat the same molecule may be capable 

 of vibrating in different ways, and thus of yielding different 

 spectra. Just as a bell may give out different notes when struck in 

 different ways." It is well to note that the bell a- a whole gives 

 but one sound, and the other sounds are not true harmonics, but 

 come from parts of the bell, either before the whole is in vibra- 

 tion or from parts badly amalgamated, flaws in the metal, air- 

 bubbles in pouring into the mould, lack of homogeneity, in- 

 equalities in the mould, &c. 



The noises in a belfry are most discordant, whereas harmonics 

 form a succession of consonances — octave, fifth, fourth, major 

 and minor thirds, seventh and treble octave. 



Wm. Chappeli, 



The Electric Light 



While so many experiments are being made on lighting by 

 the incandescence of infusible materials produced by electric 

 currents, it is well to point out that Dr. Draper, as early as 1844, 

 used a strip of platinum so heated to determine the facts that all 

 solid substances become incandescent at 977° F., that light 

 increases in refrangibility and intensity, and that the order of 

 the colours emitted followed the true prismatic order as the tem- 

 perature increases. 



Dr. Draper says: "Among writers on optics it has been a 

 desideratum to obtain an artificial light of standard brilliancy. 

 The preceding experiments fornish an easy means of supplying 

 that want, and give us what might be termed a 'unit lamp.' 

 A surface of platinum of standard dimensions raised to a 

 standard temperature by a voltaic current will always emit a con- 

 stant light. A strip of that metal one inch long and -j^th of an 

 inch wide, connected with a lever by which its expansion might 

 be measured, would yield at 2,000" a light suitable for most pur- 

 poses. Moreover, it would be very easy to form from it a pho- 

 tometer by screening portions of the shining surface. An in- 

 genious artist would have very little difficulty, by taking advan- 

 tage of the movements of the lever, in making a self-acting 

 apparatus in which \.\\t platinum should be maintained at a uni- 

 form temperature, notwithstanding any change taking place in 

 the voltaic^current." ( Vide Draper's "Scientific Memoirs," p. 45.) 



Wimbledon, Januay 11 W, H. Preece 



Force and Enerjfy ^ 



III. ' : 



In consequence of energy not being a directed quantity we 

 come at once upon an important distinction between transference 

 of energ)'^ and transference of momentum. There may be a- 

 large force exerted, i.e., a large amount of momentum rapidly 

 transferred, without there being any accompanying transference 

 of energy. In the distance Von the two sides of a given section 

 of the stressed material through which the two opposite streams 

 are flowing, there is lodged a certain amount of motion which 

 is the same in the one portion on the one side of the section 

 as in that on the other side. The momentum and the 

 energy lodged in each portion are simply different functions 

 of one and the same motion. In unit time the whole of 

 the motion in the portion on the one side of the section 

 is transferred into the portion on the other side, and vice 

 rersd. The resulting quantitative transference of the one function 

 of the motion is double what would take place if only one, 

 instead of two, opposite streams were flowing through the sec- 

 tion, the reason being that this function is a directed quantity. 

 The resulting quantitative flow of the other function of the motion 

 is'zero, because it is a function which has no direction. The rate 

 of transference of momentum, or the force, is in this case eE, 

 the sign being given by the sign of e. Suppose, now, one only 

 of these streams of motion to be flowing past the section, the 

 rate of transference of momentum being ^eE, where ^ is the 

 geometrical ratio of extension, or the strain. The rate of trans- 

 ference of energy remains to be calculated. The material may 

 be either at rest or in motion. In fact whether it is to be con- 

 sidered at rest, or at what velocity it is to be considered moving, 

 depends altogether upon the set of bodies relatively to which 

 the motion is to be measured. Its relative velocity may also 

 be either uniform or variable. The relative velocity of the centre 

 of inertia of the material lying between two given sections will 

 be uniform if the whole of the motion measured in any quanti- 

 tative way flowing in through one of these sections is equal to 

 that simultaneously flowing out at the other. 



Suppose that before the force begins to act there is a uniform 

 velocity, Vo, throughout a given length. As soon as there is a 

 uniform force, \e E, throughout this whole length, the flow 

 being only in one direction, one half the particles will have at 

 any instant the velocity, v„, while the other half has the velocity 



(r'o + z')> where ^ = ^ * /■^. 



V = 





being the velocity of stream-flow; there is in the 



length V lodged an amount of momentum (F/tz'„ + i Vfiv) 

 for unit section throughout that length. Of this amount 

 J Vfi V = ^eE is transmitted forwards per unit of time. The 

 mean velocity of the material is also {v^ + J f). 

 ' Continu'^d from p. 219 



