302 



NATURE 



{Feb. 14, 187S 



subject of" the improvement of peasant farmers is ably 

 discussed in Prof. Baldwin's report on the Irish prize 

 farms, R. W. 



OUR BOOK SHELF 



Oregon : its Resources, Climate, People, and Prodtictions. 

 By H. N. Moseley, F.R.S. (London : Stanford, 1878.) 



This little manual is the result of a visit paid in July and 

 August last by Mr. Moseley to Oregon. Mr. Moseley gives 

 not only the results of his own observations, but has taken 

 the trouble to consult carefully and give the gist of official 

 publications on the state, the result being a thoroughly 

 satisfactory, full, and trustworthy account of the present 

 condition of Oregon. Mr. Moseley has done a public 

 service in undertaking this task, and we recommend his 

 book to all who contemplate emigrating. It will answer 

 nearly every question an intending emigrant is likely to 

 ask, and gives, moreover, very definite advice as to the 

 kind of people for which the state at present is suited. 

 The book contains an excellent map of the state, 



A Handbook of Common Salt. By J. J. L. Ration, M.D., 

 M.C. Madras College. (Madras : Higginbotham and 

 Co., 1877.) 



This work is not to be judged as a scientific treatise, but 

 as a practical guide to the manufacture of common salt 

 from sea-water. The author has fulfilled the purpose 

 which he set before himself in compiling the book. 

 Starting with a brief historical introduction, he proceeds to 

 lay before the reader a concise statement of the principal 

 chemical and physical qualities of salt. The occurrence 

 of salt as a mineral is then shortly discussed ; the 

 analysis of natural salt occupies a small chapter, which is 

 succeeded by others upon the hygienic value of salt, and 

 upon the agricultural uses of the same substance. The 

 principal rock-salt deposits are described, and the mining 

 operations sketched. 



After these chapters, which must be considered as 

 introductory, the composition of sea-water is discussed ; 

 the leading facts concerning evaporation of solutions of 

 mixed salts, and fractional precipitation of the saline sub- 

 stances, are clearly laid down, and upon these the theory 

 of salt manufacture is shown to be based. 



Details of the salt manufacture are then given, followed 

 by descriptions of the growth of " spontaneous salt," of 

 the manufacture of salt from brine springs, of " earth 

 salt," and lastly, of salt lakes. The final chapter is 

 devoted to a discussion of the bearings of taxation upon 

 the salt trade. 



The book is written from the Indian view-point, and is 

 rich in local illustrations of the manufacture ; but the 

 author has endeavoured to make, and we think has 

 succeeded in making, the work a really good manual of 

 general applicability. 



The author is to be praised for the carefulness with 

 which he has gathered together and arranged a large 

 mass of facts ; the result is a most useful and convenient 

 little book of reference. M. M. P. M. 



LETTERS TO THE EDITOR 



[The Editor does not hold himself responsible for opmions expressed 

 bv his correspondents. Neither can he undertake to return, 

 or to correspond with the -writers of rejected manuscripts. 

 No notice is taken of anonymous communications. 



[ The Editor urgently rc(jtiests correspondents to keep their letters as 

 short as possible. The pressure on his space is so great that it 

 is impossible otherwise to ensure the appearance even of com- 

 munications containing interesting and novel facts ^ 



The "Phantom" Force 



The famous principles of conservation and dissipation of 

 energy, which have done so much to promote the progress of 



physical science in recent years, were undoubtedly first inferred 

 and generalised from certain similar laws in the theory of forces 

 which, as we find noticed by Prof. Tait in Nature (vol. xiv. 

 p. 462), were first propounded by Newton.^ If in any me- 

 chanical system, Newton observes, urged by any forces, to 

 which we must add those which arise from friction, the action of a 

 force reckoned as a gain in the system is measured by the product 

 of its impulse and the space through which it is pushed back, or 

 as a loss in the system when the product relates to a space 

 through which the force is allowed to act, and if as action of the 

 same kind in the system we also count its gains and losses of 

 actual energy of motion, the whole amount of action in the 

 system remains xmchanged during the motion. Viewed from the 

 standpoint of the laws of motion, force, and matter, which 

 Newton starts with in the " Principia," and keeping in mind the 

 special definition here given (coinciding with the modern term 

 " potential increase ") of the "action " of a force, obviously the 

 reverse of what would vulgarly be called the action of a force in 

 increasing a body's energy of motion, this proposition at first 

 looks like a truism ; but the idea of potential energy here coined 

 by Newton ^ is really an essential one ; and it besides allows the 

 mode of action of some forces of very common occurrence in 

 nature to be described more simply than they could be without 

 it. The force of gravitation, of attraction and repulsion between 

 two bodies permanently electrified or magnetised, and all dual 

 forces or actions and reactions directed along, and depending only 

 on the distance between two bodies, and not at all upon the 

 time, are of this kind. The force can be completely described 

 in these cases (and it may be looked upon in the first instance as 

 only a measure of convenience) by the permanent gradient of 

 energy-variation everywhere ; and hence also by the permanent 

 change of energy from one distance to another, when, as is sup- 

 posed in this example, the dual force pair acts along the line of 

 centres ; since then the changes of actual energy which it produces 

 (acting alone upon the bodies) are independent of the rotation of 

 this line, and may be regarded either as produced with the 

 natural motion of this line's rotation or by the same forces acting 

 along a fixed line of centres. When two such bodies approach, 

 or recede from each other, whatever time elapses or whatever 

 course they may pursue about their centre of mass, not only 

 are the momentary transfers between actual and potential 

 energy equal in energy value at every moment of the motion 

 (for this is general, and by this condition otily the bodies return- 

 ing twice to the same distance from each other might have very 

 different energies of motion at the two returns) ; but the whole 

 energy of motion which can be gained between two distances is 

 a definite one, and as this would not be so if the bodies returned 

 twice to the same distance with different actual energies, nor if 

 they returned twice to the same distance with different poten- 

 tial energies, it follows at once that not only is the sum of 

 the actual and potential energies at any one distance invariable 

 with the lapse of time and with any intervening motions of the 

 bodies, but since the gain of actual energy from this distance to 

 any other is the loss of potential energy, the sum of these two 

 energies is also the same at one distance as it is at another, and 

 it therefore varies neither with the time nor with the distance of 

 the bodies from each other. 



In this illustrative example of two bodies (otherwise un- 

 impelled) exerting upon each other a permanent action and 

 reaction, several points connected with the use of the term 

 "potential energy," as just described, require attention. In 

 the first place, whatever the real forces are (acting in "absolute 

 space " ^) upon the two bodies, the Newtonian laws of motion 



^ On reading the passage again (which I here desciibed from memory) 

 I find that its statement is verbally but not substantially different from 

 what I wrote above, and that in Newton's statement the signs are merely 

 taken oppositelj . Newton thus describes an "acceleration " (a gain of actual 

 energy) as a "resistance " {i.e. , a force) overcome, with a corresponding loss 

 of action in the system. This is the modern view of equivalence between 

 potential and actual "action " or energy, but wii/i the signs e/" these actions 

 changed. 



2 Newton, in fact, anticipated D'Alembert's principle ; and if we apply 

 D'Alembert's principle to the motion of a single particle, the way in which 

 it likewise coincides with the modern definition or recognition of potential 

 energy will presently be understood, although it also reverses the signs of 

 both of the energies concerned. 



3 The term "absolute space," or the simpler word "space," used in 

 Newton's enunciations of the laws o( motion as the field of action of " force " 

 is nothing more than a space whose oiigin is either the centre of mass of all 

 the bodies under actual observation, or any space in which that centre is 

 moving uniformly in a straight line. If \ve extend our observation to new 

 bodies found not to be moving uniformly in the original space, the old space 

 must be given up, and a new one must be adopted (recognising the new 

 masses), to enable us tckstate all the forces and to describe the motions com. 

 pletely, of all the bodies under observation (which is the sole problem and 



