500 



NATURE 



\Oct. II, 1877 



system of artificial culture, which unfortunately all ended 

 in failure. He then asks the question whether artificial 

 culture after the French method would be possible on the 

 German coasts of the German Ocean, and in a well- 

 written chapter arrives at the conclusion that this ques- 

 tion must be answered in the negative. An important query 

 is whether natural oyster-beds can be artificially enlarged, 

 and whether new oyster-beds can be established. Prof. 

 Moebius thoroughly ventilates this question, and an 

 attentive perusal of the little work will not leave anybody 

 in doubt as to whether any intended experiments will or 

 will not be crowned with success. The author quotes 

 several examples of natural beds which were ruined by 

 over-fisihing ; he also gives an account of the repeated 

 experiments itiade in the Baltic with a view of establishing 

 natural oyster-beds, all of which tailed, the last with 50,000 

 oysters deposited in 1843 near the Island of Kiigen, of 

 which only two years afterwards not a single one remamed 

 alive. One of the most interesting chapters in the book is 

 the one treating of the increase in the number of oyster- 

 eaters, the rise in the price and the decrease in the quantity 

 of oysters ; it contains numerous statistical data showing 

 how, in 1740, frt-sh oysters were sold at Hamburg at 3^/. 

 per hundred ! Even as late as 1830 they were sold at \s. 

 per tub (about 1,600) at Falmouth ; but Prof. Mcebius 

 doubts whether in this age of railways and steamboats we 

 shall ever return to such a state of things. A chapter on 

 the chemical constituents and the taste of oysters, and 

 another on the object and results of a rational culture 

 of oysters, form the conclusion to this clever little work. 



Die Naturkriifte. — Die Gesetzmiiszii^kcit im Gcsell- 

 schaftslcben. Statische Studien von Dr. Georg Mayr, 

 Mimsterialrath und Universitats Professor. (Miinchen : 

 R. Oldenbourg, 1S77.) 

 This small and readable volume contains a slight but 

 comprehensive sketch of the main features of political 

 and social statistics. It shows how the necessary data 

 have to be obtained, how they should be discussed, and 

 how the final results may be most clearly published, 

 whether in a graphic or a tabular form. It enters into no 

 technicalities, it is of very little value as a storehouse of 

 information, and it contains perhaps no remarks that are 

 strikingly original, but being written by a very competent 

 statistician it has the merit of giving a good, general idea 

 of the range of statistical inquiry and of its national value. 

 It is well calculated to instruct those who may desire to 

 obtain a broad and just view of the efforts, the difSculties, 

 and the achievements of modern statisticians. F. G. 



LETTERS TO THE EDITOR 

 [The Editor does not hold himself responsible for opinicns expressed 

 by 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 requests 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 app arance even of com- 

 munications containing interesting and novel facts. \ 



Potential Energy 



Most persons must agree with your correspondent, " X.," 

 that the term " potential energy" has been used with consider 

 able v.igueness and witti some difference of meaning by various 

 writers. They may even go further, and doubt at times whether 

 they are quite clear with respect to the cases to which the terms 

 "force "and "energy" are respectively applicable. But this 

 arises, I am inclined to think, from the difficulty of understand- 

 ing what is force, and would certainly not be removed by bring- 

 ing these two terms into m.ore frequent and closer juxtaposition 

 than that in which they are now found. 



Without attempting to reconcile the somewhat conflicting views 

 of different writers on the subject of po'enual energy, which, 

 however, I must own, are not so far apart from one another as 

 "X." seems to suppose, I should like to make a few remarks 



with respect to his proposal to trans'er " potential energy " from 

 the body in which it is said to exist to the force to which it owes 

 its existence. 



Energy, as generally understood, is of two kinds : the one is 

 energy of motion, and the other energy of configuration. In 

 both cases the system possessing energy has the power of doing 

 work ; in the one case actually, in the other potentially. Now 

 it is against this potential power of doing work that " X." pro- 

 tests, for he considers the expression tautological. But here I 

 disagree with him. I can see nothing incomprehensible in the 

 statement that a body has the power of acquiring the power of 

 doing work ; and, to restrict myself to the very simple illustra- 

 tion of a stone raised to a certain height, I should say that the 

 system of the stone and earth, by the action of a certain force 

 through a certain distance, had gained the power of acquiring a 

 certain amount of kinetic energy, i.e.. the power of acquiring 

 the power of doing a certain amount of work. 



This seemingly tautological phrase is more in the language 

 than in the idea ; for we often use, with respect to other sub- 

 jects than physics, similar expressions. We may say that the 

 possession of wealth confers the power of purchasing, and a 

 wealthy man is one who possesses a certain power which he may 

 have gotten himself, or which may have been transferred to him 

 from another. But a youth with a certain education, and placed 

 in a certain position, though not yet wealthy, may be said to be 

 in a position to acquire wealth, — to have in himself the power of 

 acquiring the power of purchasing. 



One other point I would notice before considering " X.'s " 

 new proposal. 



But first I would say that justice is hardly done to every 

 " dortor " (I am none myself) when we ate told, in connection 

 with the projection of a stone upwards, that "the gravitation 

 attraction is usually and conveniently conceived and spoken of 

 as all the earth's ; and the stone is usually regarded as being 

 simply attractiv/," and that "every doctor will frequently speak 

 thus." Certainly one doctor whom "X." has quoted is careful 

 to say that " when a stone has been lilted to a certain height 

 above the earth's surlace, the system of two bodies, the s'one 

 and the earth has potentiil energy, and is able to do a certain 

 amount ol work during the descent of the stone."' 



"X." finds considerable difficulty in understanding that if 

 the leaden weight of a clock is raised by winding it up the 

 energy of the clock is thereby increased. He says that " the 

 weight sets-toand work^ with E., which it has not in possession, 

 but only has the power of acquiring, and which it lo;es the 

 power of acquiring ! ! " and in a note we are told that " the 

 weight never acquires more than a quite insensible amount of 

 actual E, so called." This is quite true, because the "actual E." 

 is continuously used up as fast as it is acquired. If the weight 

 could have fallen freely it would have possessed a store of 

 "actual E. ' at its lowest point ; but instead of being able to do this 

 it has been continuously transferring its actual energy to the 

 machinery of the clock which it has set in motion. The weight 

 at Its highest point was in a position for doing work, and during 

 its descent work has been done. 



Let us see now if there are no difficulties connected with the 

 proposal to call potential energy " energy of tension," and to 

 locate it in the force rather than in the body. We are told if 

 you spend E. "in raising a stone to a certain height, you have 

 bestowed your E. on that attraction, you have transferred your 

 E. to gravity." This is not veTy clear, but " X." goes on to say 

 " that attraction was beforehand pressing at the stone as hard as 

 it could (this looks as if " X." placed the attraction in the earth 

 only), but it had no power of doing work." Well, why not? 

 Because, practically, there was no force acting on the stone. 

 The lorce of gravi ation was counteracted by the reaction of 

 the suriace on which the stone was resting. A force tree to act 

 has always the power 01 doing work ; but the existence of a force 

 presupposes the existence of a body, and the confusion is, there- 

 fore, considerably irxre.ised by speaking of ttie transference of 

 the power of doing woik from a body to a force. There seems 

 to be no harm in speaking of the energy ol a torce, but then we 

 mean the energy due to a force ; and this can be as well said of 

 kinetic as of potential energy. 



With the metaphysical dilficulty about force staring us in the 

 face, it surely would be very unwise without the gain of some 

 much more solid advantage than "X." has made out, to com- 

 plicate the idea by giving it the attribute of possessing energy ; 

 the connection between force and matter is so intimate that 

 let energy mean what it may, the idea of its transference iVom 

 ■ Clerk Maxwell, "Matter and Motion," p. 81. 



