8 



NATURE 



\Nov. 2, I 87( 



" reason to believe that this is the first specimen of a 

 wild Capercailzie which has been shot in Northumberland 

 in the memory of man." 



Probably the writer's glee was somewhat chilled by the 

 appearance in the same paper of an indignant letter from 

 the F.arl of Ravensworth, stating that the bird was reared 

 in his park at Eslington from eggs sent from the High- 

 lands, in the hope of naturalising the species in our 

 country ; the bird had strayed from its home. 



All naturalists — and especially such as know this noble 

 bird in its northern home — must hope that Lord Ravens- 

 worth will not be discouraged by this mishap, nor by 

 others like it, but that he will persevere in his experiment. 

 Eslington Park is situated in the valley of the Aln, 

 near the foot of the Cheviots. Perhaps this is not the 

 best part of the country in which to rear the birds, as, 

 although not devoid of timber, the woodland there is 

 scarcely that most suited to the Capercailzie. Pine woods 

 on rocky ground are probably the best. Not far from 

 Eslington, and partly on the Ravensworth estates, there 

 is a suoerb piece of woodland and crag. Thrunton Crags, 

 and the adjoining crags of Callaly, are very Httle known 

 except to those who live near them, for they are out of 

 the usual tourists' routes. But in all the county we do not 

 know a district better worth a visit, nor one which so 

 closely resembles the fir-clad mountain sides of southern 

 Norway. The rocks are craggy sandstones, whilst those 

 of Norway are mostly bosses of schists, and there is none 

 of the water which gives such a charm to Norwegian 

 scenery, but, in spite of these differences, the other resem- 

 blances point out this as the Capercailzie's fitting home. 



A wide area of high sandstone moorland stretches 

 through Northumberland, attaining a height of over 

 1,400 feet at Simonside, near Rothbury, and approaching 

 1,000 feet in other places. This runs to the west of 

 Alnwick, and thence north-west to Chillingham and away 

 towards the Tweed. Another similar district branches 

 from that of Simonside and spreads over a wide area 

 around Harbottle, and between the Coquet and the Reed. 

 Of the first-named range the crags of Thrunton and Callaly 

 form conspicuous features. Only a small part of this range 

 is wooded. It is much to be wished, for the sake of the 

 springs and streams, that plantations were more numerous ; 

 and for many reasons, artistic and others, we could wish 

 the same. In such parts of these wild hills as fir planta- 

 tions occur, the Capercailzie should do well ; and, as 

 plantations increase, we might hope that the birds would 

 here find a permanent home. Besides the crags of Thrunton 

 and Callaly we may note, as districts especially well suited 

 to them, the fir woods of Harbottle; parts of the Duke of 

 Northumberland's extensive park at Alnwick ; and the 

 wild forest-like park at Chillingham, the home of the 

 far-famed " wild cattle." In years to come, when the plan- 

 tations increase, SirW. Armstrong's grounds at Cragside, 

 near Rothbury, will afford the birds a shelter. Here, 

 amidst a profusion of sub-Alpine plants, the Capercailzie 

 should be quite at home. W. Topley 



NEWTON ON FORCE 

 IV/rANY EngHsh mathematicians are in the habit not 

 -'■''-»- only of using the word " force " in a certain technical 

 sense — briefly, as cause of change of motion — but of 

 regarding all other senses as loose and inaccurate. Of 

 late years there has been an increasing tendency, largely 

 due to Sir W. Thomson and Prof. Tait, to return to the 

 methods of expressing dynamical principles used by 

 Newton ; so that at the present time his statement of the 

 laws of motion is adopted to the exclusion oi others 

 which had usurped its place. This return to Newton has 

 led to a very prevalent notion that for all the statements 

 of fundamental dynamical principles current in modern 

 English mathematical literature, we have his authority, 

 and in particular for the above-mentioned restriction of 

 the use of the word " force." As the authority of Newton 



seems to me to be here claimed without warrant, and as 

 Newton's conception of force cannot be without interest, 

 I propose to examine as briefly as possible what this con- 

 ception was. In doing so I shall assume that the English 

 word " force " is the equivalent of Newton's Latin word 

 vis. 



Newton commences the Principia with eight defini- 

 tions, among which are definitions of the intrinsic force 

 of matter {vis insita or vis ijtertice), impressed force, and 

 centripetal force ; he then proceeds to state and explain 

 the laws of motion. From the chapters on the definitions 

 and the laws of motion, we are able to infer with much 

 probability the sense in which the word "force" is used, 

 To show this it will only be necessary to translate a few 

 extracts. 



" Def. III. — The intrinsic fo7'ce of matter \s its power oj 

 resistance, by which every body, as far as depends on itself, 

 persists in its state either of rest or of uniform rectilineal 

 motion." 



" Def. IV. — An intp'essed force is an action exerted on 

 a body towards changing its state either of rest or of uni- 

 form rectilinear motion." 



" Def. V. — A centripetal force is one by which bodies are 

 pulled, pushed, or in any way tend from all parts towards 

 any point as a centre." 



" Just as in cases of impact and rebound bodies are 

 equipollent whose velocities are reciprocally as theii 

 intrinsic forces ; so in moving mechanical instruments 

 agents are equipollent, and mutually support each othei 

 by their contrary efforts, whose velocities, estimated ir 

 the direction of the forces, are reciprocally as the forces.' 

 (Scholium to the Laws of Motion.) In the latter part ol 

 this quotation the word '' forces " is used in the sense oJ 

 impressed forces as defined by Def. IV. 



One more quotation must be made ; it is from Newton's 

 comment on Def. III. : " — but a body exerts this force 

 \yis i)ierticE\ only while a change is being made in its 

 state by another force impressed on it." 



In the quotation from the Scholium, Newton is con- 

 sidering two cases in which bodies are to be moved ; in 

 one the power of resisting the motion arises from the 

 intrinsic force of the body (or its vis inertics), in the othei 

 from the impressed force ; in each case he seeks a mea- 

 sure of the effort required to move the bodies. Newtor 

 argues that, in the first case, this is to be measured bj 

 the intrinsic force and velocity conjointly, and in the 

 second by the impressed force and [virtual] velocity con- 

 jointly ; implying that, in the first case, to give a body a 

 certain velocity is equivalent to giving a body of twice 

 its intrinsic force half that velocity ; and in the second 

 case — to take an example — that to lift a body vertically 

 at a certain rate is equivalent to lifting a body of twice 

 its weight at half the rate. 



In this Scholium, in the case both of intrinsic force 

 and of impressed force, the word " force " indicates some 

 power of resistance to change of state ; a general powei 

 due to the intrinsic force, and a special power due to the 

 impressed force. And throughout it will be found thai 

 Newton's use of the word in its most general sense indi- 

 cates a power of resistance, which professed metaphysi 

 cians are not alone in attributing to matter as essential 

 to the conception of it. This most general sense of the 

 word does not prevent Newton using it in any one of its 

 special senses, and in particular very frequently for im- 

 pressed force, where no confusion is likely to arise ; but his 

 language is very far from sanctioning the dictum that this 

 is the sense and the only sense in which the word " force ' 

 may be used. 



It is painful to reflect that Newton, great as he un- 

 doubtedly was, does not seem to have been sufficientlj 

 advanced to doubt whether there is such a thing as force, 

 nor to have had a proper sense of the heinousness of his 

 conduct in writing of accelerative force, motive force, and 

 — worst of all— centrifugal force. P. T. Main 



