460 



NATURE 



{April II," 1872 



sociated with the other probable causes of tubercle, and 

 it is difficult to give instances where tubercular con- 

 sumption has made its appearance whilst perfectly pure 

 iiir is continually breathed. But, we think, various con- 

 siderations render Dr. MacCormac's views untenable. We 

 will not refer to Iceland or to the inhabitants of the 

 elevated plains of the Andes, or of the Steppes of Asia — 

 all of which are sad stumbling-blocks in his way — because, 

 as he says, they are so far off, and our facts in rCL^ard to the 

 frequency of tubercle in these regions are perhaps not quite 

 satisfactorily ascertained. But we may call attention to the 

 circumstance that the disease is more common in England 

 than in almost any other country — than in France, for 

 example ; yet, surely, the hygienic relations in regard to 

 ventil.ition are superior in England to those e.xisting on 

 the other side of the Channel. 



If air that has been breathed is so certainly the cause 

 of tubercle, the poor population of London and other 

 large towns should not only be decimated, but should be 

 swept off en masse, for they all breathe through the night, 

 and through a great part of the day, air so contaminated. 

 Once more, how is it that one member of a household 

 belonging to the upper class is attacked and dies, though 

 all the rest, notwithstanding their being exposed to the 

 same conditions, are preserved ? Looking at animals, 

 again, any Indian inedical officer will tell Dr. MacCormac 

 thit monkeys kept in confinement, though they have 

 never had a roof over their heads and have consequently 

 never breathed air a second time, will die with their lungs 

 stuffed with tubercle. Lastly, the evidence is very strong in 

 favourof Virchow'3view,that tubercular matter is originally 

 composed of cells resembling the white corpuscles of the 

 blood, which are either modified white corpuscles, or, as 

 Virchow himself maintains, proceeds from the prolifica- 

 tion of connective tissue corpuscles. Whilst disagreeing, 

 therefore, with Dr. MacCormac in regarding the breathing 

 of air impel fectly freed from the products of previous 

 respiration as the exclusive cause of tubercle, we may 

 fully endorse his views upon the desirability of thorough 

 and complete ventilation, especially in our sitting-rooms 

 and sleeping apartments. The exigencies of modern 

 civilisation seem to lead unavoidably to the close herding 

 of mankind ; but we confess it is with a sigh of regret that 

 we see year by year long lines of close-packed houses, 

 springing up on what were but recently green fields on 

 every side of this great metropolis. To reach green fields 

 and breathe fresh air is now a day's work. 



H. Power 



Tlmny of Friction. By John H. Jellett, B.D., P.R.I. A. 



(Dublin : Hodges and Co. ; London : Macinillan ) 

 This book is, to a certain extent, of the character of a 

 supplement to ordinary treatises on mechanics. It deals 

 with the question of friction by the use of analytical 

 expressions very general in the possibility of their applica- 

 tion, on which account perhaps some of the significance 

 of their physical character may be apt to escape the 

 general reader, and the book is thus, perhaps, rather more 

 suitable for advanced than for junior students. 



The author brings well into prominence the radical 

 difference between problems in statical and dynamical 

 friction, namely, that the latter are determinate, whereas 

 the former are not necessarily so. He says : — 



" When a system of material particles, each of which 

 rests on a rough surface, is subject to the action of ex- 

 ternal forces, it will in general be found that, of these 

 particles, some will be in a state of inotion and others in 

 a state of rest. Everything connected with the moving 

 particles, namely, their positions, their velocities, and the 

 forces, geometrical and frictional, which act upon them, is 

 fully determined by means of the dynamical and geo- 

 metrical equations. The geometrical and frictional 

 forces which act upon the quiescent particles will also be 

 determinate, unless it be possible to form by elimination 



one or more equations between the co-ordinates of the 

 quiescent particles only. If this be possible, the geo- 

 metrical force replacing every such equation will be inde- 

 terminate in intensity." 



The character and cause of the analytical indeter- 

 minateness in the case of statical friction is enunciated in 

 the following words, which obviously apply also to forces 

 not frictional : — 



"If any one or more of the forces acting upon the 

 particles of a system be not determinate functions of the 

 co-ordinates, the number of the unknown quantities will 

 exceed the number of equations, and there will be in 

 general an infinite number of positions satisfying the 

 conditions of equilibrium, d.sposed in one or more groups, 

 in each of which these positions succeed one another 

 continuously." 



There is an interesting chapter on the distinction 

 between necessary and possible equiUbrium, arising, so 

 far as friction is concerned, from the fact that the co- 

 efficient of dynamical friction is less than that of statical 

 friction, so that " if the system be disturbed from its posi- 

 tion of equilibrium by the communication of infinitely 

 small velocities to its several points, when the friction at 

 each point will, of course, become dynamical, a finite 

 force tending to augment the displacement may at once 

 be developed at some or all of these points." The whole 

 point of distinction between this and ordinary unstable 

 equilibrium, when frict on is not taken into account, con- 

 sists in the fact of the infinitely small velocity calling into 

 play a finite force, which it would not do in the case of 

 ordinary unstable equilibrium, in the lapse of a finite time. 

 Without questioning the analytical excellence and interest 

 of the invesligation, we nny hesitate in adopting the 

 change from statical to dynamical friction as a consequence 

 of the assumption of an infinitely small velocity. We 

 would point to the following problem (page 170) as a 

 good example of the concrete application of the principles 

 of the treatise : — " Two rods, AB, CD, firmly jointed 

 together at B, rest so that A presses against a rough 

 vertical surface, and CD lies on a rough peg in the same 

 vertical ; find the limiting positions and the nature of the 

 equilibrium." 



At the end of the book there are several problems 

 worked out, namely, the well-known problem of a top 

 spinning on a rough plane, the problem of " friction 

 wheels," and one or two problems connected with the 

 driving wheels of locomotives. J. S. 



LETTERS TO THE EDITOR 



[ The Editor does not hold himself responsible for opinions expressed 

 by his correspondents. No notice is taken of anonymous 

 communications. ] 



The Adamites 

 I SHOULD not have noticed the letter of " M. A. I.," which 

 appeared in the last number of Nature, with reference to my 

 paper on " The Adamites," were it not that my silence might be 

 interpreted as an acknowledgment of the justice of the remarks 

 of the anonymous writer. If I had been silent, however, I trust 

 your readers would have had more sense than to accept the 

 dictum of a writer, anonymous or otherwise, who thinks to nega- 

 tive the conclusions of a paper, written at least in a truly scien- 

 tific spirit, by such nonsense as the reference to Paddy&r^A Taffy. 

 (jne looks for reasoning in the criticisms which appear in such 

 .1 journal as Nature, and not for a misleading statement of an 

 opponent's position, supported by reference to general con- 

 clusions and the use of weak satire. When " M. A. I." conde- 

 scends to advance an argument, I shall be happy to consider it ; 

 and if it should be unanswerable, I shall not hesitate to admit it 

 to be so. Doubtless I ought to feel thankful for the tenderness 

 with which he has trodden on my toes, but I have scant regard 

 for mere courtesy where questions of science are at stake ; and 

 in the interests of truth I would rather that the errors of my 



