5o8 



NA TURE 



[April 2, 1885 



planetary systems, and whilst the field is thus left open 

 to the nebular hypothesis or other rival theories, it is 

 submitted that tidal friction has a bearing on those 

 theories which cannot be neglected. 



A numerical comparison of the distribution of moment 

 of momentum amongst the several planetary sub-systems 

 shows that the terrestrial system differs considerably from 

 all the others, but it would hardly be logical to postulate 

 an absolutely independent mechanism in this case, and it 

 is not very easy to reconcile the genesis of the moon close 

 to the earth with the formation of a ring in the midst of a 

 planetary agglomeration of meteorites. Let us now sum- 

 marise the advantages and disadvantages of M. Faye's 

 scheme. 



The conception of the growth of planetary bodies by 

 the aggregation of meteorites is a good one, and perhaps 

 seems more probable than the hypothesis that the whole 

 solar system was gaseous, and that the influence of hydro- 

 static pressure was felt throughout. The internal annula- 

 tion of the meteorites is left unexplained, and this com- 

 pares very unfavourably with Laplace's system, where the 

 annulation is the very thing explained. The difference of 

 orbital motion of the inner and outer meteorites of a ring, 

 the development of that difference as time progresses, and 

 the consequence of direct and retrograde rotation at 

 different distances from the sun is an excellent idea. But 

 it is necessary to this idea that the inner planets should 

 have been formed the first, and we are met directly by the 

 fact that the single surviving ring, that of Saturn, is nearer 

 to the planet than are the satellites. It is, of course, 

 possible that special causes have preserved this ring, but 

 we should be driven to the startling conclusion that 

 Saturn's ring is the oldest feature of his system. 



The actual distribution of satellites in the solar system 

 is at variance with M. Faye's theory, for, according to 

 him, the internal planets were generated from rings whose 

 motion was such as would give greater moment of 

 momentum to the planetary agglomeration than would 

 the external ones. The number of satellites manufactured 

 should be greater the greater the amount of rotation in 

 the primitive agglomeration of meteorites, and thus the 

 nearer planets should be richer in satellites than the 

 remote ones. 



The celebrated experiment of Plateau, in which a drop 

 of oil rotating in alcohol and water is made to parody 

 Laplace's solar system, is worthy of attention, and it tells 

 against Faye and in favour of Laplace. It is of course 

 to be admitted that surface-tension does not duly repre- 

 sent gravity. 



On the whole, then, we must hold the opinion that 

 there are great difficulties in the acceptance of M. Faye's 

 theory, notwithstanding its excellences. The time does 

 not appear yet ripe for definite judgment on this very 

 complex subject, but science is undoubtedly the gainer 

 by such suggestive theories. Whilst a false statement of 

 fact always proves a serious detriment, the enunciation 

 of false or partially true theories is always the incentive 

 to, or initiation of, the discovery of truth. 



G. H. Dakwin 



SIR WILLIAM THOMSON ON MOLECULAR 

 DYNAMICS ' ' 

 II. 

 T N the present article Sir William Thomson's spring 

 -!■ and shell molecule will be described and its theory 

 sketched, in so far as this has been investigated with the 

 view of getting over some of the difficulties which sur- 

 round the wave theory of light. In Helmholtz's memoir 

 on anomalous dispersion, a sketch of such a theory was 

 published. But this new molecule differs from that of 

 Helmholtz in several points, chiefly in the fact that absorp- 

 tion is not accounted for by any viscous action in the 

 1 Continued from p. 463. 



molecule dissipating the energy of vibration into low 

 grade heat. Most readers who have ever visited the 

 natural philosophy lecture-room in Glasgow University 

 will recognise a very old friend in this new molecule, 

 where they have seen it vibrating, I suppose, any time 

 since the University occupied its present site. In appear- 

 ance the molecule has been changed, but its theory as 

 taught to the students there is identical. For a descrip- 

 tion of this molecule let us refer to page 10 of the 

 lectures : — 



" Imagine for a moment that we make a rude mechani- 

 cal model. Let this be an infinitely rigid spherical shell ; 

 let there be another absolutely rigid shell inside of that, 

 and so on, as many as you please. Naturally we might 

 think of something more continuous than that, but I only 

 wish to call attention to a crude mechanical explanation 

 possibly of the effects of dispersion. Suppose we had 

 luminiferous ether outside, and that this hollow space is 

 of very small diameter in comparison with the wave- 

 length. Let zig-zag springs connect the outer rigid 

 boundary with boundary number two. I use a zig-zag, 

 not a spiral, spring which has the helical properties which 

 we are not ready for yet, such properties as sugar and 

 quartz have in disturbing the luminiferous vibrations. 

 Suppose we have shells two and three also connected by 

 a sufficient number of spiral springs, and so on ; and let 

 there be a solid inclosed in the centre with spring con- 

 nections between it and the shell outside of it. If there 

 is only one of these interior shells, you will have one 

 definite period of vibration. Suppose you take away 

 everything except that one interior shell ; displace that 

 shell and let it vibrate. The period of its vibration is 

 perfectly definite. If you have an immense number of 

 such shells with moveable molecules inside of them, dis- 

 tributed through some portion of the luminiferous ether, 

 you will put it into a condition in which the velocity of 

 propagation of the wave will be different from what it is 

 in the homogeneous luminiferous ether. You have what 

 is called for, viz. a definite period ; and the relation 

 between the period of vibration in the light considered 

 and the period of the free vibration of the shell will be 

 fundamental in respect to the attempt of a mechanism of 

 that kind to represent the phenomena of dispersion. 



" If you take away everything except the one shell, you 

 will have almost exactly, I think, the view of Helmholtz's 

 paper — a crude model as it were of what Helmholtz 

 makes his paper on anomalous dispersion. Helmholtz, 

 besides that, supposes a certain degree or coefficient of 

 viscous resistance against the vibration of the inner shell, 

 relatively to the outer one. Helmholtz does not reduce it 

 to a gross mechanical form like this, but merely assumes 

 particles connected with the luminiferous ether and as- 

 sumes a viscous motion to operate against the motion of 

 the particles." 



In the lectures the action of such a molecule when 

 subjected to forced vibrations was illustrated by a model 

 of ingenious construction, which among the irreverent 

 passed by the name of the " wiggler." A steel wire was 

 hung vertically, and five or six lathes 2 feet long and 

 2 inches wide were attached in a horizontal position to 

 the wire, each one having three pins fixed in it for this 

 purpose. These lathes were loaded at their ends, the 

 weight on each lathe being less than that on the one 

 above it. The lowest lathe was attached to a pendulum 

 arrangement which impressed forced vibrations upon the 

 system, the period being adjustable. The theory of such 

 a system is the same as that of the molecule described 

 above. 



But in working out the theory a third type of vibrator 

 was used, the identical one which vibrates in the lecture 

 room at Glasgow. This is a series of weights attached to 

 each other by vertical springs which can be stretched. 

 The highest is the heaviest, and the others are arranged 

 in the order of weight. 



