N(W. 22, 1888] 



NATURE 



8r 



connected together, the direction of the current depending on the 

 direction of rotation. 



It will be noticed that, since the coil and wheels are always in 

 contact, no undulations are produced as when brushes coine in 

 contact with successive ends of coils, as in the ordinary dynamo. 



When the instrument is placed in circuit with a sensitive galvano- 

 meter, the rotation being constant, no variation in the current can 

 be detected, even when the motion is very slow. The coil 

 when arranged with one wheel at A, and a mercury contact at B, 

 will revolve when a current is sent through it, becoming in this case 

 a motor. If an iron chain, or an elastic band of iron such as a 

 measuring tape, be placed inside the ring coil, it then becomes a 



I 



distorted Gramme ring, the wheels taking the place of the brushes, \ 

 the way in which the current is produced being the same. If coils 

 approaching N produce a current upwards, then those which are j 

 leaving N produce one downwards. The same takes place on the ! 

 other side ; coils leaving the s pole produce a current upwards, 

 while those which approach it produce a current downwards ; both 

 of the ascending currents, being in the same direction, go to the 

 wheel A, while both of the. descending currents, being in an ' 

 opposite direction, go to the wheel B. 



The first coil made was of copper wire. Phosphor bronze wire 

 answers better, being less easily distorted. 



Frederick J. Smith. 



Trinity College, Oxford, November 12. 



The Use of Rotifera. 

 Can any of your readers inform me whether Rotifers are of 

 any use in removing decaying organic matter, as Infusoria do ? 



C. L. 



ON THE MECHANICAL CONDITIONS OF A 

 SWARM OF METEORITES^ 



I. 



■jy/TR. LOCKYER writes in his interesting paper on 

 ■''*-*• meteorites- as follows : — 



" The brighter lines in spiral nebuhe, and in those in 

 which a rotation has been set up, are in all probability due 

 to streams of meteorites with irregular motions out of the 

 main streams, in which the collisions would be almost 

 nil. It has already been suggested by Prof. G. Darwin 

 (Nature, vol. xxxi. p. 25)— using thegaseous hypothesis— 

 that in such nebulae ' the great mass of the gas is non- 



' Abstract of a Paper read before the Royal Society on November 15 by 

 ProtG. H. Darwin. F.R.S. 



' Nature, November 17, 1887. The paper itself is in the Rr y. Soc. Proc., 

 November 15, 1887 (No. 259, p. 117). 



luminous, the luminosity being an evidence of condensa- 

 tion along lines of low velocity according to a well-known 

 hydrodynamical law. From this point of view the visible 

 nebula may be regarded as a luminous diagram of its own 

 stream-lines.'" 



The whole of Mr. Lockyer's paper, and especially this 

 passage in it, leads me to make a suggestion for the re- 

 conciliation of two apparently divergent theories of the 

 origin of planetary systems. 



The nebular hypothesis depends essentially on the 

 idea that the primitive nebula is a rotating mass of fluid, 

 which at successive epochs becomes unstable from ex- 

 cess of rotation, and sheds a ring from the equatorial 

 region. 



The researches of Roche ^ (apparently but little known 

 in this country) have imparted to this theory a precision 

 which was wanting in Laplace's original exposition, and 

 have rendered the explanation of the origin of the planets 

 more perfect. 



But notwithstanding the high probability that some 

 theory of the kind is true, the acceptance of the nebular 

 hypothesis presents great difficulties. 



Sir William Thomson long ago expressed to me his 

 opinion that the most probable origin of the planets was 

 through a gradual accretion of meteoric matter, and 

 the researches of Mr. Lockyer afford actual evidence in 

 favour of the abundancy of meteorites in space. 



But the very essence of the nebular hypothesis is the 

 conception of fluid pressure, since without it the idea of a 

 figure of equilibrium becomes inapplicable. Now, at first 

 sight, the meteoric condition of matter seems absolutely 

 inconsistent with a fluid pressure exercised by one part of 

 the system on another. We thus seem driven either to 

 the absolute rejection of the nebular hypothesis, or to 

 deny that the meteoric condition was the immediate 

 antecedent of the sun and planets. M. Faye has taken 

 the former course, and accepts as a necessary consequence 

 the formulation of a succession of events quite different 

 from that of the nebular hypothesis. I cannot myself find 

 that his theory is an improvement on that of Laplace, 

 except in regard to the adoption of meteorites, for he 

 has lost the conception of the figure of equilibrium of a 

 rotating mass of fluid. 



The object of this paper is to point out that by a certain 

 interpretation of the meteoric theory we may obtain a 

 reconciliation of these two orders of ideas, and may hold 

 that the origin of stellar and planetary systems is meteoric, 

 whilst retaining the conception of fluid pressure. 



According to the kinetic theory of gases, fluid pressure 

 is the average result of the impacts of molecules. If we 

 imagine the molecules magnified until of the size of 

 meteorites, their impacts will still, on a coarser scale, give 

 a quasi-fluid pressure. I suggest, then, that the fluid 

 pressure essential to the nebular hypothesis is in fact the 

 resultant of countless impacts of meteorites. 



The problems of hydrodynamics could hardly be at- 

 tacked with success, if we were forced to start from the 

 beginning and to consider the cannonade of molecules. 

 But when once satisfied that the kinetic theory will give 

 us a gas, which, in a space containing some millions of 

 molecules, obeys all the laws of an ideal non-molecular 

 gas filling all space, we may put the molecules out of 

 sight and treat the gas as a plenum. 



In the same way the difficulty of tracing the impacts of 

 meteorites in detail is insuperable, but if we can find that 

 such impacts give rise to a quasi-fluid pressure on a large 

 scale, we may be able to trace out many results by treat- 

 ing an ideal plenum. Laplace's hypothesis implies such 

 a plenum, and it is here maintained that this plenum is 

 merely the idealization of the impacts of meteorites. 



As a bare suggestion, this view is worth but little, for its 

 acceptance or rejection must turn entirely on numerical 

 values, which can only be obtained by the consideration 



' Montpellier Acad. Set. Mim. 



