Feb., 1905.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



yet the doubt remains whether the cause alleged was 

 adequate to the effect produced. Afthe distance of 

 Saturn, solar tidal friction exerts only about one- 

 twenty-thousandth its power on the earth '; its efficacy 

 would, it is true, be greatly enhanced by the distension 

 of the mass subjected to it; but approximately to what 

 extent, it baffles our powers of calculation to determine. 



The one certain inference derivable from the diver- 

 sity of facts ascertained within the last hundred years 

 is that our world is not (so to speak) machine-made. 

 The modus operandi employed to disengage the planets 

 from their nebulous matrix was not of cast-iron 

 rigidity; it was adaptable to circumstances; it left room 

 for the display of boundless inventiveness in details. 

 This, nevertheless, was made to consist with the per- 

 fect preservation of the main order, both in design 

 and operation. Tlie general plan is broadly laid down 

 and unmistakable, and the springs of the machine are 

 undisturbed in their free play. And for the primary 

 reason that departures from regularity, which might 

 in any way, prove a menace to stability, affect bodies 

 of negligible mass. Tlie great swing of settled move- 

 ment goes on irrespectively of them. De minimis ncn 

 curat lex. Thus, the erratic behaviour of comets is 

 harmless only because of their insignificance. If pur- 

 sued by substantially attractive masses, it could not 

 fail to jeopardise the planetary adjustments. Even the 

 asteroids would be unsafe neighbours but for their 

 impotence; and it is remarkable that Mercury, by far 

 the smallest of the major planets, circulates along a 

 track of the asteroidal type. It would seem as if an 

 important size carried with it an obligation to revolve 

 in an orbit of small eccentricity, inclined at a low angle 

 to the principal plane of the system. The reason why 

 this should be so is not obvious; but were it otherwise, 

 the equilibrium, now so firmly established, would sub- 

 sist precariously, or not at all. 



The assertion, indeed, that it is firmly established, 

 can only be made under reserve. We are ignorant of 

 any causes tending towards its overthrow; yet they 

 may supervene, or be already subtly active. One such 

 lurking possibility is the presence of a resisting medium 

 in interplanetary space, ^^■aifs and strays of matter 

 must, at any rate, be encountered there — outlawed 

 molecules, self-expelled from the gaseous envelopes of 

 feeble globes; thin remnants of cometary paraphernalia, 

 driven off amid the fugitive splendours of perihelion; 

 products of ionic dissociation set flying by the impact 

 of ultra-violet light — and all disseminated through an 

 ethereal ocean, which " is cut away before, and closes 

 from behind," as moving bodies traverse it. That its 

 indifference is shared by ordinarv material substances, 

 when in the last stage of attenuation, is a plausible 

 but unverified conjecture. It is only safe to say that 

 retardation of velocity in what may pass for empty 

 space is insensible, or null. 



There may, nevertheless, be springs of decadence in 

 the solar system. Some of them have been discussed 

 by M. Poincare,f whose confidence in the reassuring 

 demonstrations of Laplace and Lagrange is inversely 

 proportional to the magnitude of the terms they were 

 forced to neglect. They dealt with fictitious globes, 

 devoid of appreciable dimensions, and swayed by the 

 strict Newtonian law. But the real planets and their 

 satellites are acted on by other forces as well, frictional, 

 magnetic, radio-repulsive; and their joint effects may 

 not be wholly evanescent. The tidal drag on rotation 



• G. H. Darwin, Phil. Trans. Vol. CLXXII., p. 526: Moulton, 

 Astrooh. Jour., Vol. XI.. p. no. 

 t Annuaire du Bureau des Longitudes, 1898 



undoubtedly occasions a small but irretrievable loss 

 of energy. The moon, for instance, as M. Poincare 

 states, now gains, by the reactive consequences of tidal 

 friction in widening its orbit, no more than .^ the vii 

 viva of which the earth is deprived by the infinitesimal 

 slowing down of its rotation. And the remaining "ths, 

 being dissipated abroad as heat, are finally abstracted 

 from the system. The ultimate state, we arc told, to- 

 wards which tlie planetary mechanism tends, is that 

 of the synchronous revolution, in a period of about 

 twelve years, of all its members. This might, apart 

 fiom a possibly resisting medium, have indefinite 

 permanence; otherwise precipitation to the centre would 

 gradually ensue, and one solitar}- sphere, cold, stark, 

 and unilluminated, would replace the radiant orb of 

 our cerulean skies, with its diversified and exquisitely 

 poised cortege. Unsecured drafts upon futurity, how- 

 ever, are not among the most valuable assets of science; 

 and a consummation so incalculably remote may be 

 anticipated by a score of unforeseen contingencies. 

 What can be, and has been ascertained, is the relative 

 durability of the scheme with which the visible destinies 

 of the human race are so closely connected. It will, 

 beyond question, last long enough for their accom- 

 plishment. Curiosity that would seek to penetrate 

 further is likely to remain ungratified. 



But this is not all. There are other, and incalculable 

 items in the account. The sun, although an autocrat 

 within his own dominion, is himself subject to ex- 

 ternal influences. As a star, he is compelled to follow 

 whithersoever the combined attractions of his fellow- 

 stars draw him; nor can we thoroughly interpret the 

 summons which he obeys. The immediate outcome in 

 the transport of the solar system towards the constel- 

 lation Lyra, has, it is true, been determined; but the 

 eventual scope and purpose of the journey remain pro- 

 foundly obscure. The pace is to be reckoned as leis- 

 urely; twelve miles a second is little more than half the 

 average stellar speed. We should, however, probably 

 suffer no inconvenience from being whirled through 

 the ether in the train of such a stellar thunderbolt as 

 .Arcturus. Only the excessive velocities of any ad- 

 ventitious bodies we might happen to pick up would 

 betray to ordinary experience the fact of our own swift 

 progress. As it is, our sweepings from space appear 

 to be scanty. If shreds from inchoate worlds, or dust 

 of crumbled worlds, strewed the path of our system, 

 they should be annexed by it in its passage, tem- 

 porarily or completely; and we should then expect to 

 find the apex of the sun's way marked, if no otherwise, 

 by the predominant inflow from that quarter of comets 

 and meteors. Yet there is no trace of such a prefer- 

 ence in the distribution of their orbits. Hence the en- 

 forced conclusion that the sun has attached to him, 

 besides the members of his immediate household, an 

 indefinite crowd of distant retainers, which, by their 

 attendance upon his march, claim with him original 

 corporate unity. To this rule there may be a few 

 exceptions. An occasional aerolite probably enters the 

 earth's atmosphere with hyperbolic velocity, and takes 

 rank accordingly as, in the strictest -sense, a foreign 

 intruder; but the broad truth can scarcely be challenged 

 that the sun travels through a virtual void. 



We can, however, face no necessity why he should 

 for ever continue to do so. Widely different conditions 

 seem to prevail near the centre, and out towards the 

 circumference of the sidereal world. What may be 

 designated the interior vacuity of the Milky Way is 

 occupied mainly by stars of the solar type, including 



