290 



• KNOWLEDGE ♦ 



I Nov. 9, 1883. 



starry deeps to comfort me, threw upon me a look of 

 gentlest pity and of unutterable love, so that in my 

 heart I had a sudden rapture of joy such as passes 

 all understanding, and I awoke in the tumult of my 

 happiness. 



I awoke : but my happiness survived my dream ; and I 

 exclaimed : Oh ! how beautiful is death, seeing that we 

 die in a world of life and of creation without end ! and I 

 blessed God for my life upon earth, but much more for the 

 life in those unseen depths of the universe which are 

 emptied of all but the Supreme Eoality, and where no 

 earthly life nor perishable hope can enter. 



THE EING OF SMALL PLANETS. 



By Richakd A. Proctor. 



OLBERS'S theory or suggestion that the four small 

 planets Ceres, Pallas, Juno, and Vesta were the 

 fragments of a single planet which had burst, was not hope- 

 lessly untenable ; for the mean distances of these four 

 planets are nearly equal, being 2-769, 2-771, 2-G6S, and 

 2-361 respectively, the earth's distance being unity. Indeed 

 when Olbers first suggested the theory, only two of the small 

 planets had been discovered, Ceres and Pallas, and these 

 travel at almost the same mean distance from the sun. In 

 the Connaissance des Temps for 1814, Laplace showed that 

 with a velocity of explosion not exceeding twenty times 

 that of a cannon-ball, say twelve times that of a ball 

 fired from one of our best modern cannon, the observed 

 range in the mean distances of these bodies might 

 have been obtained. It is, however, to be noticed that 

 even for this the Pallas fragment was assumed to have been 

 expelled directly forwards aud the Vesta fragment directly 

 backwards (with reference to the course of the planet at 

 the moment of the catastrophe, in order that the whole 

 effect of the change of velocity might fall on the period, 

 lengthening to the utmost that of Pallas and shortening to 

 the utmost that of Vesta. If the picture illustrating this 

 article is examined, it will be seen that the range in the 

 known mean distances of the asteroids has enormously 

 increased since then, and that even in this aspect alone the 

 explosion theory is no longer so hopeful as when Laplace 

 held it to be barely tenable. 



But another objection, tirst dealt with by Encke, has 

 always been held more serious, and has now become to all 

 intents overwhelming. It was seen by Olbers and his con- 

 temporaries that whenever such an explosion occurred the 

 new orbits of the scattered fragments must pass through 

 the point where the explosion took place. They found 

 no such point common to the orbits of the four first- 

 discovered asteroids. Professor Newcomb says it is curious 

 that they expected to find a region of intersection, as it 

 implied (meaning that their intersection implied) that the 

 explosion had occurred within a few thousand years. It 

 would have been curious, however, if in Olbers's time 

 men had expected anything else. However, whatever theii- 

 expectations may have been, it is certain they found 

 no such region of intersection. Encke showed even 

 that, judging from the present variations of the orbits of 

 the asteroids there was less approach towards a region of 

 common intersection in past ages than there is at present. 

 But a more careful investigation of certain of the asteroidal 

 orbits has shown that, apart from the attraction of the small 

 planets on each other, the orbits examined never could 

 have intersected. And as the attractions of these planets 

 on each other are exceedingly small it is practically impos- 



sible that the divergence of the system from a state of 

 things which must have existed if ever a planet burst in 

 the mid-region between Jupiter and Saturn, can te ex- 

 plained in this way. 



Hence, as Professor Newcomb rather abruptly puts it,. 

 " Olbers's hypothesis is no longer considered by astro- 

 nomers." 



The physical objection to the theory lies in the circum- 

 stance that though in a great volcanic outburst the earth's 

 internal forces suffice to eject matter with velocities com- 

 parable with those required by the hypothesis, we have in 

 these cases the energies of enormous subterranean regions 

 exerted on relatively minute bodies. To conceive that the 

 energies even of the whole earth, if all brought into action 

 at once, could suffice to propel the whole mass of the 

 earth — in fragments — from around the centre of explosion^ 

 with velocities enormously exceeding those ever observed 

 in any fragment ever shot out from a volcanic crater, is a 

 very difterent matter indeed. A certain charge of gun- 

 powder will drive a cannon-ball to a distance of two or three 

 miles, but a thousand times that charge would not scatter 

 the fragments of the cannon (if the ball had been tightly 

 fixed in) over a similar distance all around the place of 

 explosion. Nothing known about our earth's interior, 

 nothing which we can infer about the interior of any other 

 planet formed by processes such as we recognise in the 

 development of the solar system as at present understood, 

 suggests the possibility that a millionth part of the force 

 necessary to shatter a planet, as Olbers's theory requires, caii 

 ever be generated and accumulated within the planet's, 

 interior. 



But while the nebular theory gives a sufficient explana- 

 tion of the existence of a zone of small planets instead of 

 a single planet, this theory associated with the theory of 

 meteoric aggregation which recent discoveries have esta- 

 blished, gives also an account of the position of the zone of 

 asteroids in the solar system and also of certain peculiarities 

 which it presents under careful scrutiny. 



On the former points I shall make some remarks next 

 week, endeavouring to show that the zone of asteroids- 

 belongs to the region separating the sun's special family of 

 small planets, from the region where orbs which not so very 

 long ago were minor suns travelled with their attendant 

 families of worlds. 



For the present I note only the significance of that 

 strange feature which Professor Daniel Kirkwood of 

 Bloomington, Ind.,was the first to indicate, — the gaps in the 

 asteroidal system at those mean distances for which the 

 attraction of Jupiter would have been most eft'ective — in 

 past ages — in disturbing the movements of meteoric or 

 nebulous matter. It is well known that when two bodies 

 travel around a centre in commensurable periods, the per- 

 turbations w-hich with incommensurable periods would be 

 ineffective in long periods of time, become very effective, 

 especially if the periods are so related that a few 

 circuits renew the disturbing influences. We see 

 this in the case of Saturn and Jupiter, where two 

 revolutions of the outer planet are accomplished in nearly 

 the same time as five revolutions of the inner ; hence- 

 arising the Great Inequality or Irregularity. The smaller 

 body is the one always which is most disturbed. A vaporous 

 or meteoric mass moving in a period similarly synchronising 

 with Jupiter's would be very much more disturbed than 

 a larger one, for precisely the same reason that (for in- 

 stance) the moon's departures from the common centre of 

 gravity of herself and the earth are much greater than 

 the earth's. Such a mass would be driven to one side or 

 the other of its original track until it would probably come 

 into collision with one moving in a longer or a shorter 



