136 



Near f Cygni... 

 ,, )3 Piscium 



NATURE 



\yune 6, 1889 



Meteor- Showers, 

 R.A. Decl. 



250 

 318 

 345 



20 S. 

 30 N. 

 o 



Very swift. 



NOTES ON METEORITES^ 



IX. 



Did these Swarms or Comets always belong to the 

 System ? 



IWrUST we assume that the members of the swarms to which 

 we have referred and of all the other swarms similar to 

 it have always been thus crossing the earth's orbit periodically ; 

 that the November swarm, to take an instance, has always been 

 crossing it every thirty-three years ? Must they of necessity 

 have started their existence with the planets and other more 

 stable members of the system ? 



This point has been well inquired into, and it is certain that 

 it is not at all necessary that such a state of things should have 

 existed from all time. 



It is a matter of common knowledge that all stars are in 

 motion. The so-called " fixed " stars are not really fixed : they 

 are only relatively fixed. The sun is a star, and therefore like 

 the other stars it is also in movement with its attendant bodies 

 in space. 



If we have a swarm of meteorites moving in space, as 

 the sun is doing, at a very considerable distance from the 

 sun, the directions of movement being not parallel but inclined to 

 each other, a time will come when the two bodies, taking the 

 swarm as representing one body, and the sun another, will begin 

 to have an attractive influence on each other. If the attractive 

 energy of the sun is considerable as compared with that of the 

 swarm, the swarm will begin to change its direction obviously 

 towards the sun. If, in changing its direction towards the sun 

 and increasing its velocity in consequence of this increased 

 gravitational stress, that swarm can get round the sun without 

 any loss of momentum the two bodies will say good-bye to each 

 other and will go different ways ; but supposing there has been 

 a loss of momentum the loss may mean that for the future the 

 swarm of meteorites must perform its journey round the sun. 



It does not therefore follow that when a particular group of 

 meteorites has been watched for 900 years that these meteorites 

 which give rise to the appearance of shooting stars always formed 

 part of ihe solar system. What we do know is that at the 

 presetit j?io!nent this particular swarm to which the November 

 meteors are due and another swarm which is called the Biela 

 swarm, to mention two instances, do really move round the sun 

 in closed cometary orbits, and the chronicle of the appearances 

 of both these swarms is so complete' that very definite statements 

 may be made about them. 



With regard to the November swarm it is known that a 

 thousand millions of miles of its orbit have been pierced by the 

 earth in its successive passages through it since the year 902, 

 each time the earth must have filched many millions of the 

 small constituents of the swarm and used them up as shooting 

 stars, and yet the swarm does not seem to be very much the 

 worse, and enormous though the numbers are, it is known that 

 the distances between the meteorites is so considerable that no 

 obvious mutual gravitational effect can be noted, so that their 

 combined or common movement is a clear indication of a 

 common origin. 



In the case of the orbit of the Biela swarm we know that 

 more than half of it, or a length of 500 million miles, contains 

 these meteorites ; a long thin line, say a mile long and an inch in 

 section, represents, according to Prof. Newton, the distribution 

 of the meteorites along the orbit. 



The great Laplace was the first to suggest that many comets, 

 especially those of high inclination and great eccentricities, 

 represented introductions of matter into the solar system from 

 external space. But on this, as on many other points, we owe 

 our present views chiefly to Schiaparelli, who, in 1867, attacked 



' Continued from vol. xx.xix. p. 402. 



the problem ^ in connection with his researches on the November 

 swarm. 



He commenced by referring to the point made by Laplace as 

 to the phenomena presented by cometary orbits, suggesting that 

 the planets are truly indigenous to the system ; have always 

 followed the sun in his movement through space ; and had taken 

 part in all the evolutionary changes which have finally brought 

 the solar system to its present condition. In these characters 

 common to planets the comets are lacking, while the eccen- 

 tricities of their orbits generally is so great that the greater part 

 of their journey is performed outside the known limits of our 

 system. Schiaparelli considers that these facts demonstrate that 

 the comets were not members of the solar system during its 

 early stages, but that they are really messengers from the stellar 

 void. Cloudlike masses wandering in parts of space where 

 there was no star sufficient to dominate them have fallen gradually 

 under the empire of our own by the effect of their movement rela- 

 tively to our system. This movement, combined with the accelera- 

 tion produced by the large mass of the sun has determined the 

 relative orbits of these bodies in relation to the sun, which is very 

 different from their absolute orbit in space. He next examined all 

 the circum.stances of the movement of the external mass under 

 these conditions. First, there is no doubt that the movement 

 of the solar system in space is comparable to that of the 

 planets in their respective orbits, while it is possible — indeed 

 certain — that many of the stars are in more rapid movement 

 than our sun. Hence when it is affirmed that the relative 

 movement of the sun and of other bodies disseminated through 

 space is comparable in rapidity to the orbital movement of the 

 planets, the statement is not a surprising one. 



That being so, let us next supp:)se that one of these 

 cloudlike masses — let us call them external swarms — wander- 

 ing in space in consequence of its initial movement, penetrates 

 eventually into a region where the attraction of the sun is 

 much greater than that of any other star. It might be situated 

 at a very great distance from the sun, where the annual 

 parallax is only a small number of seconds. The relative move- 

 ment will take place in a conic section. To define it, let us 

 suppose the sun stopped, and let us give to the comet, instead 

 of its real velocity in space, its relative velocity to the sun ; and 

 let us further imagine a perpendicular dropped from ihe sun in 

 the direction of this relative velocity. It is evident that the area 

 described by the comet round the sun in unit time will be 

 equal to the half of the product of this perpendicular by its 

 relative velocity. 



Now as in general this velocity is of the order of planetary 

 velocities, and since most frequently the perpendicular in questioa 

 will be very much greater than the distance of the planet from the 

 sun, we must conclude that the areas described by the comet round 

 the sun in unit time will be incomparably greater than the- 

 corresponding areas described by the planets. But when many- 

 bodies move in conic sections round a central body, the areas 

 described in unit time are, among themselves, as the square 

 roots of the parameters of their respective orbits ; therefore, the 

 ratio of the parameters of cometary orbits to those of planetary 

 orbits will be much greater than the ratio of the areas described 

 by comets to the areas described by planets in unit time. 

 Whence it follows that, in general, cometary orbits will have 

 enormous dimensions in every direction, and that bodies which 

 describe them will remain perpetually invisible to us, in con- 

 sequence of their enormous distance. Nevertheless, among the 

 infinite combinations possible in cometary orbits, there are two 

 which may bring the cometary cloud within our ken : one, when 

 the comet is moving directly towards the sun, describing a 

 hyperbolic orbit very little different from a right line ; and the 

 other, when the relative movement of the comet and sun is 

 almost zero, that is, when the two bodies are moving through 

 the stellar space along parallel lines with nearly equal velocities. 



Schiaparelli then goes on to show that when these cosmic 

 clouds are attracted by the centre of our system, the con- 

 stituent particles of the cloud must be drawn out into a para- 

 bolic current ; thus, for instance, supposing a cosmic cloud equal' 

 in volume to the sun and at such a distance that its apparent 

 diameter is i', the sun's attraction upon this would result in the 

 formation of a parabolic chain or stream of such a length that 

 it would require 636 years to pass through perihelion. When 

 the centre was close to the sun, the beginning and the end of it 

 would be distant from it 263 times the earth's distance from the- 



Les MondeSy vol. xiii. p. 147. 



J 



