494 



NATURE 



{March 27, 1879 



If meleorites have been projected fi om volcanoes, on what body or 

 bodies in the universe must those volcanoes have been located ? 



Let us first take up a few of the principal celestial bodies 

 jmrt/m and consider their claims totheparentageof the meteorites. 

 We begin with the sun. It has been abundantly shown that 

 there exists upon the sun tremendous explosive energy. It is not 

 at all unlikely that that energy would be sufficiently great under 

 certain circumstances actually to drive a body from the sun never 

 to return. We might therefore find upon the sun adequate explo- 

 sive power for the volcano, but the projectiles are here the 

 difficulty. There are a number of circumstances (notably the 

 breccia-like appearance of some meteorites) which show con- 

 clusively that the meteorites have beeen torn from rocks which 

 were already nearly, if not quite, solid, and as it seems in the 

 highest degree improbable that rocks of this nature should exist 

 in the sun, we may conclude that the sun has not been the source 

 of the meteorites. 



Can the meteorites have come from the moon? Owing to the 

 small mass of the moon the explosive energy required to carry a 

 body away from the moon is comparatively small. Can such 

 a body fall upon the earth ? 71? simplify queUions of this kind 

 •we shall suppose various disturbing influences absent. We shall 

 suppose that the projectile is discharged from a volcano on the 

 moon with sufficient velocity to carry it therefrom. We shall then 

 omit all account of the disturbing influence both of the sun and 

 moon on the projectile, and we shall suppose that the projectile 

 is really revolving round the earth as a satellite. This pro- 

 jectile will fall upon the earth if its distance from the earth's 

 centre whtn in perigee be less than the radius of the earth 

 (augmented, perhaps, by the thickness of the earth's atmosphere), 

 it should how ever be observed that if the projectile otue escaped 

 the earth it would never fall thereon, hence the question as to 

 whether the moon can be the source of the meteorites now falling 

 appears to be connected with the question as to whether the 

 lunar volcanoes are now active. But it is generally believed that 

 the lunar volcanoes are not now active to any appreciable 

 extent (even if the suspected indications of recent change 

 were thoroughly established). It follows that even if the 

 moon has been a source of meteorites in ancient times, 

 we no longer receive a supply from that quarter. There is of 

 course just a possibility that projectiles from the moon which 

 have been revolving round the earth as satellites in elliptic orbits 

 ever since their ejection may, under the infiuence oi the disturbing- 

 causes previously excepted, gradually change their orbits until they 

 become entangled in the atmosphere and descend as meteorites. 

 It therefore appears to be not quite impossible that even still a 

 meteorite which had its origin in the moon in past ages may 

 occasionally tumble on the earth. 



Passing from the sun and the moon let us now bring under review 

 some of the other celestial bodies and see how far they will fulfil 

 the conditions of the question. Is it possible that the meteorites 

 can have been projected from the surface of a planet ? In order 

 to get over the difficulties of the great initial velocity which would 

 be necessary to overcome the gravitation of a large planet, it 

 seems natural to inquire if a volcano placed upon one of the 

 small planets could accomplish the task. 



It is clearly impossible that a projectile should ever fall on the 

 earth unless the orbit of the projectile cuts the plane of the 

 ecliptic in a point which lies in the narrow ring between 8,000 

 and 9,000 miles wide which the earth traces out on the ecliptic, 

 but if a meteorite with an elliptic orbit intersect this ring, then, in 

 the lapse of time, it may happen that the earth and the meteorite 

 meet at the intersection of their orbits, in which case of course 

 the long travels of the meteorite will come to an end. 



We shall therefore consider the circumstances under which it 

 would be possible to discharge a projectile from the surface of a 

 planet (say Ceres), so that the projectile shall intersect the ecliptic 

 m the ring we have just referred to. The planet being small the 

 initial velocity that would be required to carry a projectile from 

 its surface presents no difficulty ; perhaps an ordinary cannon 

 would be sufficient so far as the mere gravitation to the planet is 

 concerned. But when we consider the necessity that the projectile 

 must be driven through the ring we have been considering, a 

 vastly more powerful instrument would be required. 



Ceres is moving in an orbit (supposed circular and in the ecliptic) 

 with a velocity of about eleven miles per second. A projectile 

 discharged from Ceres will have an actual velocity which is com- 

 pounded of the velocity of Ceres, with the velocity which is im- 

 parted by the volcano. But simple dynamical considerations show 

 that if the projectile have an initial velocity perpendicular to the 



radius vector, differing from about eight miles per second, it caa 

 never intersect the nng, no matter in what direction it be dis- 

 charged.^ The volcano on Ceres must therefore be adeqjate to 

 the abatement of the velocity perpendicular to the radius vector 

 from eleven miles per second to eight miles per second, i.e.y 

 the volcano must be at the very least adequate to producing an 

 initial velocity of three miles per second. As this is quite inde- 

 pendent of the additional volcanic power requisite to carry the 

 projectile away from the attraction of Ceres, it is obvious that 

 after all there may be but little difference between the volcano 

 which would be required on Ceres, and that (of six mile power) 

 which would project a body away from the surface of the earth 

 for ever. 



Admitting, however, that a volcano of sufficient power were 

 placed upon Ceres, would it be likely that a projectile driven 

 therefrom would ever cross the earth's track ? This is a question 

 in the theory of probabilities, and it is not easy to state the 

 problem very definitely. If the total velocity with which the 

 projectile leaves the orbit of Ceres be less than eight miles 

 per second, then the projectile will fall short of the earth's 

 track ; on the other hand, if the total initial velocity exceeds 

 sixteen miles per second, the orbit in which the projectile moves 

 will be hyperbolic, and though it may cross the earth's track 

 once, it will never do so again. Taking a mean between these 

 extre^ne velocities we may investigate the following problem : — 

 Suppose that a projectile is discharged from a point in the orbit 

 of Ceres in a random direction with the total initial velocity of 

 twelve miles per second, determine the probability that the orbit 

 of the projectile shall cross the earth's track. When this 

 problem is solved in accordance with the calculus of probabili- 

 ties it is found that the chances against the occurrence are about 

 50,000 to I, i.e., out of every 50,000 projectiles discharged at 

 random from a point in the orbit of Ceres, only a single one can 

 be expected to cross the earth's track. 



It is thus evident that there are two objections to Ceres (and 

 the same may be said of the other minor planets) as a possible 

 source of the meteorites. Firstly, that notwithstanding the small 

 mass of Ceres, a very powerful volcano would be required j 

 and secondly, that we are obliged to assume that for each 

 meteorite which could ever fall upon the earth, at least 50,000 

 must have been ejected. 



It thus appears that if the meteorites have been originally 

 driven from any planet of the solar system, large or small, the 

 volcano must from one cause or another be a very powerful 

 one. 



There i?, however, one planet of the solar system which has a 

 special claim to consideration. On that planet it is true that a 

 volcano would be required which was capable of giving an 

 initial velocity of at least six miles per second ; but every projec- 

 tile launched from that volcano into space would, after ac- 

 complishing an elliptic orbit round the sun, dash through the 

 track of the earth, and again pass through the same point at 

 every subsequent revolution. It is not here a case of one solitary 

 projectile out of 50,000 crossing the earth's track, but every one 

 of the 50,000 possesses the same property. The planet of which 

 we are speaking is, of course, the earth itself. If in ancient times 

 there were colossal volcanoes on the surface of the earth which 

 had sufficient explosive energy to drive missiles upwards with a 

 velocity sufficient to carry them away from the earth's surface, 

 after making allowance for the resistance of the air, these 

 missiles would then continue to move in orbits rotmd the 

 sun, crossing at each revolution the point of .the earth's track 

 from which they were originally discharged. If this were the 

 case, then doubtless there are now myriads of these projectiles 

 moving through the solar system, the only common feature of 

 their orbits being that they all intersect the earth's track. It 

 will, of course, now and then happen that the earth and the 

 projectile meet at the point of crossing, and then we have the 

 phenomenon of the descent of a meteorite. This theory, that the 

 meteorites have originated in the earth, was so far as I know first 

 put forward by Dr. Phipson. Mr. J. Lawrence Smith in a letter 

 I received from him some months ago inclines to the same view 

 as at all events one of the probable sources. 



It is well to note here the great difference between the lunar 

 theory of meteorites and the terrestrial theory. For the lunar 

 theory to be true it would probably be necessary that the lunar 

 volcanoes should be still active. In the terrestrial theory it is 

 only necessary to suppose that the volcanoes on the earth once 



^ Disregardinj; an obvious^xcepticn. 



