The World of Comets

tors inhabited Arcadia before the moon existed. But such a belief furnishes a very poor argument. The arguments, or rather the calculations, by which Dionys du Séjour has annihilated the hypothesis of Maupertuis are more difficult to refute. I will briefly state them as given by Pingré : ‘Subjected to the test of analytical reasoning,' he observes, 'the whole theory falls to the ground. Dionys du Séjour has proved :— 1st. That it is absolutely impossible for a comet moving in a trajectory either parabolic or hyperbolic to become a satellite of the earth; 2nd. That for a comet whose orbit is elliptic to become a satellite of the earth it would be necessary that when it entered into the sphere of the earth's attraction, its relative motion, that is to say, the difference between its velocity and that of the earth, should be only 2,176 feet per second. But is it possible that a comet whose orbit, although elliptic, approaches, nevertheless, very nearly to a parabola should have a velocity relative to the earth of only 2,176 feet per second, whilst it is demonstrated that the relative velocity of a parabolic comet placed at the same distance, that is to say, at the distance of the earth from the sun, must, under the most unfavourable circumstances, be 39,000 feet per second? Besides,' adds Pingré, 'even if this were so, the comet, when transformed into our moon, would pass in each of its revolutions to the extremity of the sphere of the earth's attraction, and the least force would suffice to detach it from us. . . It would then recommence its orbit about the sun.'

These reasons are derived from the laws of cometary and planetary motion, and from the principle of gravitation, of which the laws are the expression; but it is also evident that in physical constitution nothing can be more unlike a comet than the moon. Everything leads us to believe that our satellite is entirely, at its surface at least, reduced to a solid condition. If it has an atmosphere it is the least vaporous possible

and is of extremely slight density. Now, all known comets, those at least which have been subjected to telescopic scrutiny, appear to have been characterised by a predominance of nebulous atmosphere about the nucleus. The savants of the eighteenth century, who regarded comets as planetary globes, nevertheless recognised the entire absence of analogy between the physical constitution of the moon and that of a comet. Maupertuis, in order to explain how our satellite, an ancient comet disguised, might have lost its coma and tail, had only to invent a new hypothesis, to the effect that some other comet might in its passage have swept away the atmosphere of the moon. If I remember rightly, it is not Maupertuis who makes this new supposition, but some other author whose name has escaped me.