Page:The Scientific Monthly vol. 3.djvu/551

 WHAT WE KNOW ABOUT COMETS 545

comef s nucleus is thought to be a collection or cluster of small bodies^ such as have been observed to collide with our atmosphere and to pro- duce the meteor showers. They are held together, so to speak, while they are far away from the sun, because of their own very small but sufficient attraction for each other; but when they come within our planetary system, and especially when they come relatively close to the great planets Jupiter and Saturn, the sun and the planets attract the nearer particles of the comets more strongly than they do the farther particles. The nearer particles forge ahead on smaller orbits, the farther particles lag behind on larger orbits, and in the course of cen- turies the cometary material is strewn along a great stretch of the orbit. Other separative forces — of magnetic or electric natures, for example — ^may develop amongst the particles composing the nucleus as a comet approaches the sun. The intensity of the reflected light in all parts of the scattered comet structure becomes too small to let us see the remains of the comet, except as the remnants collide with the earth's atmosphere. There is certainly no reason to doubt that a very great many of our shooting stars are the remains of disintegrated comets. Tens of millions of little meteors enter our atmosphere every twenty- four hours and with rare exceptions are consumed by the heat of fric- tion with the atmosphere when they rush through it at tremendous speeds. The gases from the combustion enter the atmosphere, and the ash and other unconsumed parts fall down to the earth's surface in due time. Accumulated meteoric dust is found in the perpetual snows at the tops of high mountains, and Sir John Murray found it in the ooze brought up from the depths of the oceans. Whether the meteorites which penetrate our atmosphere and are found and placed in our mu- seums are parts of ancient comets can not safely be asserted, but it seems entirely possible that some of them are. However, it is not cer- tain that any meteorite found on the earth has come from a meteor stream of recognized cometary origin. It is pretty well established that many of the sporadic meteors which plunge into our atmosphere were traveling on hyperbolic orbits.

We discover only a certain proportion of the comets which come close to the sun and to the earth. The numbers which course through the planetary system and remain undiscovered by the observers on the earth must be exceedingly great. The supply of cometary material in the remote outskirts of the planetary system must be enormous. This material is probably in the nature of remnants of the nebula or other mass of matter from which the sun, its planets and their moons devel- oped. This idea is to a certain extent speculative; but that the come- tary material is now out there in abundance we can not doubt. Much of it naturally consists of matter in the solid state; and, the sun's attraction at that great distance being almost zero, neighboring masses

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