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sun and solar system through space while tliey were still outside that system.

The view which now seems established by a study of the whole subject is that these objects have their regular orbits, differing from those of the planets in their great eccentricities. Their periods of revolution are generally thousands, and sometimes tens of thousands, and even hundreds of thousands of years. During this long interval they fly out to an enormous distance beyond the confines of the system. If, as they return to the sun, they chance to pass very near a planet, two things may happen: Either the comet may be given an additional swing that will accelerate its speed, throw it out to a greater distance than it ever had before or possibly to a distance from which it can never return, or the speed may be retarded and the comet made to move in a smaller orbit. Thus it is that we have comets of so many different periods. If comets come from the regions of the fixed stars, there is no reason why the motion of one might not be directly toward the sun, so that it would fall into our central luminary. But such an occurrence is hardly possible when the comet belongs to our system, because one of these bodies nearing an orbit passing through the sun would have fallen into the sun on its first round, long ages ago, and never could have a chance to fall in again.

The very bright comets which appear from time to time are of the greatest interest to every beholder. It is purely a matter of chance, so far as our knowledge ex-