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THE ORIGIN AND NATURE OF COMETS
I do not think that the time has yet arrived when we can give a complete solution of the problems of the nature and origin of comets. Considerable progress has, however, been made in this direction during the last half-century, and it seems worth while to collect and put in order some considerations, which at all events limit the field within which the solutions must lie; this limitation is really a help to us in framing hypotheses, since it enables us to eliminate those that have little chance of proving fruitful.
It is now about fifteen years since M. Fabry published an essay concerning the true significance of the parabolic form in cometary orbits. The essay does not seem to be very widely known, as is shown by the fact that Prof. C. L. Poor, in «The Solar System» p. 281 argues as if the prevalence of parabolic orbits shows that the majority of comets come to the Solar System from outside. In reality, as M. Fabry shows, if comets really came to our system from outside, quite a large number would have strongly marked hyperbolic orbits. It is easy to see that this is so, if we reflect that the velocity at any point of a parabolic orbit is the same as that due to a fall from an infinite distance, (or, practically, from any distance that is very great compared with the actual distance of the body from the sun) under the influence of the sun’s attraction; for a hyperbolic orbit the speed is greater than this, for an elliptical orbit less. Hence to assert that a comet moving in an apparently parabolic orbit has come to our system from outside is equivalent to saying that it entered the sun’s sphere of influence, (by which we mean the region where the sun’s influence is