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large majority of the comets is this. They appear to us as if falling toward the sun from some great distance, we know not what. If a comet fell exactly toward the sun, it would fall into it, but this is a case which has not been known to occur and which, for reasons to be explained later, cannot be expected ever to occur. As it approaches the sun, it acquires greater and greater velocity, speeds around the central body in a great curve, and, by the centrifugal force thus generated, flies off again, returning to the abyss of space nearly in the direction from which it came.

Owing to the faintness of these objects they are visible, even in powerful telescopes, only in that part of their orbit which is comparatively near the sun. This is what makes it so difficult in many cases to determine the exact period of a comet which has only been seen once.

The first of these objects which was found to return in a regular period is celebrated in the history of astronomy under the name of Halley's comet. It appeared in August, 1682, and was observed for about a month, when it disappeared from view. Halley was able, from the observations made upon it, to compute the position of the orbit. He found that the latter was in the same position as that of a bright comet observed by Kepler in 1607.

It did not seem at all likely that two comets should move precisely in the same orbit. Halley therefore judged that the real orbit was an ellipse, and that the comet had a period of about seventy-five years. If this