The paths that these erratic visitors follow in their journeys around the sun bear not the slightest resemblance to the paths of the planets, which are almost perfect circles. The orbits of comets are ellipses that are greatly elongated or parabolas. If the orbit is a parabola the comet makes one and only one visit to the sun, coming from interstellar space and returning thereto after a brief sojourn within our solar system. Donati's comet of 1858, one of the greatest comets of the nineteenth century, had a period of more than two thousand years and its aphelion (the point in its orbit farthest away from the sun) was five times more distant than the orbit of Neptune. There is, however, a class of comets known as _periodic_ comets that have extremely short periods of revolution around the sun. To this class belongs Halley's comet whose period of seventy-five years exceeds that of any other short period comet. Encke's comet, on the other hand, has a period of three and a third years which is the shortest cometary period known. Most of the periodic comets are inconspicuous and only visible telescopically even when comparatively near to the earth. Halley's comet is the only one of this class that lays any pretensions to remarkable size or brilliancy and it also is showing the effects of disintegration resulting from too frequent visits to the sun. Comets are bodies of great bulk or volume and small total mass. Their tails, which only develop in the vicinity of the sun, are formed of the rarest gases, and the best vacuum that man can produce would not be in as tenuous a state as the material existing in the tails of comets. There are many proofs of the extreme tenuity of comets. The earth has on a number of occasions passed directly through the tails of comets without experiencing the slightest visible effects. Stars shine undimmed in luster even through the heads of comets. If the earth should encounter a comet "head on" it is doubtful if it would experience anything more serious than a shower of meteors which would be consumed by friction with the earth's atmosphere, or a fall of meteorites over a small area of a few square miles. It is possible, however, that matter in the nucleus, the star-like condensation in the head of a comet, may consist of individual particles weighing in some instances a number of tons, surrounded by a gaseous envelope and held together by the loose bonds of their mutual attraction. If the earth should encounter the nucleus of a comet considerable damage might be done over a portion of the earth's surface, but the chances of such an occurrence are less than one in a million. Since the total mass of a comet is so small, a close approach to one of the planets, especially Jupiter, produces great changes in the form of the comet's orbit, though the motion of the planet is not disturbed in the slightest degree by the encounter.