Comets contrasted with Planets in Nature as well as in their Movements--Coggia's Comet--Periodic Returns--The Law of Gravitation--Parabolic and Elliptic Orbits--Theory in Advance of Observations--Most Cometary Orbits are sensibly Parabolic--The Labours of Halley--The Comet of 1682--Halley's Memorable Prediction--The Retardation produced by Disturbance--Successive Returns of Halley's Comet--Encke's Comet--Effect of Perturbations--Orbit of Encke's Comet--Attraction of Mercury and of Jupiter--How the Identity of the Comet is secured--How to weigh Mercury--Distance from the Earth to the Sun found by Encke's Comet--The Disturbing Medium--Remarkable Comets--Spectrum of a Comet--Passage of a Comet between the Earth and the Stars--Can the Comet be weighed?--Evidence of the Small Mass of the Comet derived from the Theory of Perturbation--The Tail of the Comet--Its Changes--Views as to its Nature--Carbon present in Comets--Origin of Periodic Comets. In our previous chapters, which treated of the sun and the moon, the planets and their satellites, we found in all cases that the celestial bodies with which we were concerned were nearly globular in form, and many are undoubtedly of solid substance. All these objects possess a density which, even if in some cases it be much less than that of the earth, is still hundreds of times greater than the density of merely gaseous materials. We now, however, approach the consideration of a class of objects of a widely different character. We have no longer to deal with globular objects possessing considerable mass. Comets are of altogether irregular shape; they are in large part, at all events, formed of materials in the utmost state of tenuity, and their masses are so small that no means we possess have enabled them to be measured. Not only are comets different in constitution from planets or from the other more solid bodies of our system, but the movements of such bodies are quite distinct from the orderly return of the planets at their appointed seasons. The comets appear sometimes with almost startling unexpectedness; they rapidly swell in size to an extent that in superstitious ages called forth the utmost terror; presently they disappear, in many cases never again to return. Modern science has, no doubt, removed a great deal of the mystery which once invested the whole subject of comets. Their movements are now to a large extent explained, and some additions have been made to our knowledge of their nature, though we must still confess that what we do know bears but a very small proportion to what remains unknown. Let me first describe in general terms the nature of a comet, in so far as its structure is disclosed by the aid of a powerful refracting telescope. We represent in Plate XII. two interesting sketches made at Harvard College Observatory of the great comet of 1874, distinguished by the name of its discoverer Coggia.