Comets, according to Sir Isaac Newton, are compact, solid, fixed, and durable bodies: in one word, a kind of planets, which move in very oblique orbits, every way, with the greatest freedom, persevering in their motions even against the course and direction of the planets; and their tail is a very thin, slender vapour, emitted by the head, or nucleus of the comet, ignited or heated by the sun. There are _bearded_, _tailed_, and _hairy_ comets; thus, when the comet is eastward of the sun, and moves from it, it is said to be _bearded_, because the light precedes it in the manner of a beard. When the comet is westward of the sun, and sets after it, it is said to be _tailed_, because the train follows it in the manner of a tail. Lastly, when the comet and the sun are diametrically opposite (the earth being between them) the train is hid behind the body of the comet, excepting a little that appears around it in the form of a border of hair, or _coma_, it is called _hairy_, and whence the name of comet is derived. For the conservation of the water and moisture of the planets, comets (says Sir Isaac Newton) seem absolutely requisite; from whose condensed vapours and exhalations all that moisture which is spent on vegetations and putrefactions, and turned into dry earth, may be resupplied and recruited; for all vegetables increase wholly from fluids, and turn by putrefaction into earth. Hence the quantity of dry earth must continually increase, and the moisture of the globe decrease, and at last be quite evaporated, if it have not a continual supply. And I suspect (adds Sir Isaac) that the spirit which makes the finest, subtilest, and best part of our air, and which is absolutely requisite for the life and being of all things, comes principally from the comets. Another use which he conjectures comets may be designed to serve, is that of recruiting the sun with fresh fuel, and repairing the consumption of his light by the streams continually sent forth in every direction from that luminary-- "From his huge vapouring train perhaps to shake Reviving moisture on the numerous orbs, Thro' which his long ellipsis winds; perhaps To lend new fuel to declining suns, To light up worlds, and feed th' ethereal fire." THOMSON. Newton has computed that the sun's heat in the comet of 1680,[2] was, to his heat with us at Midsummer, as twenty-eight thousand to one; and that the heat of the body of the comet was near two thousand times as great as that of red-hot iron. The same great author also calculates, that a globe of red-hot iron, of the dimensions of our earth, would scarce be cool in fifty thousand years. If then the comet be supposed to cool a hundred times as fast as red-hot iron, yet, since its heat was two thousand times greater, supposing it of the bigness of the earth, it would not be cool in a million of years.