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."