ur goes, the sun waxing bigger and bigger with each stage of
approach, until at last the little star has become a fiery globe,
filling up half the heavens with its vast proportions, and stretching
from the horizon to the zenith of the visible concave. The great comet
of 1680 came in this way from a region of space where the sun looked
but half as wide as the planet Mars in the sky, and where the solar
heat was imperceptible, the surrounding temperature being 612 degrees
colder than freezing water, into another in which the sun filled up
140 times greater width of the sky than it does with us, and where the
heat was some hundred times higher than the temperature of boiling
water. It was then only 880,000 miles away from the solar surface, and
would have fallen to it in three minutes, in obedience to its
attraction, if the impetus of its motion in a different direction had
been on the instant destroyed or arrested. But this impetus proved too
great for the attraction, light as the material of the moving body
was. When the comet has approached comparatively near to the grand
source of attraction, the speed of its accelerating motion has become
so excessive, that it is able to withstand the augmented solicitation
it is subjected to, and move outwards in a more direct course. It
goes, however, slower and slower, and curving its journey less and
less, until at last its motion in remote obscurity is again so
sluggish, that the sun's attraction is once more predominant, and able
to recall the truant towards its realms of light. Such is the history
of the comet's course.

Thin comet vapours drift through space, sustained by exactly the same
influences that uphold dense planetary spheres. They are supported in
the void by the combined effects of motion and attraction. Their own
impetus strives to carry them one way, while the sun's attraction
draws them another, and they are thus constrained to move along paths
that are intermediate to the lines of the two impulses. Now, when
bodies are driven in this way by two differently acting powers, they
must travel along curved lines, if both the driving forces are in
continued operation, for a new direction of motion is then impressed
on them at each succeeding instant. There are three kinds of curved
lines along which bodies thus doubly driven may move: the _circular_
curve, which goes round a central point at an unvarying equal
distance, and returns into itself; the _elliptical_ curve,