from us, are beyond the reach of telescopic observation.()

(x) The proportional distances, and the times of the periodic revolutions of the six satellites of Uranus, are as follows: the diameter of the planet being assumed as unity.

LETTER XXII.

ON COMETS, AEROLITHS, AND METEORS.

HAVING in my last letter explained to you the most interesting particulars relating to the planets, I shall now proceed to give you some account of the comets, those erratic bodies, which the ancients I considered as enormous meteors, formed in the atmosphere, and sent as harbingers of divine vengeance. This was the prevailing opinion as early as the time of Homer, who speaks of

"The red comet, by Saturnia sent,

To fright the nations with a dire portent;
A fatal sign to armies on the plain,

Or trembling sailors on the wat'ry main."

And a similar allusion is also given by Milton, who compares the indignation of Satan, at being opposed in his passage by Death, to the burning of a comet,

"That fires the length of Ophiuchus huge

In th' arctic sky, and from his horrid hair
Shakes pestilence and war.”

Tycho Brahe, and Dominique Cassini, were the first among the modern astronomers who gave these bodies a place in our system; but they appear to have been unacquainted, both with their motion round the sun, and the true figure of their orbits. These particulars were left for the deter

mination of Newton, who has discovered the paths they describe, and the laws to which they are subject. Their revolutions are now known to be performed in very long ellipses, whose lower focus is in or near the sun, being governed throughout by the same law, of describing equal areas in equal times, which is known to regulate the motions of all the other bodies in the system.

By observations of parallax it is also found, that at their first appearance they are nearer to us than Jupiter; from which it is concluded that they are, in general, less than that planet; for if they were as large, they would be seen as far off. In their motions round the sun they are also subject to the same irregularities as the planets; but as their orbits are extremely eccentric, those variations are much more considerable. When they are near the sun, their motion is very rapid, and in the more distant parts of their orbits extremely slow; so that their vicissitudes, in this respect, are as much in the extreme as what they undergo from heat and cold.

When a comet arrives within a certain distance of the sun, it emits a fume or vapour called its tail; which shows that these bodies contain a portion of matter considerably more rare and volatile than any on the earth; for the tail begins to appear when they are yet in a higher, and consequently a colder region, than Mars. In every situation of the comet the tail is always directed to

that part of the heavens which is nearly opposite

to the sun; and is always greater after the comet has passed its perihelion, than during its approach towards it, being greatest of all when it has just left that point.

The head of the comet is also surrounded with a substance similar to the tail, which is called the coma; the head itself being easily distinguished from it by its shining with a much greater lustre ; and the nucleus, which is the body of the comet, is still brighter than the head, but very small, and not distinguishable except by means of the best telescopes.

That part of a comet's orbit, which comes under our inspection, is so small in proportion to the whole, that it differs but little from a parabola; for which reason the dimensions of their orbits and periodical times cannot be ascertained, with any degree of precision, from a single observation. But from the re-appearance of several comets, after long intervals of time, in the same region of the heavens; and from their being found to move in the same curve, it is evident that their revolutions must be performed in certain stated times, like those of the planets. This indeed has been shown by Halley, who, from the theory of Newton, has calculated tables for determining the orbits of the comets, which, in some instances, have been found agreeable to observation.

But it is very difficult to determine with accuracy the elements of their orbits, which are so extremely eccentric, that a very small error in the

observation, will change the computed orbit into a parabola or hyperbola. And as the thickness and inequality of the atmosphere, with which the comet is surrounded, render it impossible to ascertain, with any degree of precision, when either the limb, or centre, pass the wire of the telescope at the time of observation, much uncertainty must necessarily attend the result thus deduced. The only safe method therefore to determine the periods of comets, is to compare the elements of all those that have been computed, and where any remarkable coincidence is perceived, an identity may be inferred; it being extremely improbable, that two different comets should have the same inclination, the same perihelion distance, and the places of the perihelion and the node the same. By this means, the periodic time being determined, the major axis of the orbit becomes known from the laws of Kepler, and the perihelion distance being likewise obtained from observation, will also give the minor axis of the orbit.

It was thus that Dr. Halley was enabled to foretell the return of the comet in 1759; he having, by comparison, found that it had before appeared in the years 1456, 1531, 1607 and 1682, and therefore, that its period was about 75 years. But as the comets sometimes pass very near the planetary bodies of our system, a considerable variation may frequently happen in their periods of revolution, as was the case with the comet above-mentioned; and therefore, if the ele