By knowing the parallax of any celestial object, its distance from the centre of the earth may be easily obtained by trigonometry. Thus, if the distance of G from T be sought in the triangle STG, ST being known, and the angle SGT determined by observation, the side TG is thence known.

The parallax of the moon may be determined by two persons observing her from different stations at the same time; she being vertical to the one and horizontal to the other. It is generally concluded to be about 57'.

But the parallax most wanted is that of the sun, whereby his absolute distance from the earth is known; and hence the absolute distances of all the other planets would be also known, from the second Keplerian law. But the parallax of the sun, or the angle under which the semidiameter of the earth would appear at that distance, is so exceeding small, that a mistake of a second will cause an error of several millions of miles.

OF REFRACTION.

As one of the principal objects of astronomy is to fix the situation of the several heavenly bodies, it is necessary, as a first step, to understand the causes which occasion a false appearance of the place of those objects, and make us suppose them in a different situation from that which they really have. Among these causes, refraction is to be reckoned.

By this terin is meant the bending of the rays of light, as they pass out of one medium into another.

The earth is every where surrounded by an heterogeneous fluid, a mixture of air, vapour, and terrestrial exhalations, that extend to the regions of the sky. The rays of light from the sun, moon, and stars, in passing to a spectator upon earth, come through this medium, and are so refracted in their passage through it, that their apparent altitude is greater than their true altitude.

Let AC, plate 7, fig. 3, represent the surface of the earth; T, its centre; BP, a part of the atmosphere; HEK, the sphere of the fixed stars; AF, the sensible horizon; G, planet; GD, a ray of light proceeding from the planet to D, where it enters our atmosphere, and is refracted towards the line DT, which is perpendicular to the surface of the atmosphere; and as the upper air is rarer than that near the earth, the ray is continually entering a denser medium, and is every moment bent towards T, which causes it to describe a curve, as DA, and to enter a spectator's eye at A, as if it came from E, a point above G. And as an object always appears in that line in which it enters the eye, the planet will appear at E, higher than its true place, and frequently above the horizon AF, when its true place is below it, at G.

This refraction is greatest at the horizon, and decreases very fast as the altitude increases, insomuch that the refraction at the horizon differs from the refraction at a very few degrees above the horizon, by

about one-third part of the whole quantity. At the horizon, in this climate, it is found to be about 33'. In climates nearer to the equator, where the air is the refraction is less; and in the colder clipurer, mates, nearer to the pole, it increases exceedingly; and, is a happy provision for lengthening the appearance of the light, at those regions so remote from the sun. Gassendus relates that, some Hollanders who wintered in Nova Zembla, in lat. 75o, were surprized with a sight of the sun seventeen days before they expected him in the horizon. This difference was owing to the refraction of the atmosphere in that latitude. To the same cause, together with the peculiar obliquity of the moon's orbit to the ecliptic, some of these very northern regions are indebted for an uninterrupted light from the moon, much more than half the month; and, sometimes, almost as long as it is capable of affording any light to other parts of the earth.

Through this refraction, we are favoured with the sight of the sun about three minutes and a quarter before it rises above the horizon; and also as much every evening after it sets below it; which, in one year, amounts to more than forty hours.

It is to this property of refraction that we are also indebted for that enjoyment of light from the sun, when he is below the horizon, which produces the morning and evening twilight. The sun's rays, in falling upon the higher part of the atmosphere, are reflected back to our eyes, and form a faint light, which gradually augments till it becomes day. It is

owing to this, that the sun illuminates the whole hemisphere at once: deprived of the atmosphere, he would have yielded no light, but when our eyes were directed towards him; and even when he was in meridian splendor, the heavens would have appeared dark, and as full of stars as on a fine winter's night. The rays of light would have come to us in straight lines; the appearance and disappearance of the sun would have been instantaneous; we should have had a sudden transition from the brighest sunshine to the most profound darkness, and from thick darkness to a blaze of light. Thus, by refraction, we are prepared gradually for the light of the sun, the duration of its light is prolonged, and the shades of darkness softened.

To it we must attribute another curious phenomenon, mentioned by Pliny; for he relates, that the moon had been eclipsed once in the west, at the same time that the sun appeared above the horizon in the east. Mæstlinus, in Kepler, speaks of another instance of the same kind, which fell under his own observation.

OF THE FIXED STARS.

No part of the universe gives such enlarged ideas of the structure and magnificence of the heavens, as the consideration of the number, magnitude, and distance of the fixed stars. We admire, indeed with propriety, the vast bulk of our own globe; but, when we consider how much it is surpassed by most

of the heavenly bodies, what a point it degenerates into, and how little more even the vast orbit in which it revolves would appear, when seen from some of the fixed stars, we begin to conceive more just ideas of the extent of the universe, and of the boundaries of creation.

The most conspicuous and brightest of the fixed stars of our horizon is Sirius. The earth, in moving round the sun, is 190 millions of miles nearer to this star in one part of its orbit, than in the opposite; yet the magnitude of the star does not appear to be in the least altered, or its distance affected by it; so that the distance of the fixed stars is great beyond all computation. The unbounded space appears filled, at proper distances, with these stars; each of which is probably a sun, with attendant planets rolling round it. In this view, what and how amazing, is the structure of the universe!

Though the fixed stars are the only marks by which astronomers are enabled to judge of the course of the moveable ones, and we have asserted their relative positions do not vary; yet this assertion must be confined within some limits; for many of them are found to undergo particular changes, and perhaps the whole are liable to some peculiar motion, which connects them with the universal system of created nature. Dr. Herschel even goes so far as to suppose, that there is not, in strictness of speaking, one fixed star in the heavens; but that there is a general motion of all the starry systems; and, consequently, of the solar one, among the rest,