appears to stream up from the sun a little after sunset and before sunrise. It appears nearly in the form of a cone, its sides being somewhat curved, and generally but ill defined. It.extends often from 50° to 100° in the heavens, and always nearly in the direction of the plane of the ecliptic. It is most distinct about the beginning of March; but is constantly visible in the torrid zone. The cause of this phenomenon is not known.

In Almanacks, the sun is usually represented by a small circle, with the face of a man in it.

SECT. 3. Of Mercury.

17. Proceeding from the sun, the grand centre of the system, the first planet is Mercury. It revolves round the sun at nearly the mean distance of 37 millions of miles, and completes its revolution in about 3 months. The time, in which it turns on its axis, is about 24 hours. It emits a brilliant white light; but because it is near the sun, and consequently seldom out of twilight, it is not often noticed. Its greatest apparent distance from the sun, or its greatest elongation, is never more than 28°. . When viewed through a good telescope, it exhibits all the different appearances or phases, which the moon does, and they are to be accounted for in the same manner. Of this we shall treat hereafter.

18. The distance of Mercury from the sun is to that of the earth nearly as 3 to 8. Therefore the degree of heat and light at Mercury is to that at the Earth, nearly as (8 x 8) 64 to (3 x 3) 9; which is very nearly as 7 to ì. Consequently, at Mercury, heat and light are 7 times greater than with us. Water would there fly off

in steam and vapour.

SECT. 4. Of Venus.

19. Next to Mercury, in the Solar system, is Venus. This planet revolves round the sun at the mean distance of 68 millions of miles. It completes its revolution in about 7 months; and turns on its axis in little less than 24 hours. The light reflected by this planet is very brilliant, and often renders it visible to the naked eye in the day time.) Its greatest elongation is about 47°. It exhibits phases similar to those of Mercury and the moon. Spots are sometimes seen on its surface; the appearances of which, and its phases, are exhibited in the annexed wood-cut. Heat and light at Venus are nearly double what they are at the earth./

20. This planet is brightest, when she is about 40° distant from the sun and then only about part of her disk is illuminated. Her brightness in this position is surprising. Her lustre far exceeds that of the moon, at the same apparent distance from the sun. For though, on account of her apparent magnitude, the moon reflects more light to us than Venus does, yet this light is incomparably more dull, and has none of the life and briskness which attend the beams of Venus. This difference arises probably from the circumstance of Venus having a very dense atmosphere, while the moon has a very rare one.)

21. Mercury and Venus are called interior planets, because they are nearer the sun than the earth is; while those that are farther from the sun than the earth is, are called exterior.* They exhibit some peculiarities,

* In most books on astronomy, what are here called interior planets, are styled inferior; and what are here called exterior, are there denominated superior. But why this distinction of superior and inferior was ever made, it is difficult to see. In what proper sense can the word superior be applied to Mars in comparison of the Earth or Venus? Since every natural blessing of existence is derived from the neat and light of the sun, we should suppose that planets would be

arising from their situation; but as Mercury is seldom seen, those of Venus only will be noticed. During a part of its revolution, Venus rises and sets before the

it is then called morning star) During another part of its revolution, it rises and sets after the sun; it is then called evening star.* (Pl. II. fig. 2.) Let S be the sun, BDEC the orbit of Venus, A the earth, AL a part of its orbit, while Venus is moving from C, (which point is called its superior conjunction) through B to D, it will appear to the inhabitants of the earth at A to be above, or eastward of the sun; it will consequently be visible after the sun has set. But while passing from D, (which point is called its inferior conjunction,) through E to C, it will appear below or westward of the sun, and will consequently set before the sun.

22. If the earth were stationary at A, it is obvious that Venus would be above the sun, and be evening star in half its orbit; and be below the sun, and be morning star in the other half.) But because the earth is in motion, Venus is above and below the sun alternately, in much more of its orbit. For let Venus emerge above the sun at C, when the earth is at A; while it is coming through B to D, the earth passes from A to F; consequently Venus must pass from D to d, before it is seen below the sun. So while Venus moves from d to x, (half its orbit,) the earth has come to o; consequently Venus must move on from a to v before it emerges again above the sun. This effect is very much greater than is represented on the figure. For while Venus passes

superior according to the degree of heat and light which they received; that is, according to their proximity to the sun. This distinction of interior and exterior is not new, though but few have adopted it; but being, (as I conceive,) much the most appropriate, I feel desirous of having it adopted.

*The Ancients called the morning star, Phosphorus; and the evening star, Hesperus. These names are now often used, especially in poetry.