tion being first subtracted from the parallax, the remainder is the correction of the moon's altitude; which being added to the apparent altitude, gives the true altitude of the moon.

If the moon had no parallax, like the stars; her refraction would be her correction. As the greater the distance from the earth the less the parallax, the sun's parallax is very small; his greatest, that is when in the horizon, being only nine seconds; and his refraction greater, the difference between the refraction and parallax will be the correction of the sun's altitude.

The moon's horizontal parallax is given in the VII page of the month of the Nautical Almanac, for every noon and midnight, and may be proportioned to any time as before mentioned.

The moon's parallax in altitude may be found by increasing the index of the proportional logarithm of the horizontal parallax by 10, and subtracting the co-sine of the moon's altitude; the remainder is the proportional logarithm of the parallax in altitude, or the sine of the moon's zenith distance taken from the same proportional logarithm will give the proportional logarithm of the moon's parallax in altitude; and the refraction in altitude of the moon, taken from her parallax in altitude, will be her correction. Or by adding the proportional logarithm of the moon's horizontal parallax to the secant of her apparent altitude, the sum, rejecting the indexes, will give the proportional logarithm of the moon's parallax in altitude; the refraction being substracted from this will, as before, give the moon's correction.

E The earth. F he moon's orbit. G The orbit of a superior planet,

Plate III. is the same as plate II. but, to prevent confusion, it is to show that the further the planet is from the earth, the less the parallax. Here the moon's parallax is omitted.

The greatest parallax of the moon is when she is in her perigee or opposition; that is when she is in that part of her orbit nearest the earth, and at the full; her least parallax is when she is in her apogee or conjunction; or when she is in that part of her orbit which is furthest from the earth, and at the change. In the Nautical Almanac the horizontal parallax is given generally from 53 to 62 minutes.

On the Dip or Depression of the Horizon at Sea.

THE depression of the horizon at sea, or as it is commonly called, the dip, is the angle contained between the horizon of the observer and the furthest visible point on the surface.

Fig. 2.

For if an observer, whose eye Fis situated at D fig. 2, takes the altitude of a celestial object by a quadrant, and brings the object to the surface of the water B, instead of the horizon F, he evidently makes the altitude too great by the

angle FDB.

Sensible or Visible Horizon.

THE sensible or visible horizon is that circle which terminates our view all round, of which we are the centre, and where, in a clear day, the sea and sky seem to meet.

The rational or true horizon is an imaginary plane passing through the centre of the earth parallel to the sensible horizon.

The sensible horizon extends only a few miles. For example :—if a man of six feet high were to stand on the surface of

the sea, the utmost extent of his view of the water would be about three miles; so that from the deck of a common sized ship, where the eye is from twelve to twenty feet above the level of the sea, the regular dip will be about four miles, aud the horizon distant about six miles. It may be important to know, that at the distance of six miles from the land the fore observation of the sun's meridian altitude will answer, as the horizon will not be affected by the land. I have myself, when passing small islands at six miles distance, found no difference between the same meridian altitude in the openings between the islands, as when the land was between me and the sun; from which it is plain the horizon was not more than six miles distant, and the observation good.

On the Refraction of the Heavenly Bodies.

THE rays of light from a heavenly body, in passing to a spectator on our earth, come into the atmosphere, which, being a dense medium, bends them out of their course, and shows the body from whence they proceed more elevated than it really is. This will be rendered familiar to any one, by putting a piece of money in the bottom of a bowl or basin, and retiring back from it till it just disappears; then, without moving the money, pour the basin full of water, and the money will be seen clearly by the refraction, which the denser medium (water) will cause to take place.

It must be remembered that it is only the rays which fall obliquely that are thus refracted; for a ray which falls perpendicularly is equally attracted on all sides, and therefore suffers no refraction at all.

To illustrate this by the experiments which has just been mentioned, you must know that it is by the light reflected from it to your eye, that any object is rendered visible; you see the piece of money in the basin, therefore, by the rays of light which are reflected from its surface. Now the angle of incidence, and the angle of reflection, are equal, and as you stand in

an oblique direction to the piece of money, you see it while the basin is empty, by the rays of light which fall upon it in a direction exactly as oblique as that in which your eye is situated towards it. The piece of money then, which before was hid from your sight, is rendered visible by pouring in the water, because the rays of light, which serve to render it then visible, are bent out of their course. Thus the ray of light A C, fig. 12, which passes obliquely from the air into water at C, instead of continuing its course to B, takes the direction C a; and consequently an object at a, would be rendered visible by rays proceeding in that direction, when they would not have touched it, had they proceeded in their direct course.

By this figure you will understand that the angle of refraction P Ca, is not so large as the angle of incidence p Č A, but bears a certain proportion to it, and this proportion varies with respect to different mediums. Thus when a ray of light passes from air into water, the angle of incidence is to that of refraction in the ratio of four to three; from air into glass, as three to two; from air into diamond, as five is to two, and the contrary proportion holds good in passing back again, as when light passes from water into air, the ratio was three to four, and from all this you will clearly understand the more oblique a ray falls, the greater is the refraction. It is also necessary to remember that the light is refracted or drawn towards the perpendicular as in fig. 12, it passes out of rare into a denser medium, and it is refracted from the perpendicular, or in a more oblique direction, when it passes from a dense medium into one which is rare; and the denser the medium, the greater is the refraction; thus a diamond is found to refract most powerfully. This principle will explain several of the common phenomena of nature. Mr. Walker observes, that many a school boy has lost his life, by supposing the bottom of a clear river to be within his depth, as (when he stands on the bank) the bottom will appear one-fourth nearer the surface than it really is; a skilful marksman, who shoots a fish in the water, will take his aim somewhat below the fish as it appears, (perhaps a foot) because it appears much nearer the top of the water than it really is. But the most excellent use to which this principle has been applied, is the construction of opucal glasses; for by grinding these glasses thinner at the edges than in the middie, those rays of light which would strike upon it in a straight line, or perpendicularly if it was plain, would strike upon it obliquely, and consequently suffer a refraction, and be made to converge and on the contrary, by making the glass thinner in the middle than at the sides, the rays are refracted the contrary way, and are made to diverge: the former are called convex glasses, the latter concave. See fig. 13 and 14.

N. B. To converge the rays of light, is to cause them to meet in a point, after passing through the convex glass; and to diverge them, is the contrary.

In working for the apparent time to be used for lunar observations, the following table is not necessary, but it may be used in rating a chronometer which requires more exactness.