3d. THE ANOMALISTIC YEAR is the interval between two succeeding passages of the sun through the same apsis: it is found to be 4 min. 42 sec. longer than a sidereal year. This fact indicates that the line of apsides, or the line joining the greatest and least distances of the earth from the sun, has a motion of 12′′ in consequentia, or according to the order of the signs among the fixed stars.

SECTION VIII.

THE SUN-DIAL.

280. THE most simple instrument for measuring time is the sun-dial; upon the construction of which, in former times, great pains were bestowed. Since the more general diffusion of portable time-keepers, and the introduction of the telescope in astronomical observations, dials have been less in repute as, however, a good dial, well fixed, will always give a near approximation to solar or apparent time at any hour of the day, and by the application of the equation will thus serve to regulate a common clock or watch, and since its construction illustrates the doctrine of the sphere, a few words on the subject of dialing may not be amiss.

Let us suppose NSD, Plate VIII. fig. 51, to be a glass globe, with meridians drawn through every 15° on the equator: these will be hour-circles. Let this globe be placed in the sunshine, with its axis, which we will suppose to be opaque, elevated to the latitude of London. The meridian which points due south must

be numbered XII., as the sun will be on it at that hour. Now, since the distance of the sun is so great that the earth viewed from him would appear no larger than Mercury does to us, no perceptible error will arise from the centre of the glass globe not coinciding with that of the earth, which theoretically it ought to do.

Suppose the opaque plane D B to pass through the centre, so that the angle N EB = 5140: this will represent the horizon of London. As the sun advances to the first meridian from the central one, viz. F, the opaque axis will throw a shadow on the horizontal plane, which will point out the direction of the oneo'clock hour-line. One hour afterwards the sun will be over the next meridian G, and the shadow of the opaque axis will indicate the position of the two-o'clock hour-line in like manner, the shadow of the opaque axis will point out the other hour-lines as long as the sun is above the horizon. In practice, the meridian lines are done away with; but the principle of dialing is better understood by supposing them visible.

The spaces on the horizon between the hour-lines are the hour-arcs: these are the bases of spherical triangles, whose sides meet at the pole. Thus, NC B is a spherical triangle; the angle N BC is a right angle; NB is equal to the latitude 51°. The vertical angle CNB is the hour-angle, measured on the equator by the arc Ea in this case it is equal to 15° x 6 or 90°, whence the base CB may be found by spherical trigonometry, which will be the distance in degrees from the XII. o'clock, or south line.

A vertical dial differs from a horizontal, in that the opaque plane passes through the zenith.

281. Plate VIII. fig. 52, represents a very elegant universal dial, by Adams: it may be adapted to any latitude, and will shew the time very correctly, if rectified with due care. Here the hour-circle is made to coincide with the plane of the equator, and is therefore divided into twenty-four equal parts; each of these represents an hour of solar motion, which, when measured on the equator, will always be equable. The gnomon n s is parallel to the axis of the earth, and may be elevated or depressed by the graduated arc bs to correspond to the latitude of any place between the cquator and either pole: the shadow of the gnomon will then indicate the time on the circle abcd, representing the equator. The foot of the dial may be placed perfectly horizontal by means of the foot-screws e,f,g, and, which is of the utmost consequence, due north and south by means of the COMPASS shewn at m.

As this is the first time we have had occasion to allude to that valuable auxiliary to nautical science, it will be appropriate to spend a short time in describing its construction, and pointing out its use.

SECTION IX.

ON THE COMPASS.

282. IF a bar of steel be nicely balanced on its centre, and afterwards rubbed with a magnet, it becomes endued with the property of pointing in the direction of the north; disturb it as you may, it will, after vibrating from side to side, at last remain steady in its original

direction. Knowing the northern point in the heavens, the south will be opposite to it; the east on your left hand as you face the south, the west on your right These are the four cardinal points. The portions of the circle described with the pivot of the balanced needle for a centre, intercepted between these, are subdivided into eight, making in all thirty-two points of the compass. Independently, then, of any observation of the heavenly bodies, or of known objects on land, the sailor is thus provided with the means of ascertaining the direction in which his ship is sailing, or is able to direct her in the proper course; even in the darkest night this infallible monitor never forsakes him; in reliance upon its directions he still steers with confidence towards his destined haven.

283. We should fall into error, however, if we considered the north indicated by the needle (for such is the balanced bar called) to be identical with the meridian line determined astronomically, or by the method pointed out in § 222. It is a singular circumstance, that the magnetic north deviates from the true north by a quantity which varies, and that by no fixed law, in different latitudes indeed, the VARIATION OF THE

:

The points of the compass, which appear in general so complicated to landsmen, are easily understood by attending to the following simple rules:

1st. When the letters indicating two points are united, the point meant is half way between the two: thus, N.E. is half way between N. and E.; S.S. W. half way between S. and S.W.

2d. When the letters are joined by the word by, the point meant is the one which comes next after the first, going towards the second: thus, S. by W. is next to S. going west; N.E. by N. is next to N.E. going north.

NEEDLE, as it is called, differs in the same place in various years; and if more minutely watched, it will be seen, during twenty-four hours, to vibrate slightly on each side of its mean position. Hence, for the more exact purposes of astronomy, its aid is of no avail; and on board ship the compass, from time to time, iş corrected by comparing the sun's azimuth, indicated by it, with that deduced from astronomical observation at the same instant.

In 1850, at Greenwich, the variation of the needle was 22° 30' to the west of north; so that, to find the true north, we must bring the needle over 2210 west, by turning the compass card, dial, globe, or whatever instrument we wish to rectify; then the line marked north and south will be nearly in the plane of the meridian of the place we may be in, provided its latitude differs only by a small quantity from that of London.

284. The following table will shew the variation of the needle for certain periods during the last 270 years it arrived at its maximum of westerly variation about the year 1820; since which time it has approached the true north two degrees.

285. The variation of the needle is not the only

singular and inexplicable circumstance attending it. If