represented by the action of a blue mass on the north side and a red mass on the south side, or by one of these alone. The result at which we have arrived is analogous in form to that which we have obtained in other magnetical investigations; and, to preserve the analogy of language, we shall call E the Earth's magnet-power. It is important to remark that, on account of the assumed magnitude of c and c', the angular momentum here is expressed by a single term, a multiple of B, without any of the additional terms which occur in Article 20.

SECTION IV.

ON TERRESTRIAL MAGNETISM, AS ACTING IN THE HORIZONTAL PLANE AT EACH PLACE OF OBSERVATION.

22. Definition of Local Magnetic Meridian and Magnetic Variation or Declination; instruments and methods for ascertaining roughly the Declination: Azimuth Compass; observation of Sun's Azimuth, Variation Theodolite; Declination Charts.

The 'Local Magnetic Meridian' is the direction, on the horizontal plane of any place of observation, which is taken by the compass-needle. The 'Magnetic Variation' (a very bad term, still commonly employed by nautical men, but for which, among men of science, the term 'Magnetic Declination' is usually substituted) is the angle made by the Local Magnetic Meridian with the Astronomical Meridian: it requires to have appended to it the word east" or west" as applied to the north point of the needle. Thus the Magnetic Declination at Greenwich at the present time is about 20° West: meaning that the north point of the magnetic needle points west of the astronomical north meridian by 20° nearly.

66

The general direction of the Local Magnetic Meridian may be obtained by merely observing the direction of the needle: but the Astronomical Meridian is not so obviously visible: and therefore the Declination cannot be easily found. Navigators however require the Declination in order to enable them to adapt their compass-steering to ordinary maps and charts. The

Fig. 17.

Fig. 17*.

element required was obtained by the use of an

azimuth compass' represented in Figures 17 and 17*.

(The jimbal-rings, by which the compass-card retains its horizontality in all motions of the ship, are omitted in these Figures.) It must be remarked that in ships' compasses the compass-needle is never exhibited naked, but is inclosed in the thickness of a 'compass-card,' a circular card on which points of the compass and degrees of azimuth are engraved, and which, being firmly connected with the compass-needle, is so directed by the magnetic power of the earth acting on the compass-needle that the N, E, S, W on the card point truly to magnetic north, east, south, west. In the compass, nothing touches the circumference of this card, but there are, rising from the compass-box, two small frames carrying vertical wires; by directing the eye-view along the two wires and turning the box till that eye-view sees a distant object in the same line as the two wires, the line of wires is made to coincide with the direction of the object and then the observer must read the graduations of the card which correspond to points in the box below the two wires. In this way he obtains the azimuth of the object as referred to the Local Magnetic Meridian.

In the best modern instruments, a horizontal ring is expressly provided to carry the vertical wire-frame: and, instead of having a wire next to the eye, a glass prism acting by internal reflection is placed there; so arranged, that one half of the pupil of the eye can observe the wire on the further side of the horizontal ring, and the distant object; and the other half of the pupil can see the graduations of the compass-card by

internal reflection in the prism. (Small instruments of this kind, called "prismatic compass," are to be obtained at little expence, and are very convenient.)

The azimuth of an object being obtained as referred to the Local Magnetic Meridian, the next point is to find its azimuth as referred to the Astronomical Meridian. The power of doing this depends on the choice of the object. Navigators almost invariably choose the rising or setting Sun. The latitude of the ship being known with sufficient accuracy for this purpose, and the Sun's declination and consequent distance from the celestial pole being taken from the Nautical Almanac, the solution of a quadrantal equation gives the Sun's azimuth at rising or setting as referred to the Astronomical Meridian. The difference between this and the azimuth as referred to the Local Magnetic Meridian gives the Magnetic Declination. (The poetic reader will find much of this operation correctly described in Falconer's "Shipwreck.")

For determinations on shore, a "Variation-theodolite" was sometimes used, consisting of a common theodolite adapted to the measure of horizontal angles, the axis of its telescope being so much raised that the telescope could be pointed to the pole-star, by which the reading of the horizontal circle, for astronomical meridian could therefore be found; and that the telescope could also be pointed down to a compass-needle immediately below the theodolite-frame, to the ends of which the telescope-object-glass could be directed, and which ends were made distinctly visible by affixing

1