magnetic intensity, which are copied, without essential change, in Figures 28 and 29. These maps are on the

stereographic projection. The numbers upon the lines give the value of E in Metric measure. Remarks on the peculiarities of these curves will be given below, in Article 42.

SECTION V.

ON TERRESTRIAL MAGNETISM, AS ACTING IN THE VERTICAL AT EACH PLACE OF OBSERVATION; AND ON THE COMBINATION OF THE HORIZONTAL AND VERTICAL FORCES, AND THE TOTAL TERRESTRIAL MAGNETIC FORCE AT EACH PLACE OF OBSERVATION.

35. First evidence of the existence of a vertical magnetic force.

When a needle is prepared, in the unmagnetized state, for mounting in a compass, with its center of gravity very little below its point of support, and is adjusted to horizontality; on being magnetized, its red end (in northern latitudes) dips considerably. This proves that (in northern latitudes) the terrestrial horizontal magnetic force towards the north is accompanied with a vertical force downwards, and the terrestrial horizontal force towards the south is accompanied with a vertical force upwards.

When the same compass is carried into southern latitudes, the blue end dips. This proves that, while the sign of the terrestrial horizontal force in the north direction or in the south direction has not changed, the

sign of the vertical force has changed. This is so well known that, in the best compasses, a sliding weight is provided, which in north latitudes can be applied to the blue end of the needle, and in south latitudes can be applied to the red end of the needle.

The instrument with which this vertical force is most conspicuously exhibited and most accurately examined will be described in the next article.

36. Description of the Dipping Needle.

The function of this instrument is limited strictly to the determination of the direction which a needle will. take under the action of the total terrestrial magnetic force, when it is constrained to move in an arbitrary vertical plane. This limitation permits the construction of an instrument possessing great simplicity, and, in consequence (viewing the nature of its action) great accuracy.

The needle must be carried by a horizontal axis, passing as nearly as practicable through its center of gravity. This condition, though convenient, is not necessary: for, as will be shewn in the next Article, we can so arrange the observations as perfectly to eliminate the effects of error of position of the axis: and indeed, for some observations, the place of the center of gravity is purposely moved to a sensible distance from the axis. The axis must terminate in two delicate pivots; and it is mainly in the formation of these that the utmost skill of the artist is required. It is very difficult so to arrange the observations that the injurious effect of an

oval or otherwise ill-formed pivot can be entirely removed. To make these pivots turn with the least possible friction is of the utmost importance: and for this object, the pivots must not turn in Ys like those of a transit-instrument, but must roll upon two edges of a very hard substance, usually agate. In the direction parallel to the plane in which the needle moves, these edges must be straight and perfectly horizontal; in the vertical section at right angles to that plane, or in the direction of the needle-axis, the section of each edge is rounded; a form very desirable for permitting the escape of particles of dust, &c. Great attention is required for the satisfactory polishing of the edges. When due care is given to these preparations, the friction is extremely small.

It is necessary now to describe the method of observing the position of the needle.

The needles employed are always pointed: and, till within a few years past, the needle was allowed to swing within a graduated ring of brass, and the divisions opposite to the points of the needle were read. The reading was very rough, and there was risk of error from the close proximity of the needle to the brass, which is seldom perfectly free from iron. Lately, a far superior form has been introduced, known as the Kew pattern (from the circumstance that it was invented and first used at the Kew Observatory). A view of that instrument is given in Figure 30. Several auxiliary parts, unimportant to the general principle, are omitted in this drawing. There is no metal near the needle: the

points of the needle are observed by means of microscopes which are attached to a revolving frame that carries verniers by which the graduations of an external

Fig. 30.

vertical circle are read. The true position of the needle (including all effects of friction, uncertainty of reading, &c.) is rarely doubtful to the extent of 2'. A modified form of the instrument, adapted to the use of needles of different lengths, and with other fittings, is mounted as a permanent instrument at the Royal Observatory, Greenwich.

In all cases, the instrument is so mounted that it