interfering with the free movements of the needle. On this account, the former is employed in preference in very perfect apparatus. This circle or vertical limb rests upon a foot which is movable round a vertical axis, the prolonged direction of which passes through its centre, and consequently through the axis of suspension of the needle. An azimuth or horizontal circle enables us to determine at each instant the angles described by the vertical limb, and consequently of placing the latter in all azimuths, and particularly in the direction of the magnetic meridian. The magnetised needle then of itself takes a position according to the line of inclination. The angle is measured by means of the division of the vertical limb, which is generally furnished with a magnifier. Several observations must be made, and their mean must be taken in order to arrive at an accurate result; because, whatever precautions may be taken, the suspension is always imperfect, and because we are never very sure that the axis passes exactly through the centre of gravity of the needle. To be secure against this cause of error, we should make four kinds of observations, and take the mean of the results of each of them. After the first observations fresh ones are taken, after having turned the faces of the bar round, without changing the poles; then two other similar series of observations are made, after having changed the poles of the needle by magnetising it in the contrary direction, namely, by rubbing it against the same pole of a magnet, in a contrary direction to that in which it was formerly rubbed, so that the extremity that directed itself towards the south is found to be directed towards the north, and that which was directed to the north is directed to the south.

The inclination (or dip) which was 75° at Paris in 1671, has from that time been continually diminishing; in 1835, it was 67° 14', and in 1849, 66° 44'. It varies like the declination, not only with the epochs of observation, but still more with the places; at London, it was 70° 27' in 1720; 69° 2′ in 1833; and 68° 51' in 1849. At Geneva, it was 65° 48' 30" in 1825, and 64° in 1849. It increases the more we approach the north pole: thus there exists a place in 80° of north latitude

discovered by Captain Parry, when the inclination is 90°. On the other hand, the inclination decreases as we approach the equator: there is also on the surface of our globe a series of points where it is absolutely nothing, and which form a curve around the earth, called the magnetic equator. This curve is very regular in one part of its course, and may be regarded as a great circle inclined 12° or 13° to that of the terrestrial equator. Beyond the equator the inclination begins again, and increases in proportion as we approach the south pole; but it is then the south pole and not the north pole of the needle that inclines below the horizon.

When we do not know the declination, and have to make an observation of inclination, we may, without a declination compass, easily determine the plane of the magnetic meridian by looking for the plane in which the dipping needle holds itself perfectly vertical: this plane is perpendicular to that of the meridian. When, therefore, we have determined the former, we know the latter, and place the needle in it by making the vertical circle describe an angle of 90° on the azimuthal circle. This very simple connection between the two planes arises from the magnetised needle placing itself naturally, when it is free, in the plane of the magnetic meridian; and, not being solicited to go out of it, it must evidently, in order to obey this law, take the vertical position when the plane in which it is obliged to move is perpendicular to that of the magnetic meridian; for in this manner it is found to be at once in both planes.

The force which determines the direction of the magnetised needle is in fact neither attractive nor repulsive, but simply a directive force, incapable of impressing upon the magnet any movement of translation. This may be proved by different experiments: thus a magnetised needle floating upon water by means of a piece of cork to which it has been fixed, does not experience any onward movement; it simply takes the direction of the magnetic meridian like the needles which, resting on their point, are retained at their centre. A magnetised needle, fixed crosswise, and at the extremity of a horizontal slip of wood, with a counterpoise at the other extremity

to maintain equilibrium (Fig. 75.), also takes a direction exactly in the plane of the magnetic meridian, when the slip of wood is suspended by its centre of gravity to a silk thread without torsion, although its centre is not that of the needle. Finally, if we ballast a magnetised needle by means of a piece of platinum, so as to make it remain in equilibrio in the interior of a mass of mercury, and thus to withdraw it from the action of gravity, we see it place itself in the magnetic meridian, and take a direction inclined to the north, perfectly similar to the direction of the dipping needle. This direction is therefore. that which results naturally, abstraction being made of every mode of suspension, from the action exercised upon the magnetised needle by the forces or the force termed terrestrial magnetism.

Fig. 75.

It was not seen at first that the force, which impresses upon the magnetised needle the direction that we have just determined, emanates from the earth. Some placed the seat of this force in a small star, forming the tail of the Great Bear; others placed it further on still. Gilbert, at the end of the sixteenth century, in a very remarkable work, entitled Physiology of the Magnet*, was the first to show that it must be sought for in the terrestrial globe, a result that evidently arose from observations made in various places on the surface of the earth. We shall, in fact, see further on that by means of these numerous and varied observations, it becomes easy to determine the directions of the forces by which the needle is attracted; and that these directions are such, that it is evident these forces themselves emanate from the earth. We have, in like manner, become aware that the intensity of terrestrial magnetism varies, as well as the declination and inclination, with time and with places; and that it increases from the equator to the poles.

Physiologia nova de Magnete, magneticisque Corporibus: London, 1600.

With regard to the nature of the magnetic forces that emanate from the earth, the conjectures that have been formed in this respect are connected with the properties of magnets: we shall take care to point them out when studying these properties, but we shall not be able to examine their value until in the Third Chapter of Part the Fifth, when we shall be occupied upon natural electricity and terrestrial magnetism.

The discovery of the compass is much less ancient than that of the magnet. It appears that the first European navigator who made use of it was Vasco de Gama, in his first expedition into India. However, express mention is made in a Chinese dictionary that was completed in the year 121 of the Christian era; and in another dictionary, completed under the reign of Kang-hi, of the fact that, under the dynasty of Tsin (419 before Christ), vessels were directed towards the South by means of the magnet. The discovery of the declination appears to go back to Christopher Columbus; he was the first to perceive, in 1492, when he was traversing the Ocean to go and discover the New World, that the needle did not turn directly to the north in all places of the earth, as had hitherto been supposed. With regard to the change of declination in the same place, it was discovered in 1622, by Gunter, a professor in Gresham College. The dip was discovered in 1576 by Normann.

Different Properties of Magnets and magnetic Bodies.

Hitherto we have only been considering a magnetised needle or bar as insulated; we have recognised in it the property of attracting iron to its poles; that of being directed, when it is movable, by the action of the earth; that, finally, of communicating the virtue it possesses to other steel bars or needles. But magnets possess other properties, which are especially manifested when they are brought near to each other; such, for example, as a mutual attraction or repulsion. If we bring near to the north pole of a magnetised needle, freely suspended, the north pole of another needle or bar held

in the hand, we immediately perceive a very decided repulsion established between the two poles: it is the same if we present the two south poles to each other; these repulsions occur at a distance, and are very energetic. But if to the north pole of the movable needle is presented the south pole of the fixed needle, or to its south pole the north pole of the other, there is no longer a repulsion, but a very strong attraction takes place; the pole of the movable needle rushes to the pole of the fixed needle, and an adherence is made manifest between them, which lasts so long as we do not detach them from each other by a violent effort; and this establishes a very characteristic difference between these attractions and electric attractions, which latter cease at the very moment of contact. The repulsion of the two poles of the same needle upon the two poles of the same name of another needle is manifested in a very sensible manner when we approach parallel to each other two needles, having the north and south direction that is given them by terrestrial magnetism.

Magnetic attractions and repulsions are in no degree influenced by the medium interposed between the two magnets, or between the magnet and the magnetic body, when it is acted upon by the simple attraction exercised by a magnetised body upon one that is not so. These actions occur with the same force, whether the interposed medium be air, vacuum, or any substance whatever. It is only necessary that the medium itself be not magnetic. Thus, a plate of wood, glass, non-magnetic metal, a sheet of paper or cotton, a stratum of liquid or of any gas, the interposition of a flame, in no degree modify this kind of action; distance alone exercises an influence upon its intensity, into the investigation of which we shall very shortly enter.

The experiments that we have been relating, joined to observations relative to the needle, magnetised by the influence of the terrestrial globe, establish the important fact that, even while the two poles, situated at the opposite extremities of a magnetised bar, equally attract iron, they present essential differences in their properties, and a kind of antagonism analogous to that which exists between the two