Lyon found that the dip in the latitude and longitude mentioned, very near the magnetic pole, was 86° 32', and Captain Segelke determined it to be 69° 38' at Woolwich in 1830. According to Captain Sabine, it appears to have been decreasing for the last fifty years at the rate of three minutes annually.

If a magnetised needle freely suspended, and at rest in the magnetic meridian, be drawn any number of degrees from its position, it will make a certain number of oscillations before it resumes its state of rest. The intensity of the magnetic force is determined from these oscillations in the same manner that the intensity of the gravitating and electrical forces are known from the vibrations of the pendulum and the balance of torsion, and in all these cases it is proportional to the square of the number of oscillations performed in a given time; consequently a comparison of the number of vibrations accomplished by the same needle, during the same time, in different parts of the earth's surface, will determine the variations in the magnetic action. By this method Humboldt and Rossel have discovered that the intensity of the magnetic force increases from the equator to the poles, where it is probably at its maximum. It appears to be doubled in the ascent from the equator to the western limits of Baffin's Bay. Accord

ing to the magnetic observations of Professor Hansteen, of Christiania, the magnetic intensity has been decreasing annually at Christiania, London, and Paris, at the rate of its 235th, 725th, and 1020th parts, respectively, which he attributes to the revolution of the Siberian magnetic pole. There is, however, so much uncertainty in the magnetic phenomena of the earth, that the results require to be continually corrected by new observations.

The inventor of the mariner's compass, like most of the early benefactors of mankind, is unknown; it is even doubted which nation first made use of magnetic polarity to determine positions on the surface of the globe; but it is said that a rude form of the compass was invented in Upper Asia, and conveyed thence by the Tartars to China, where the Jesuit missionaries found traces of this instrument having been employed as a guide to land travellers in very remote antiquity. From that the compass spread over the east, and was imported into Europe by the Crusaders, and its construction improved by an artist of Amalfi, on the coast of Calabria. It seems that the Romans and Chinese only employed eight cardinal divisions, which the Germans successively bisected till there were thirty-two, and gave the points the names which they still bear.

The variation of the compass was unknown till Columbus, during his first voyage, observed that the needle declined from the meridian as he advanced across the Atlantic. The dip of the magnetic needle was first noticed by Robert Norman, in the year 1576.

Very delicate experiments have shown that all bodies are more or less susceptible of magnetism. Many of the gems give signs of it; cobalt, titanium, and nickel sometimes even possess the properties of attraction and repulsion; but the magnetic agency is most powerfully developed in iron, and in that particular ore of iron called the loadstone, which consists of the protoxide and the peroxide of iron, together with small portions of silica and alumina. A metal is often susceptible of magnetism if it only contains the 130000th part of its weight of iron, a quantity too small to be detected by any chemical test.

The bodies in question are naturally magnetic, but that property may be imparted by a variety of methods, as by friction with magnetic bodies, or juxtaposition to them, but none is more simple than percussion. A bar of hard steel, held in the direction of the dip, will become a magnet on receiving a few smart blows with a hammer on its upper extremity; and M. Hansteen has ascertained that every substance has magnetic poles

when held in that position, whatever the materials may be of which it is composed.

One of the most distinguishing marks of magnetism is polarity, or the property a magnet possesses, when freely suspended, of spontaneously pointing nearly north and south, and always returning to that position when disturbed. Another property of a magnet is the attraction of unmagnetised iron. Both poles of a magnet attract iron, which in return attracts either pole of the magnet with an equal and contrary force. The magnetic intensity is most powerful at the poles, as may easily be seen by dipping the magnet into iron filings, which will adhere abundantly to each pole, while scarcely any attach themselves to the intermediate parts. The action of the magnet on unmagnetised iron is confined to attraction, whereas the reciprocal agency of magnets is characterized by a repulsive as well as an attractive force, for a north pole repels a north pole, and a south repels a south pole; but a north and a south pole mutually attract one another, which proves that there are two distinct kinds of magnetic forces, directly opposite in their effects, though similar in their mode of action.

Induction is the power which a magnet possesses of exciting temporary or permanent magnetism in such bodies in its vicinity as are capable of

receiving it. By this property the mere approach of a magnet renders iron or steel magnetic, the more powerfully the less the distance. When the north pole of a magnet is brought near to, and in the line with an unmagnetised iron bar, the bar acquires all the properties of a perfect magnet, the end next the north pole of the magnet becomes a south pole, while the remote end becomes a north pole. Exactly the reverse takes place when the south pole is presented to the bar; so that each pole of a magnet induces the opposite polarity in the adjacent end of the bar, and the same polarity in the remote extremity; consequently the nearest extremity of the bar is attracted, and the farther repelled, but as the action is greater on the adjacent than on the distant part, the resulting force is that of attraction. By induction, the iron bar not only acquires polarity, but the power of inducing magnetism in a third body; and although. all these properties vanish from the iron as soon as the magnet is removed, a lasting increase of intensity is generally imparted to the magnet itself by the reaction of the temporary magnetism of the iron. Iron acquires magnetism more rapidly than steel, yet it loses it as quickly on the removal of the magnet, whereas the steel is impressed with a lasting polarity.

A certain time is requisite for the induction of magnetism, and it may be accelerated by anything