magnet disturbs a compass, another magnet may be so placed as to neutralize the disturbance. Poles of a horse-shoe magnet.

The experiments proving these general laws are the easiest of all. Turn the red end of a magnet held in the hand towards the red end of a suspended needle or compass-needle, and it repels the needle's red end. In like manner, the blue end of the hand-magnet repels the blue end of the needle. On the contrary, turn the red end of the hand-magnet towards the blue end of the needle, and it attracts the needle's blue end; and in like manner, the blue end of the hand-magnet attracts the red end of the needle.

The same principle may be exhibited in various forms. If the red end of the hand-magnet be pointed, from a distance, at right angles towards the middle of the needle, it attracts the blue end and repels the red end; shewing (in addition to the law which we have before us) that the ends of a magnet can act obliquely: an important remark on which we will speak further. If the hand-magnet be placed, at a distance, with its center in the line of the needle produced, and its direction transversal to that of the needle, it disturbs the needle according to the same law. If the handmagnet be placed with its center vertically above or vertically below the center of the needle, and its direction transversal to that of the needle, the same remark holds. All these experiments lead to the Second Law of Magnetism; that there is repulsion

between magnetisms of similar character and attraction between magnetisms of dissimilar character.

An additional result, of some importance, is gained by holding the hand-magnet in a vertical direction and bringing it sideways towards one end of the needle. It will be found that the energy of the attraction (or repulsion, as the case may be) varies as the handmagnet is moved up and down; and that it is greatest when a part of the hand-magnet near to its end but not at its end (distant from it perhaps by of the magnet's whole length) is nearest to the needle. This suggests the idea that the whole of the magnetism peculiar to that end of the magnet is collected into that one point: and that point is called a "Pole." But in fact it is found that, in varying the experiment, no point can be fixed on as strictly corresponding to this idea of a pole; still the language and the idea are so convenient that we shall make use of them, in general description, and even in some investigations.

It is easily found that the effect of one magnet may be neutralized by that of another magnet. Thus, if one magnet be below the needle, a similar magnet above the needle with its poles in opposite positions will neutralize it. The reader will have no difficulty in varying this experiment, so as to make it applicable to the other cases of magnetic disturbance.

If a horse-shoe magnet be held in a vertical position, and if its ends be separately presented sideways to a suspended magnet, it will be found that they possess

respectively red and blue poles, exactly similar to those of a straight bar-magnet.

11. The action of the Earth is exactly the same as that of a large magnet, whose red end is on the south side and whose blue end is on the north side. For experimental purposes, the Earth's action may be neu-` tralized over a large space. Or its action on a special magnet may be rendered insensible by use of the astatic needle.

In Figure 3, suppose the needle A to be turning freely on a fine point and the magnet B to be delicately suspended above it, both magnets taking the position given to them by the Earth's magnetic power, and therefore parallel, with their red ends pointing to the north. In this state, the needle A is maintained vigor

ously in its position; and, if it is drawn aside for a moment, it returns rapidly to that position. Lower B gradually at a certain elevation of B, the needle A will become indifferent to position, and if drawn aside will not return to its former direction. Lower B still more, and A will reverse its position, its red end pointing to the south, as in Figure 4.

It is evident here that, at the second or intermediate position of B, the action of B is sensibly neutralized by the Earth's action. But, as we have remarked in the last article, the action of B may be neutralized by that of another magnet, at a proper distance, with its red pole to the south. Consequently, the Earth's action is exactly the same as that of a magnet whose red pole is south, and for magnetic

purposes the Earth may be represented by such a magnet.

The importance of this inference for theories of magnetism cannot be over-estimated. It shews, not only that the Earth's red and blue poles must be considered to be on the south and north sides, but also that the quality of the Earth's magnetism is the same as that of a steel magnet.

Advantage may be taken of this principle, in experiments, for removing terrestrial influence. If, as in Figure 5, a large magnet be placed at a proper distance below a table, magnetic experiments may be performed upon that table without disturbance by terrestrial magnetism.

Fig. 5.

S

N

There is however another way of neutralizing the Earth's action, by use of the "astatic needle." This instrument, represented in Figure 6, consists of two needles of equal magnetic power, attached to the same central axis, with their poles in opposite positions. In this state, the action of the Earth on one needle is