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the magnet, it will incline less and less, and when over e resume the horizontal position. Now move it in the direction of the pole N; the result, as regards inclination or dip, is the same as before; but whereas in the former instance the point tended downwards, now the eye (or south-seeking) end will dip towards the pole N, and the needle will again assume a vertical position immediately above N. We may here conceive the bar-magnet to represent a portion of the earth's surface extending along a given magnetic meridian between the magnetic pole of the northern hemisphere and that of the southern hemisphere.

The earth's action, on a freely suspended needle, is this: at the magnetic pole of the northern hemisphere, the needle stands (or dips) vertically, the north-seeking end downwards; when carried thence in the direction of the magnetic equator, the dip becomes less and less, until, when on that equator, it rests horizontally; proceeding into the other hemisphere, dip commences again, but it is now the south-seeking end that dips, and the inclination continues to increase until, at the magnetic pole of the southern hemisphere, the needle again stands vertically, but with the south-seeking end downwards.

The experiment also illustrates-on the principle that like poles repel, and unlike poles attract, each other-that the part of the earth which resembles in its magnetism the north indicating pole of the compass needle is the southern hemisphere.

But to avoid confusion and to dissipate difficulties in connection with the application of the terms "north" and "south," in so far as they relate to geographical direction and position, and to magnetic poles, the Astronomer Royal judiciously proposed that the magnetism of the end of the magnet which pointed nearly to the geographical north should be called red magnetism, and that of the opposite end should be called blue magnetism, but that these words "red" and "blue" were to convey no meaning except as distinguishing the two ends. On this principle it follows that the magnetism of the northern hemisphere is blue, and that of the southern hemisphere is red; and as blue must repel blue, and red repel red, while blue must attract red and vice verså, the red end of the compass needle, which seeks the blue pole of the northern

magnetic hemisphere, is properly indicated north, since it is chiefly, if not wholly, used for geographical purposes.

As regards the force that directs a magnet to the north or south horizon, it is not simply attraction of one end or the other, nor simply repulsion, but the forces are equal, as much attraction as repulsion.

These experiments relate to the magnetism of permanent magnets and to terrestrial magnetism. There is yet another kind.

8. If a length of soft iron having no magnetism be brought very near to one pole of a magnet, a temporary state of magnetism, of a quality opposite to that of the pole, is induced in it. Thus if A be the bar-magnet, and the soft iron be placed as at B, in the upper part of Fig. 4, then the unmarked end S, that is, the blue pole of the magnet will induce in the soft iron a red pole at n,

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and as a consequence a blue pole will be produced at 8: a second piece of soft iron, as C, will also receive magnetism by induction on being placed contiguous to the piece at B; and so on, through several lengths. When the magnet A is removed all semblance of magnetic property will disappear from the soft iron. But if, as in the lower part of the fig., the magnet A be turned, end for end, and the position of its poles changed, then the marked end N, that is, the red pole of the magnet, will instantly induce, in the ends of the soft iron, poles of a character opposite to those they previously had. You can test this by holding the suspended magnetic needle successively to the outermost ends of the soft iron; and the experiment is an illustration of the effect of

transient induced magnetism; it also explains the previous experi ments (p. 641), for the pole of a magnet, by induction, produces a pole of the opposite quality in the nearest part of the iron, and between unlike poles there is attraction.

You can try the effect, on the suspended needle, of a single long bar of soft iron, or of the poker, when each end of the iron is successively brought close to a pole of the bar-magnet; you will find the magnetic action greater than before, because the same mass of iron divided into several parts generally produces a smaller effect than an undivided mass.

You remember the experiment of the tacks and the two poles of the magnet (p. 641); try it again; this is the result of induction; every tack while so situated has two poles; similarly, by dipping a pole of the magnet among the iron filings, a brush of them is formed at the pole, each particular grain of iron being a temporary magnet, inducing magnetism, and attracting other particles.

9. You can now conceive that on placing two magnets in the same line, but a short distance apart, with unlike poles turned towards each other, if a soft iron rod be laid in the intervening space and in the line of the magnets, then, the rod will be magnetised by induction the red pole of one magnet will produce a blue pole in the contiguous end of the soft iron, and, similarly, the blue pole of the other magnet will produce in the rod a red pole. On turning the rod about its centre, its magnetism will gradually become less and less, until, when at right angles to the line of magnetisation, it will be neutral: if the rod be further turned until the position of the ends has been changed with respect to the magnet, the magnetism of the ends will be the opposite of that they had before. By proper manipulation the earth will do this for us, and show the result on a suspended needle. Thus

If a soft iron rod be presented end ways to the centre of a freely suspended magnetic needle, the direction of the rod being east and west, i.e., at right angles to the needle, and also exactly horizontal, there will be neither attraction nor repulsion. If it be presented endways to either pole of the magnet it will slightly attract that pole, by induction, as just explained. It is indifferent which end of the rod is presented to the magnet. But if the rod be held in a

vertical position, then, on successively raising and depressing it, the end which is downwards will repel the red or N-seeking pole of the magnet and attract the blue pole; but the upper end of the rod will attract the red and repel the blue pole. The soft iron rod has become, in its vertical position, a true magnet, but the magnetism is only transient, for upon inverting the rod the properties of its ends are also inverted, and all magnetism will vanish when it is placed horizontally in an east and west direction, though it will retain a small amount if placed horizontally in a north and south position.

The earth, which is a powerful magnet, has induced this magnetism into the vertical rod. The direction of the magnetic force in London, this year, is the same as if there were a blue pole 184° west of true north, and 674° below the horizon; and also a red pole 184° east of true south and 67° above the horizon: this is the direction of the line of force, or the line of dip, and the more nearly a soft iron rod is held or supported in this direction, the more powerful is its effect on the poles of a magnetic needle.

You can readily try this experiment, and others of a similar character; take the poker, and present it vertically before the ends of the suspended magnetic needle; first the knob, and then the lower point: if, in any of the positions, either end acts sluggishly on the needle, this shows that the iron has not quite lost the magnetism it had acquired in a previous direction; but you can greatly increase and accelerate the magnetic action by sharply striking with a hammer the end of the poker. Thus, with knob uppermost and presented to the N.-seeking (red) end of the needle, strike the knob, and the red pole will be briskly drawn towards it; similarly, strike the lower end of the poker and present it (downwards) to the same pole, and it will now turn away with accelerated speed. Reverse the ends of the poker; owing to the hammering the poles of the needle will not be so speedily acted on as before; but again apply the hammer, and there will be quick repulsion and attraction of the proper poles.

10. Terrestrial magnetic force will temporarily magnetise a sphere of soft iron in the same way as it does a rod or bar. The diameter in the line of dip will be the magnetic axis; the lower and

northern half of the surface will have the same magnetic quality as the N.-seeking end of the needle, red; the upper and southern half will be blue.

It is hoped that you will try these experiments, and vary them in many different ways; you will then be prepared to understand the action of an iron ship on the compass.

(To be continued).

W. H. R.

THE TAY BRIDGE.

HE straightforward and exhaustive report of the Wreck Commissioner on the Tay Bridge disaster, will be accepted by the public generally as correct, although

professional men will probably dispute some of the premises. However this may be, the astounding fact remains that English engineers have, as a body, disregarded wind pressure as a factor in construction, or attached so small a value to it as to render the results worthless. In some measure this has arisen from the erroneous impression that even in large estuaries the heavy squalls only impinged on a small portion of the structure exposed. There cannot be a greater mistake, as anyone may convince himself of if he will take his stand on the shores of the Firths of Tay or Forth during a very heavy gale. At such times the whole surface of the water is covered with spoondrift, demonstrating that throughout the pressure cannot be under 40 lbs. to the square foot, often more. In very narrow gorges the force far exceeds the above figure, but in the Tay the configuration of the land prevents any unusual accumulation over a small space. If the law which the designer of this bridge acted on held good, it would be quite possible for particular ships, when sailing in line of battle, to escape being struck by unusually fierce squalls, although their neighbours might be dismasted. We know that such is not the case, and that the whole are alike exposed to their influence. Although this is well known to every seaman, engineers appear to

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