point at right angles to the direction in which a long thin magnet hung by a single silk fibre there places itself. One of these magnets is placed, as shown in Fig. 1, with its length in that line, and at such a distance that a convenient deflection of the needle is produced. This deflection is noted and the deflecting magnet turned end for end, and the deflection again noted. Make in the same way a pair of observations with the magnet at the same distance on the opposite side of the magnetometer, and take the mean of all the observations. These deflections from zero ought to be as nearly as may be the same, and if the magnet is properly placed, they will exactly agree; but the effect of a slight error in placing the magnet will be nearly eliminated by taking the mean of all the deflections as the deflection of the magnet for that position. The exact distance in cms. of the centre of the deflecting magnet from the mirror is also noted. The same operation is gone through for each of the magnets, which are carefully kept apart from one another during the experiments. The results of each of these experiments give an equation involving the ratio of the magnetic moment of the magnet to the value of H. Thus if m denote the magnetic moment of the magnet, m' the magnetic moment of the needle, 2 r the distance of the centre of the magnet from the centre of the needle, 2 / the distance between the poles of the magnet which, for a uniformly magnetised magnet of the dimensions stated

m (22 73)2 H

2r

tan

.. (1) The angle is to be measured thus:-The number of divisions of the scale which measures the deflection divided by the number of such divisions in the distance of the scale from the mirror, is, if the scale is placed

* The convention according to which magnetic polarity of the same kind as that of the earth's northern regions is called blue, and magnetic polarity of the same kind as that of the earth's southern regions is called red, is here dopted. The letters B, R, b, r in the diagrams denote blue and red.

as described above in the magnetic north and south line, equal to tan 20.

2

I

Instead of in the east and west horizontal line through the centre of the needle, the magnet may be placed, as represented in Fig. 2, with its length east and west, and its centre in the horizontal north and south line through the centre of the needle. If we take m, m', l, l', and r to have the same meaning as before, we have for the distance of either pole of the magnet from the needle, the expression, Let us consider the force acting on one pole, say the red pole of the needle. The red pole of the magnet exerts on it a repulsive force, and the blue pole an attractive force. Each of these forces has the value m m' 21° 21' ' z2 + 12° But the diagram shows that they are equivalent to a single force, F, in a line parallel to the magnet, tending to pull the red pole of the needle towards the left. The magnitude of this resultant force is plainly m m' In the same way 21°21'' (zi2 + 1"): 21' (x2 + 12)?" it can be shown that the action of the magnet on the red pole of the needle is a force of the same amount tending to pull the blue pole of the needle towards the right. The needle is, therefore, subject to no force tending to produce motion of translation, but simply to a "couple" tending to produce rotation. The magnitude of this couple when the needle has been turned through an angle mm' 21' cos 0 e, is 21 (r2 + 12) ??

2

or

or

m m'

m m'

(y2+12) ?

cos 8. If there be equilibrium for the deflection 6, this couple must be balanced by that due to the earth's horizontal force, which, as before, has the value m' H sin 0. Hence equating these two couples we have

Still another position of the deflecting magnet relatively to the needle may be found a convenient one to adopt. The magnet may be placed still in the east and west line, but with its centre vertically above the centre of the needle. The couple in this case also is given by the formula just found, in which the symbols have the same meaning as before.

The greatest care should be taken in all these experiments, as well as in those which follow, to make sure that there is no movable iron in the vicinity, and the instruments and magnets should be kept at a distance from any iron nails or bolts there may be in the tables on which they are placed.

We come now to the second operation, the determination of the period of oscillation of the deflecting magnet when under the influence of the earth's horizontal force alone. The magnet is hung in a horizontal position in a double loop formed at the lower end of a single fibre of unspun silk, attached by its upper end to the roof of a closed chamber. A box about 30 cms. high and 15 cms. wide, having one pair of opposite sides, the bottom and the roof made of wood, and the remaining two sides made of plates of glass, one of which can be slided out to give access to the inside of the chamber, answers very well. The fibre may be attached at the top to a horizontal wire which can be turned round from the outside so as to wind up or let down the fibre when necessary. The suspension-fibre is so placed that two vertical scratches, made along the glass sides of the box, are in the same plane with the magnet when the magnet is placed in its sling, and the box is turned round until the magnet is at right angles to the glass sides. A paper screen with a small hole in it is then set up at a little distance in such a position that the hole is in line with the magnet, and therefore in the same plane as the scratches. The magnetometer should be removed from its stand and this box and suspended needle put in its place. If the magnet be now

I

(To be continued.)

THE ITALIAN EXPLORATION OF THE MEDITERRANEAN

BELIEVE it will interest the numerous readers of NATURE, especially those who have studied the important subject of the deep-sea fauna, or who are geologists, to learn that the further exploration of the Mediterranean this year, on the part of the Italian Government, has not been fruitless, although it has been short. I have just received a letter from Prof. Giglioli, of Florence, the purport of which I will, with his permission, now give :

It seems that this summer the surveying-vessel, Washington, had to undertake a search (which proved unsuccessful) for some imaginary coral-banks in the shallow sea between Sicily and Africa, besides her usual hydrographical work, and that consequently very little time could be devoted to deep-sea exploration. However, Prof. Giglioli was allowed to accompany the hydrographer, Capt. Magnaghi, with the chance of taking any favourable opportunity that might occur. He thus got three deepsea hauls the first near Marittimo, in 718 metres, or about 389 fathoms; the second, half-way between Sicily and Sardinia (lat. 38° 38′ N., long. 10° 40' E.), at a depth of 1583 metres, or about 857 fathoms, when a very rare and peculiar abyssal fish (Paralepis cuvieri), was obtained. That day (August 15) was also appropriated to hydrographical researches, and particularly to the successful trial of Capt. Magnaghi's new water-bottle, as well as to the marvellous work of his new currentometer, a most valuable discovery, by means of which the direction and force of sub-marinè currents can be accurately determined at any depth. A large new trawl was used, and brought up a block of newly-for ned limestone, which had been hardened with recent shells of Pteropods embedded in its mass. The third and last deep-sea dredging was made on September 1, between Tavolara in Sardinia, and Montecristo, in 904 inetres, or about 490 fathoms, with indifferent results. He will send me the shells for examination. The Italian Ministry have promised him that a whole month next year will be allowed for deepsea exploration. J. CWYN JEFFREYS

IT has been necessary to dwell thus at length on the hoop method of construction in order to contrast it with the wire system, which we now proceed to describe.

A wire gun consists first of an internal tube, the function of which is to contain the rifling, and to transmit the internal pressure to the wire which is coiled upon it, and which gives the strength. This tube no doubt has a certain amount of strength of its own, but this is not its real function. The gun may be so designed and constructed that the tube is never in a state of tension. It may therefore be made, and possibly with advantage, of hard cast iron. In the 3 inch breech-loading gun made by the writer in 1860, the tube was of cast-iron inch thick, and this gun has been severely tested without injury. Hard cast-iron possesses many advantages, and amongst others that of great economy as compared with the steel tubes now generally used; but whatever be the 1 Continued from p. 14.

material of the tube, its principal function is to contain the rifling and transmit the strain to the wires coiled around it.

Upon the inner tube is wound steel wire, square or rectangular in section. The tube is mounted in a machine similar to a lathe, and the wire is coiled upon one or more cylindrical drums, which are fixed horizontally on axes parallel to the tube and provided with proper apparatus for regulating the feed and tension. The tensions having been first calculated, the coiling begins from the breechend where the end of the wire is made fast. When the muzzle end is reached the wire is coiled back again to the breech, and this process goes on till the whole of the coils are in place. The end of the wire is then made fast, and the gun, so far as strength to resist a bursting strain, which is called circumferential strength, is concerned, is complete.

Before proceeding to show how the longitudinal strength the substitution of coils of wire for the hoops above is provided for, it will be well to devote a little time to described, pointing out as we go along the superiority of the wire system. It has already been shown how important it is in the hoop system that the initial tensions

of each hoop should be accurately calculated and applied. This is no less necessary with the wire coils, and it would at first appear that this must involve very intricate and tedious calculation. In the case of the gun represented in Diagram C, it was stated that the same strength which was given by 4 coils of steel, making with the tube a total thickness of 22 inches, might be obtained by 6 inches of wire, but supposing the wire to be 18th inch square in section there would be required no less than 67 different coils and tensions, and as it is desirable to use even smaller wire for the first portion of the coils, there would probably be not less than 80 or 90 coils and the same number of tensions to be calculated. A formula has, however, been found which makes these calculations comparatively simple. In order to make this intelligible we must resort to another diagram, E, which represents the state of strain of the interior of a wire gun, or rather of the wire portion of it, on which alone we depend for circumferential strength. Assuming the wire to be very small, say th of an inch square in section, the strains are represented very nearly by the curved line BN M. The coils between the inner circumference, i.e. the first coil, and the point N are all in compression, the maximum being at C; at N is the neutral point, when the wire is neither in compression nor tension; and from N to F all the coils are in tension, the maximum being at F.

Now if we consider the case of any one coil, such as that at the position K, we see that when the gun is completed it is under considerable compression, but whilst the construction is proceeding, when the coil at this point is being laid on, it is laid on under tension, which tension is reduced by every successive additional coil until it attains the state of compression shown in the diagram of the finished structure. The question therefore to be solved is this, What is the proper tension for putting on the coil at K, so that when all the other coils are put on, it may be in the required state of compression? This problem must be solved for every individual coil. This having been done, each coil is laid on by automatic machinery with its proper tension, and the final result is that shown in the diagram.

When the full internal pressure of the explosion operates, the result is as follows:-Every coil is brought up to the same tension simultaneously and exerts the same resistance per square inch of section throughout the whole thickness of the gun as denoted by the line H O.

, 1882 The ultimate strength therefore of such a gun increases in the simple ratio of the number of coils, a result not attainable by any other mode of construction, and this is the first advantage over the hoop system. The second is, that there is no fear of error through inaccurate workmanship or unequal shrinkage. The tensions of the wire coils are actually measured by the machine by which they are laid on, instead of being inferred from presumed accuracy of workmanship or uniform shrinking power of the material. In the next place there is no danger from latent defects. The wire is not subject to such defects as thick hoops are, and can moreover be easily tested before it is applied. Then again the process of construction is simple and expeditious, it is the substitution of accurate automatic machinery for very highly skilled labour. Beyond this it is much less costly, for although the wire itself costs a high price per ton as compared with the raw material used in the hoops of the Woolwich guns, yet when the labour and work in the latter is taken into account, it will be found that it largely exceeds that of

the wire gun ton for ton, and as was before pointed out, the wire gun of equal strength can be made very much lighter.

In a paper read before the Institution of Civil Engineers in 1879 the writer estimated the cost of a muzzleloading 20 inch gun weighing 150 tons, constructed on the wire principle, at £5,041, or £33 16s. per ton. We believe that the price paid by Government to Sir Wm. Armstrong for the 100-ton guns produced from his firm was £16,000 each, or £160 per ton.

We now proceed to the question of longitudinal strength. In the old guns, as well as the present Woolwich guns, the Armstrong, Whitworth, and Krupp guns, the longitudinal strain between the breech and the trunnions is borne by the chase of the gun itself, that is to say, that the same material which has to resist the enormous circumferential strain bas at the same time to resist a very intense longitudinal strain. Now it has been generally maintained that although this is very large in the gross, yet when it is divided by the sectional arm of the chase, it is comparatively small per square inch of section. This

is a very great mistake as was pointed out several years ago by the late Sir William Palliser. The fact is, that this strain is no more uniformly divided over the sectional area of the chase than is the circumferential strain between the inner and outer circumferences.

Sir Wm. Palliser devised a method of breech construction which has since been adopted at Woolwich, by means of which the longitudinal strain is much more equally distributed, and since then the accident of a breech blowing out has been comparatively rare, and we believe has never occurred in Sir Wm. Palliser's own guns. It has always been a difficulty with many people to understand how the breech is to be secured in a wire gun. It is obvious that the coils of wire afford no longitudinal strength, and the general idea has been that it was therefore necessary to resist the whole longitudinal strain by the inner tube.

The writer has always maintained that no real difficulty exists, and that the connection between the breech and the trunnions should be by means of material quite independent of and placed outside of the coils of wire.