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point at right angles to the direction in which a long thin as described above in the magnetic north and south line, magnet hung by a single silk fibre there places itself. equal to tan 2 . One of these magnets is placed, as shown in Fig. 1, with
Instead of in the east and west horizontal line through its length in that line, and at such a distance that a the centre of the needle, the magnet may be placed, as convenient deflection of the needle is produced. This represented in Fig. 2, with its length east and west, and its deflection is noted and the deflecting magnet turned end centre in the horizontal north and south line through the for end, and the defection again noted. Make in the centre of the needle. If we take m, m', 1,1', and r to have same way a pair of observations with the magnet at the the same meaning as before, we have for the distance of same distance on the opposite side of the magnetometer,
either pole of the magnet from the needle, the expression, and take the mean of all the observations. These de- a që + 7. Let 11s consider the force acting on one pole, flections from zero ought to be as nearly as may be the say the red pole of the needle. The red pole of the same, and if the magnet is properly placed, they will magnet exerts
on it a repulsive force, and the blue pole an exactly agree; but the effect of a slight error in placing attractive force. Each of these forces has the value the magnet will be nearly eliminated by taking the mean
m' of all the deflections as the deflection of the magnet for ' 21' 21" g2 + 12°
But the diagram shows that they are that position. The exact distance in cms. of the centre of the deflecting magnet from the mirror is also noted. equivalent to a single
force, F, in a line parallel to the magThe same operation is gone through for each of the left.
net, tending to pull the red pole of the needle towards the magnets, which are carefully kept apart from one another
The magnitude of this resultant force is plainly
m' during the experiments. The results of each of these
In the same way experiments give an equation involving the ratio of the
27' 21' (que + 1) 2 l' (7+ 1%) magnetic moment of the magnet to the value of H. Thus
it can be shown that the action of the magnet on the if m denote the magnetic moment of the magnet
, m' the red pole of the needle is a force of the same amount magnetic moment of the needle, 2 r the distance of the tending to pull the blue pole of the needle towards the right. centre of the magnet from the centre of the needle, 21 the The needle is, therefore, subject to no force tending to distance between the poles of the magnet which, for a
produce motion of translation, but simply to a “couple" uniformly magnetised magnet of the dimensions stated tending to produce rotation. The magnitude of this
couple when the needle has been turned through an angle m m' 21' cos
m m' 0, is
If there be 2ľ (" + 1")?'
equilibrium for the deflection 0, this couple must be B
balanced by that due to the earth's horizontal force,
(2) above is nearly enough equal to its length, and 2l the
H distance between the poles of the needle, r, l, and l Still another position of the deflecting magnet relatively being all measured in cms., we have for the repulsive force to the needle may be found a convenient one to adopt. (denoted by F in Fig. 1) exerted on the blue pole of the The magnet may be placed still in the east and west line, needle by the blue pole of the magnet, supposed nearest to but with its centre vertically above the centre of the needle. the needle, as in Fig. 1, the value of
The couple in this case also is given by the formula just
21. 21. Tv-1)7 found, in which the symbols have the same meaning as since the value of l' is small compared with l. Similarly
before. for the attraction exerted on the same pole of the needle The greatest care should be taken in all these experiby the red pole of the magnet, we have the expression ments, as weil as in those which follow, to make sure that
there is no movable iron in the vicinity, and the instru21 Hence the total repulsive force exerted
ments and magnets should be kept at a distance from any by the magnet on the blue pole of the needle is
iron nails or bolts there may be in the tables on which
they are placed. mm'
We come now to the second operation, the determina(22 - 19)
tion of the period of oscillation of the deflecting magnet
when under the influence of the earth's horizontal force Proceeding in a precisely similar manner, we find
alone. The magnet is hung in a horizontal position in a that the magnet m exerts an attractive force equal to
double loop formed at the lower end of a single fibre of on the red pole of the magnet. The needle unspun silk, attached by its upper end to the roof of a
closed chamber. A box about 30 cms. high and 15 cms. is therefore acted on by a couple which tends to turn it wide, having one pair of opposite sides, the bottom and the round the suspending fibre as an axis, and the amount of roof made of wood, and the remaining two sides made of this couple, when the angle of deflection is 8, is plainly plates of glass, one of which can be slided out to give access equal to mm' cos 0. But for equilibrium this
to the inside of the chamber, answers very well. The fibre (r* - 1)
may be attached at the top to a horizontal wire which can couple must be balanced by m' H sin 0; hence we have be turned round from the outside so as to wind up or let the equation :
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 The angle 8 is to be measured thus :--The number of
box is turned round until the magnet is at right angles to divisions of the scale which measures the deflection
the glass sides. A paper screen with a small hole in it is divided by the number of such divisions in the distance
then set up at a little distance in such a position that the of the scale from the mirror, is, if the scale is placed
hole is in line with the magnet, and therefore in the "The convention according to which magnetic polarity of the same kind same plane as the scratches. The magnetometer as that of the earth's northern regions is called blue, and magnetic polarity should be removed from its stand and this box and susof 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. pended needle put in its place. If the magnet be now:
0 = A
= 2 TT
m H = 47° 12 W
deflected from its position of equilibrium and then allowed For a small angular deflection o of the vibrating to vibrate round a vertical axis, it will be seen through magnet from the position of the equilibrium the equation the small hole to pass and re-pass the nearer scratch, of motion is and an observer keeping his eye in the same plane as
do o the scratches can easily tell without sensible error the
f instant when the magnet passes through the position of equilibrium. Or, a line may be drawn across the bottom
where w is the moment of inertia of the vibrating magnet of the box so as to join the two scratches, and the ob- round an axis through its centre at right angles to its server keeping his eye above the magnet and in the piane | length. The solution of this equation is of the scratches notes the instant when the magnet going
B in the proper direction is just parallel to the horizontal line. The operator should deflect the magnet by bring; and therefore for the period of oscillation T we have ing a small magnet near to it, taking care to keep the small deflecting magnet always as nearly as may be with its length
T= in an east and west line passing through the centre of
H Н the suspended magnet. If this precaution be neglected Hence we have the magnet may acquire a pendulum motion about the
4 πμ point of suspension, which will interfere with the vibra
T? tory motion in the horizontal plane. When the magnet | Now, since the thickness of the magnet is small compared has been properly deflected and left to itself, its range
12 of motion should be allowed to diminish 'to about with its length, if W be the mass of the magnet u is w,
3 3o on either side of the position of equilibrium be
and therefore fore observation of its period is begun. When the amplitude has become sufficiently small, the person ob
(3) serving the magnet says sharply the word “Now," when
combining this with the equation (1) already found we
8 7 ? 12 W H2
(5) 3 T? (y2 – 7?)tan If either of the other two arrangements be chosen we have from equations (2) and (3) mn
(7° + 72)! W tan 8 . (6)
the oscillation experiments; the frictional resistance of the nearer pole of the magnet is seen to pass the plane the air to the motion of the magnet ; the virtual increase of the scratches in either direction, and another ob- of inertia of the magnet due to motion of the air in the server notes the time on a watch having a seconds chamber ; and the effect of induction in altering the mohand. With a good watch having a centre seconds ment of the magnet. The correction for an arc of osciihand moving round a dial divided into quarter-seconds, lation of 6° is a diminution of the observed value of T of the instant of time can be determined with greater accu
only od per cent, and for an arc of 10° of my per cent. racy in this way than by means of any of the usual of the other corrections the last is no doubt the most appliances for starting and stopping watches, or for regis- important; but even its amount for a magnet of glasstering on a dial the position of a seconds hand when a hard steel, nearly saturated with magnetism, and in a spring is pressed by the observer. The person observing field so feeble as that of the earth, must be very small. the magnet again calls out “ Now" when the magnet has The deflection-experiments are, as stated above, to be just made ten complete to and fro vibrations, again after performed with several magnets, and when the period of twenty complete vibrations, and, if the amplitude of vibra. oscillation of each of these has been determined, the magnetion has not become two small, again after thirty; and the tometer should be replaced on its stand, and the deflection other observer at each instant notes the time by the watch. experiments repeated, to make sure that the magnets have By a complete vibration is here meant the motion of the not changed in strength in the mean time. The length of magnet from the instant when it passes through the position each magnet is then to be accurately determined in centiof equilibrium in either direction,
until it next passes through metres, and its weight in grammes; and from these data the position of equilibrium going in the same direction. and the results of the experiments, the values of m and of The observers then change places and repeat the same
H can be found for each magnet by the formulas investioperations. In this way a very near approach to the true gated above. Equation (5) is to be used in the calculaperiod is obtained by taking the mean of the results of a tion of H when the arrangements of magnetometer and sufficient number of observations, and from this the value deflecting magnet, shown in Fig. 1, is adopted, equation of the product of m and H can be calculated.
(7), when that shown in Fig. 2 is adopted.
20 TONS PER SQ. INCH. TENSION.
The object of performing the experiments with several material of the tube, its principal function is to contain magnets, is to eliminate as far as possible, errors in the the rifling and transmit the strain to the wires coiled determination of weight and length. The mean of the around it. values of H, found for the several magnets, is to be taken Upon the inner tube is wound steel wire, square or as the value of H at the place of the magnetometer. We rectangular in section. The tube is mounted in a machine have now to apply this value to the measurement of similar to a lathe, and the wire is coiled upon one or more currents.
ANDREW GRAY cylindrical drums, which are fixed horizontally on axes (To be continued.)
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 breechTHE ITALIAN EXPLORATION OF THE
end where the end of the wire is made fast. When the MEDITERRANEAN
muzzle end is reached the wire is coiled back again to the I BELIEVE it will interest the numerous readers of breech, and this process goes on till the whole of the coils NATURE, especially those who have studied the the gun, so far as strength to resist a bursting strain,
are in place. The end of the wire is then made fast, and important subject of the deep-sea fauna, or who are
which is called circumferential strength, is concerned, is geologists, to learn that the further exploration of the
complete. Mediterranean this year, on the part of the Italian
Before proceeding to show how the longitudinal strength Government, has not been fruitless, although it has been
is provided for, it will be well to devote a little time to short. I have just received a letter from Prof. Giglioli,
the substitution of coils of wire for the hoops above of Florence, the purport of which I will, with his permis
described, pointing out as we go along the superiority of sion, now give :
the wire system. It has already been shown how imIt seems that this summer the surveying.vessel, Wash
portant it is in the hoop system that the initial tensions ington, 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-marine currents can be accurately deter nined 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,
E with indifferent results. He will send me the shells for examination. The Italian Ministry have promised him of each hoop should be accurately calculated and applied. that a whole month next year will be allowed for deep- This is no less necessary with the wire coils, and it would sea exploration.
J. Cwyn JEFFREYS 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 WIRE GUNS 1
was given by 4 coils of steel, making with the tube a total
thickness of 224 inches, might be obtained by 67 inches II.
of wire, but supposing the wire to be ifth inch square in It
section there would be required no less than 67 differen: hoop method of construction in order to contrast it coils and tensions, and as it is desirable to use even smaller with the wire system, which we now proceed to describe. wire for the first portion of the coils, there would probably
A wire gun consists first of an internal tube, the function be not less than 8o or 90 coils and the same number of of which is to contain the rifling, and to transmit the tensions to be calculated. A formula has, however, been internal pressure to the wire which is coiled upon it, and found which makes these calculations comparatively which gives the strength. This tube no doubt has a simple. In order to make this intelligible we must resort certain amount of strength of its own, but this is not its to another diagram, E, which represents the state of real function. The gun may be so designed and con- strain of the interior of a wire gun, or rather of the wire structed that the tube is never in a state of tension. It portion of it, on which alone we depend for circumferential may therefore be made, and possibly with advantage, of strength. Assuming the wire to be very small, say noth hard cast iron. In the 3 inch breech-loading gun made of an inch square in section, the strains are represented by the writer in 1860, the tube was of cast-iron } inch very nearly by the curved line B NM. The coils between thick, and this gun has been severely tested without the inner circumference, i.e. the first coil, and the point n injury. Hard cast-iron possesses many advantages, and are all in compression, the maximum being at c; at n is amongst others that of great economy as compared with the neutral point, when the wire is neither in compression the steel tubes now generally used ; but whatever be the nor tension; and from N to F all the coils are in tension, i Continued from p. 14
the maximum being at F.
15 TONS PER SQ.INCH. COMPRESSION
Now if we consider the case of any one coil, such as The ultimate strength therefore of such a gun increases that at the position K, we see that when the gun is com- in the simple ratio of the number of coils, a result not pleted it is under considerable compression, but whilst the attainable by any other mode of construction, and this is construction is proceeding, when the coil at this point is the first advantage over the hoop system. The second being laid on, it is laid on under tension, which tension is is, that there is no fear of error through inaccurate work. reduced by every successive additional coil until it manship or unequal shrinkage. The tensions of the wire attains the state of compression shown in the diagram of coils are actually measured by the machine by which they the finished structure. The question therefore to be are laid on, instead of being inferred from presumed solved is this, What is the proper tension for putting on accuracy of workmanship or uniform shrinking power of the coil at K, so that when all the other coils are put on, it the material. In the next place there is no danger from may be in the required state of compression? This latent defects. The wire is not subject to such defects as problem must be solved for every individual coil. This thick hoops are, and can moreover be easily tested before having been done, each coil is laid on by automatic it is applied. Then again the process of construction is machinery with its proper tension, and the final result is simple and expeditious, it is the substitution of accurate that shown in the diagram.
automatic machinery for very, highly skilled labour. When the full internal pressure of the explosion Beyond this it is much less costly, for although the wire operates, the result is as follows :-Every coil is brought itself costs a high price per ton as compared with the up to the same tension simultaneously and exerts the raw material used in the hoops of the Woolwich guns, yet same resistance per square inch of section throughout the when the labour and work in the latter is taken into whole thickness of the gun as denoted by the line 0. account, it will be found that it largely exceeds that of
the wire gun ton for ton, and as was before pointed out, , is a very great mistake as was pointed out several years the wire gun of equal strength can be made very much ago by the late Sir William Palliser. The fact is, that lighter.
this strain is no more uniformly divided over the sectional In a paper read before the Institution of Civil Engineers area of the chase than is the circumferential strain between in 1879 the writer estimated the cost of a muzzle- the inner and outer circumferences. loading 20 inch gun weighing 150 tons, constructed on Sir Wm. Palliser devised a method of breech constructhe wire principle, at £5,041, or £33 16s. per ton. We tion which has since been adopted at Woolwich, by means believe that the price paid by Government to Sir Wm. of wbich the longitudinal strain is much more equally Armstrong for the 100-ton guns produced from his firm distributed, and since then the accident of a breech blowwas £16,000 each, or £160 per ton.
ing out has been comparatively rare, and we believe has We now proceed to the question of longitudinal strength. never occurred in Sir Wm. Palliser's own guns. It has In the old guns, as well as the present Woolwich guns, always been a difficulty with many people to understand the Armstrong, Whitworth, and Krupp guns, the longi- how the breech is to be secured in a wire gun. It is tudinal strain between the breech and the trunnions is obvious that the coils of wire afford no longitudinal borne by the chase of the gun itself, that is to say, that strength, and the general idea has been that it was therethe same material which has to resist the enormous cir- fore necessary to resist the whole longitudinal strain by cumferential strain bas at the same time to resist a very the inner tube. intense longitudinal strain. Now it has been generaily The writer has always maintained that no real diffimaintained that although this is very large in the gross, culty exists, and that the connection between the breech yet when it is divided by the sectional arm of the chase, and the trunnions should be by means of material quite it is comparatively small per square inch of section. This independent of and placed outside of the coils of wire.
It was in this way that his gun made in 1860 was con- admit of a doubt that this is far preferable to subjecting structed, and we believe the same principle has been the same material to two strains at right angles to each adopted by Capt. Schultz in the wire guns built under his other at one and the same time. directions by the French Government. Thus the circum Another objection has been taken to wire guns, and it ferential strain is provided for by one portion of material, is this. It is well known that guns become heated by and the longitudinal strain by another, and it does not firing, and it is thought that this heating would disturb
the tensions to such an extent as to render all the calcu- | heating of guns depends chiefly on the heat absorbed by lations of strain useless. Now if this be an objection, it the metal from the powder gases. Though this heat is applies with far greater force to the system of hoop con very intense, its application is for a very small fraction of struction than to that of wire, but as there is much mis- a second, and it may be shown that in this very short conception on this point it is desirable to say a few words time only a small amount of heat can be absorbed by about it.
the surface of the gun exposed to it.
It may further In the first place, it is a mistake to suppose that the be shown, that during the very short time the heat is
applied, it can only be transmitted by internal conduction | the gun, the internal pressure is removed, the mechanical to a very small depth into the metal of the gun. But as energy is thus given back, but as it does no external work, guns do beat by firing, how is this to be accounted for ? it appears in the form of heat, which remains in the metal
The reason seems to be the following. By the explosion of the gun, until it is dissipated by convection through the of the powder, a considerable amount of mechanical surrounding air. energy is absorbed in expanding the gun against the We are quite aware that this explanation does not elastic form of the material. When the projectile leaves agree with the views of some physicists of great reputa