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THE "THUNDERER" GUN.

UR naval ordnance is a subject which will engage the early and serious attention of Parliament, and a searching investigation into what is known as the Woolwich system of gun construction is imminent. The public anxiety created by the Thunderer explosion was abated for a time by the reassuring report of the committee of inquiry; but the bursting of Sir William Armstrong's 100-ton gun on board the Italian turret ship Duilio has reawakened fears which were only partially lulled. From the first there have been two

opinions, among men capable of judging, on the subject of our own disaster; some agreeing with the committee that the balance of evidence points to the gun having given way under a double charge, while others believe, with Sir William Palliser, that the fracture was due to causes which are always present and may at any moment develop similar accidents. In the case of the Duilio, it is clear that the system of construction failed; and it will not be forgotten that both Woolwich and Elswick build their heavy ordnance on precisely similar principles, while there has never been a question as to Sir W. Armstrong's excellent workmanship; on the contrary, it is sometimes said that we allow other nations to purchase from him guns which are superior in this respect to those turned out of the national arsenal. After what has occurred on board the two ships, the country will demand that the Woolwich system be put upon its trial; and, in anticipation of a new "battle of the guns," involving larger issues than were at stake when the rival systems of Sir W. Armstrong, Mr. Whitworth, and others were competing for adoption, it may be well shortly to review the story of the Thunderer disaster, taking a preliminary glance at the general principles which govern the construction of modern naval ordnance.

These are fully illustrated by the 38-ton gun, which is built up in the following manner :-A steel tube, 12 inches bore and 16 feet long, is tightly clasped externally by three coiled tubes of wrought iron, each of which occupies about one-third the length of the steel tube. One coil surrounds the muzzle, another the middle of the gun, while a third encircles the breech, and the latter is again clasped

by a second coil having the trunnions forged upon it. The "coils" are so named because they consist of rectangular bars of wrought iron wound into great spirals, which are brought to a welding heat and placed end-up under a powerful steam-hammer, whose blows weld each convolution of the spiral to its neighbour, and thus form a tube. The three tubes are next turned in suitable lathes, and bored to a diameter slightly too small to admit the steel lining, over which, however, they will just slide when expanded by heat, and upon which they shrink with great force on cooling. The second or outer coil is shrunk over the breech-piece, and the gun is completed by rifling the steel tubes with nine grooves. Under the old system, guns were cast either of iron, bronze, or steel, in a single piece; and it is important to understand clearly why the plan of shrinking coils over a central tube has replaced this method. If a thin tube, which would rend easily under the explosion of a charge of powder, is tightly bound around with wire, its resistance to disruption is greatly increased. This is not the case if the wire encircles the tube without strain; but when the wire is already in a condition of tension, it is, so to speak, on its guard, and prepared to aid the tube in resisting the explosive force within.

The coils of a gun reinforce the steel lining in just the same way : and the second coil, embracing the breech-piece, gives additional support in the region of the powder chamber, where the strain is greatest. The proper tensions for distributing the pressure of the powder-gases evenly throughout the material of the gun are ascertained by calculation; and we may regard any piece built on this system as a tube whose walls are in a permanent state of strain, decreasing from the bore outward. A solid gun, on the other hand, may be considered, for the sake of comparison, as a tube similarly composed of annuli, embracing each other without any initial tension; and in view of the theory of reinforcement, to which attention has been called, it is easily conceivable that the bore in such a gun might suffer local disruption before the outer rings of metal-being, so to speak, off their guard-could come to the assistance of the inner layers.

Two kinds of projectiles are used in the 38-ton gun; one, known "common" shell, is made of ordinary cast iron, weighs 600 lbs., and is fired with an 85-pound charge: this shell is too soft to penetrate armour plates. The other-called, after its inventor, the "Palliser" shell-is also made of cast iron, which, being rapidly cooled or "chilled" in the mould, becomes harder than the hardest steel, and is consequently used for piercing plates. This shell weighs

700 lbs., and is fired with a charge of 110 lbs. Both projectiles are furnished with brass studs, which loosely fit the rifle grooves; and we may here call attention to a fact the importance of which will become clear hereafter, that these studs differ slightly in common and Palliser shell, both in their weight and method of fixing.

The object of first importance in constructing heavy ordnance is to obtain a very high velocity in the shot without unduly straining the gun, and this is secured by the use of coarse-grained powder. As every grain of powder burns from without inwards, large cubes consume more slowly than finer particles; with a charge of this kind, therefore, the shot begins to move, and the space within which the gases are confined increases, before the whole is ignited; with a fine-grain charge, on the other hand, the time required for complete combustion is so short that the shot has not moved appreciably before all the powder is burnt. The velocity of a shot at the moment of leaving the muzzle is of course determined by the average pressure which has been exerted upon it throughout the length of the barrel; and it is easy to understand that a quick-burning powder, producing a pressure very great at first but rapidly decreasing, may be inferior in propulsive power to one less initially intense but more continuous in its action.

Hence coarse-grain or "pebble powder" has been universally adopted for heavy ordnance as a means by which the destructive effects of high initial pressures may be avoided without loss of muzzle velocity. The Palliser shell of a 38-ton gun is propelled at the rate of 1,400 feet per second by an average pressure of 5 tons, the maximum strain with "pebble" being 24 tons per square inch on the breech of the gun; whereas, with fine-grain powder and the same velocity, the initial strains reach up to 30 tons. It is in the highest degree important to ascertain accurately what are the pressures exerted in every region of a heavy gun, in order that a suitable provision of metal may be made for meeting the varying strains; and this information is obtained, firstly, by the use of "crusher gauges," and, secondly, by calculations derived from the weight and velocity of the projectile.

Crusher gauges are small cylinders of copper about one-eighth of an inch in diameter and half an inch long. Ten of these cylinders occupy suitable recesses drilled at intervals into the steel lining-tube, where they lie flush with the bore of the gun. Upon firing, the explosion acts on each gauge like the blow of a hammer, compressing it with a force which is proportional to the pressure prevailing in that part of the gun; the soft copper gives way, and the amount of

shortening which it undergoes forms a measure of pressure when the gauges are afterwards compared with similar cylinders whose behaviour under known loads has been experimentally determined.

The weight and velocity of a moving body being given, the propelling force can be calculated, so that the pressure on a shot at any given point in the barrel can be stated if its velocity at that point is known. An ingenious device, known as the "chronoscope," invented by Captain Noble, furnishes the required information, and enables the artillerist to ascertain with extreme accuracy the rate at which a projectile is travelling through any section of the gun. It would detain us too long to describe this apparatus at length; suffice it to say that the shot in its passage cuts successively ten wires which are inserted at as many points in the bore. Each severance breaks an electric circuit, the moment of rupture being instantaneously recorded upon a sheet of paper moving with a high and known velocity. The intervals between each of these records, which are registered by dots on the paper, are measures of time, and indicate the speed of the projectile at ten different points in the barrel. From data thus obtained, diagrams are constructed showing the amount and variation of the strains to which the gun is subjected throughout its whole length; and in this way it was found that the maximum stress on the breech of the Thunderer gun was 24 tons per square inch, dropping rapidly to 5 tons at the centre, and about 2 tons at the muzzle. Under the system of coiled construction, advantage is taken of the information afforded by the chronoscope and crusher gauges to adjust the amount of metal to the strains throughout. Thus, the heavy pressure on the powder chamber is met by clasping the breech by two encircling coils, while the lighter strains about the centre and muzzle are met by thinner and thinner coils, so that a full charge of powder could not be exploded in any part of the gun except the breech without causing its disruption.

Having said so much on the principles which govern the construction of heavy ordnance at Woolwich, we must now pass on to consider the general arrangement of the Thunderer's armament and the means adopted for loading.

The ship has a fore and after turret, the former containing two 38-ton guns-one of which burst-and the latter two 35-ton guns, which are identical with the larger arms in every particular excepting length. Being three feet shorter than the 38-ton guns, the latter can be loaded by hand within the turret ; but the 38-ton guns themselves are too long to be withdrawn sufficiently for this purpose, and need therefore to be charged mechanically from without the turret. After

firing, the gun is retired into the turret, partly by its own recoil and partly by hydraulic power; the muzzle is then depressed, and now looks through a hole immediately below the firing port into the space between decks, where it is envisaged by the hydraulic loading gear. This consists of a cylinder containing a hollow plunger, within which is a second plunger, the whole being like a telescope with three joints. The inner plunger carries the rammer, and the action of the apparatus is as follows: A charge and shell having been placed in the gun, the rammer advances, carrying on its head a papier-mâché wad. Of the two plungers, the larger always starts forward first, because of the greater area which it exposes to the water pressure; when fully extended, the smaller plunger takes up the movement, driving charge, projectile, and wad home.

Want of space alone compels the adoption of this telescopic arrangement of loading gear, which has the singular disadvantage of not permitting the motion of the second plunger to be seen; so that a mechanical indicator is provided to tell when the charge is sent home; and it is important to remember that when the explosion occurred this indicator was injured and out of use. While loading, the gun inclines downward at an angle of about 12 degrees, and the shell would slide out of the gun on the removal of the rammer, but for the papier-mâché wad, which keeps it in place.

We are now in a position to appreciate the occurrences of the disaster. All four guns were loaded with a charge of 100 lbs. of powder and a Palliser shell, those of the after turret receiving no wads, which are unnecessary when guns are loaded in a horizontal position. This broadside was fired electrically, and a misfire undoubtedly occurred in one of the 35-ton guns, for the unexploded charge was subsequently withdrawn. The captain of the vessel, one of the officers, and a sailor, who were watching the firing, stated before the committee that three shots in all came from the two turrets on this broadside. On the other hand, five sailors were confident that only two shots- one from each turret--left the four guns. After the electric broadside, orders were given for each gun to be fired separately, and the charge was now 85 lbs. of powder and an empty common shell, upon firing which the left-hand gun of the fore turret burst.

After a protracted investigation, the committee of inquiry record their belief that a misfire occurred in one of the 38-ton as well as in one of the 35-ton guns during the first, or electrical, broadside; that a second charge was then placed over the first, and that the simultaneous explosion of the two charges destroyed the gun. With