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consisting of only three essential parts: the disc, hinge, and handle.

The carriage is rigidly built of steel plates, stiffened by angle and channel irons. Two hollow cheek pieces support the trunnion bearings and are mounted on four I-beams that run crosswise and tie them together. The whole box structure is mounted on four castings containing six conical wheels upon which the carriage turns and four wheels with vertical axes which center the carriage in the bed ring. The bed ring, upon which the wheels run, is set deep into the concrete foundation and is secured by twenty-four 2-inch bolts. It contains a circular rack for traversing.

The carriage contains an electric motor with suitable gearing for traversing and elevating. The gunner, who sights the piece, stands upon a platform at the left side of the carriage and controls the electric motor by means of a hand wheel connected with a rheostat located underneath the platform upon which he stands. A handle within reach of the gunner puts the electric motor in gear with the elevating worm or the traversing pinnion by means of friction clutches. Should any accident occur to the electrical system, two crank handles on the right and front side of the carriage are provided for traversing and elevating the piece by hand power.

The compressed air for propelling the projectile is contained in eight wrought iron flasks located in two chambers under the ground, one on each side of the gun platform, four flasks being placed in each chamber. The pipes leading from these flasks up through the carriage to the barrel also form a part of the air reservoir. The air flasks are about twenty-five feet long, sixteen inches outside diameter and nine-sixteenths inches thick. They are made from lapwelded iron plates with a solid head welded into one end, the other end having a flange with a large opening. Four of these air flasks are connected together by a manifold casting and the two manifold castings are connected by long. pipes to an inverted T casting located directly underneath the center of the gun. From this T casting the air is conducted by a pipe, sixteen inches in diameter, up through the carriage to the hollow trunnions. Since the gun must be moved in a horizontal and vertical plane, it is necessary to have three movable joints in the air pipe. Two of these joints are at the trunnions and a third at the center of the racer ring. These movable joints are of large dimensions, sustaining an air pressure of one thousand pounds per square inch and they must allow the gun to

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move without great expenditure of power. It would be difficult to describe their construction without the assistance of detail drawings, but I may say that they are packed with double "cup leathers" that have oil between them, under a pressure somewhat higher than the pressure of the compressed air. This excess of pressure is automatically maintained by supplying the oil from a small differential accummulator located in the carriage. The air pressure forces the plunger of the accummulator down upon the oil, and since the diameter of the upper end of the plunger upon which the air acts is a little larger than the lower end bearing upon the oil, the pressure of the oil per square inch is greater than the pressure of the air. Similar hydraulic packings are used about the main valve.

The valve, already referred to, that controls the escape of air from the reservoir into the barrel is located at the breech. Its operation when started is entirely automatic and it might properly be termed a time valve; that is to say, it can be adjusted to partially or wholly open, and remain open for a definite length of time and then close; thus it is possible to allow five, ten, fifty or any number of cubic feet of air to escape by a simple adjustment. This is analogous to a variable "cut off" in a steam engine. We can set the valve to close when the projectile has traveled one-quarter, one-half, three-quarters or any other fractional part of the length of the barrel. It will be readily understood that this affords a means of changing the range of the projectile without changing the elevation of the piece, for the muzzle velocity depends upon the energy stored in the projectile and this energy depends upon the amount of air that enters the barrel while the projectile is traveling through it. It is, in some respects, analogous to varying the amount of powder used in mortar firing. The adjustment of the valve is indicated by an index wheel located within reach of the gunner and the valve is opened or set in operation by pulling a lever at the left trunnion within reach of the gunner when he stands upon the platform to sight the gun.

The valve is too intricate in its action to be described without drawings and such a description would be out of place in this article; suffice it to say that the large sleeve, or the main valve, covering the ports in the barrel, before mentioned, is moved by unbalancing the pressure on its edge and this is done by a proper movement of a small auxiliary valve. The time of open

ing and closing depends upon the time required for air to flow through a small crifice and fill a chamber of fixed dimensions.

By changing the size of this orifice the time is changed and the size of the orifice is adjusted by a micrometer screw. Turning the index wheel, which can be reached from the platform, turns this micrometer screw and opens or closes the orifice. The operation is termed "setting the valve." The micrometer screw is graduated into 900 equal and arbitrary divisions which are neither directly proportional to the areas of the orifice nor to the losses of pressure, but require calibration experimentally by discharging the gun and determining the loss of pressure corresponding to each division of the micrometer. When this relation is known, it can be represented by a curve plotted with "losses of pressure" as ordinates and "valve settings" as abscissas. Such a curve is shown on Plate I. This relation between the "loss of pressure" and "valve setting" is not constant, but varies with the temperature, the condition of the valve, etc., but it is nearly constant for any single day and it varies only slightly from day to day.

The auxiliary valve interlocks with the breech block by means of a sliding bolt thus making it impossible to discharge the gun when the breech is open.

Should the main valve leak from any cause, such as dirt lodging on its seat, the escaping air would, after the gun is loaded and the breech closed, slowly push the projectile along the bore were it not for an escape valve that is provided to prevent such an accident. This valve is attached to the barrel just forward of the breech block and normally stands open to allow the free escape of any leaking air. When the gun is discharged the sudden rush of air closes this valve until the pressure in the barrel falls nearly to atmospheric when it again opens.

For permanent emplacements a circular track encircles the gun upon which the loading carriage runs. At Shoeburyness only a short arc of this track was built in rear of the gun, raised to the proper level for loading. The gun is always brought to an angle of seven degrees with the horizontal for loading, the height and slope of the loading carriage having been designed for this angle.

A platform fifty feet in length and from six to ten feet in width extended in rear of the gun. Upon the end of the platform, farthest from the gun, were racks for the storage of projectiles and a narrow-gauge track was laid on the platform extending from the loading carriage to the racks with a switch and two branches on either side of the racks. Two loading trays ran up this track conveying the projectiles from the racks to the

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