were soon completed. They called for the mounting of the guns on a special flat car, the bed of which was not dropped, but which had a heavy under-frame so that no bracing of the car, other than a few light outriggers, would be necessary when the gun fired. This called for the construction of a new mount in which the maximum elevation of 30° was possible, and in which the trunnion pressure was reduced to a value of about 65,000 pounds. In the new mount, the recoil was used to elevate the entire gun up an inclined runway in addition to overcoming the resistance of the hydraulic brake.

Word was received from abroad while these designs for a railway mount were being completed that better use of these seven-inch guns could be made if a suitable field mounting were developed for them, and, as they were navy guns, their operation I would then be entrusted to the marines.

The allied armies required, and needed badly, a major caliber field piece of a range of 20,000 yards or so that was thoroughly mobile, and could travel over any country over which a tractor could go, and yet could be transported from place to place without disassembly. All of the large field guns in use by the British and French, as well as many of the smaller and less powerful howitzers, were so made that in order to transport them for even short distances the entire gun and mount had to be taken apart, the pieces loaded on separate trucks or carriages and transported as separate units, and then reassembled before fire could be recommenced. There was no such thing as a really mobile highpowered large caliber field piece that could be transported to the front on a moment's notice, and would arrive there ready to commence fire on the enemy without several hours hard work of preparation.

Rear Admiral Ralph Earle, chief of the Navy Bureau of Ordnance, recognized this need for a really mobile mount, and in a letter written to the Naval Gun Factory early in March, 1918, pointed out that the design and construction of such a mount for the navy seven-inch gun would mark a distinct advance and would fill a great need, and instructed them to start the work of design.

Work on the design of a mobile field mounting for the seveninch gun was immediately started at the Naval Gun Factory, the date being March 15, 1918.

The Bureau of Ordnance was confronted with a problem which can be outlined about as follows:

A mobile mounting was desired for a seven-inch gun weighing 30,000 pounds-a mount that would allow the gun to be transported as a unit so that no preliminary preparation was necessary before firing. The seven-inch gun was the heaviest and hardest hitting gun for which a mobile field mount of this kind had ever been requested by any nation or army. The mounts were wanted in France before the close of the year 1918-a date less than nine months distant-so the time for design and construction was short.

With these conditions in mind, the gun designers of the Naval Gun Factory proceeded to study existing designs of field artillery that were available.

A brief examination of these designs showed that a wheeled mount for the seven-inch gun was not practicable. The weight of the gun and mount complete would be in the neighborhood of 70,000 pounds, or 35 tons. Assuming a six-foot wheel, and an arc of 25° in contact with the ground at any time, the total bearing surface to carry the load would be about six square feet, giving a ground pressure of about six tons per square foot, or 88 pounds per square inch. This weight was, of course, prohibitive, for the gun when moving over a road would probably leave its path marked by broken roadbed, and further, when the piece left the road and attempted to move over soft ground, there was every reason to believe that it would soon become hopelessly mired. Another doubtful point, also, was whether or not a wheeled mount of the ordinary type would prove satisfactory on firing, as wheeled mounts have a tendency to roll back on firing, and the weight of a large wheeled mount makes accurate aiming a different task.

A wheeled mount was clearly not practicable for the seven-inch gun, so a search was made for something better. At this point the idea of utilizing the principles of the so-called "caterpillar " belt for transporting the mount was suggested. was suggested. Instead of wheels, it was proposed to use a steel frame with rollers carrying a link belt presenting a large flat surface to the ground. Similar material had been used with success on commercial tractors and on adaptations of them, etc., mounting small pieces of artillery, but never on a gun mount for a major caliber gun.

Yet there appeared no practical objection. Sufficient strength could be given to the construction of the tractor belt so that it would withstand the stresses brought to bear, and preliminary calculations showed a ground contact area of 28 square feet would be obtained giving a bearing pressure of about 18 pounds per square inch, which is about one-half of that exerted by a horse's hoof. Designs were accordingly commenced by the Naval Gun Factory on a mount of this type, to carry the seven-inch guns. In order to save time in construction, it was hoped that it might be possible to incorporate into the caterpillar mount for the seven-inch gun a number of parts of the mount used on board ship, particularly the gun slide and the recoil and counter-recoil mechanism. It appeared more logical on considering the question, however, to lengthen the recoil of the gun as much as possible, thus reducing the trunnion pressure when firing and in turn the weight of the mount, than to build a heavy mount capable of standing the shock of firing with the short length of recoil set by conditions on board ship. Further, the counter-recoil mechanism of the marine mount was designed to return the gun to battery at elevations up to 15° only. An elevation of 40° was contemplated in the new land mount, making the design of a new counter-recoil mechanism necessary. It was soon found, therefore, that the designers must work from the ground up-every part of the mount had to be newly designed. Only the gun itself, with its breech mechanism and yoke, could be used of the material taken from the battleships.

Work on the designs was rushed at the Naval Gun Factory, and, in spite of the great pressure of other work, progressed rapidly. Preliminary designs were submitted within a fortnight they called for a mount, the features of which were caterpillar belt wheels, a structural steel carriage, and a gun slide equipped with hydraulic recoil and pneumatic counter-recoil systems. These recoil systems allowed a recoil of 32 inches, which reduced the trunnion pressure from the value of 195,000 pounds in the original navy marine mount, to a value of 120,000 pounds. Although entirely new, and at least a year ahead of the times, the designs were so carefully worked out and the engineering facts were so strikingly presented that immediate approval was given to the Navy Bureau of Ordnance by the chief of Naval Operations to proceed with the details and construction.

On May 25, the designs were pronounced complete, and, shown on 164 separate drawings, were ready for submission to the bidders.

DETAILED DESIGN OF THE SEVEN-INCH CATERPILLAR MOUNT

THE CATERPILLAR WHEELS

The construction of the caterpillar wheels can best be shown by the photograph.

In itself, the wheel consists of an endless belt of cast steel links, connected by hardened pins, each link carrying a corrugated forged steel plate which makes contact with the ground. The plates overlap when horizontal, so that a continuous surface is presented. Detachable grousers are provided to prevent the tread from slipping when the mount encounters soft ground.

The track links run over two large sprocket wheels and II truck and idler wheels as shown, the shafts on which these wheels turn being supported by a steel beam of special design. The sprocket wheels carry but little of the load except when the gun is descending a grade or when the brake is applied to the mount.

For smooth running and reliability, roller bearings are provided in the truck and idler rollers. The ends of the bearing housings are closed by steel plates to prevent the entrance of dirt, sand, etc., when the caterpillar is hauled through mud, sand, and soft earth.

A brake is provided to permit the control of the mount when descending hills, etc., and also to lock the caterpillar in position when the gun is set up for firing. The brake consists of a toggle joint operating on the rim of the sprocket wheel, the tension. applied being controlled by an adjustable spring. This brake, although exceedingly simple, has been very satisfactory in operation in controlling the heavy mount on steep grades and in checking any tendency of the mount to move on firing.

THE AXLE OF THE MOUNT AND THE METHOD OF ATTACHING THE

CATERPILLAR WHEELS

The axle is a steel forging, nine inches in diameter with a five-inch central hole. It is supported in the wheel by a hub bracket, which in turn is carried by the structure of the girder on which the sprocket and truck wheels are mounted. This bracket is held by oscillating bearings and is spring supported so

that the caterpillar may adjust itself to any unevenness in the road when the gun is in motion. The springs are taken up by means of holding down screws when the gun is placed in firing position, in order that the mount may keep steady on the point of aim while firing.

THE GUN CARRIAGE

The carriage which supports the gun is a structural steel framework built up of standard steel shapes. It consists essentially of two main side girders cross-braced at each end to form a single unit with a central well into which the gun recoils.

To permit of accurate aim, a traversing gear is provided at the rear end of the gun carriage. A cast steel plate which rests on the ground is built into the structural work at the rear end of the trail and is held to the trail by clips. A worm shaft operated by ratchet wrenches shifts the trail with reference to the plate and enables the gun to be accurately trained.

The trunnion seats are located at the upper end of the carriage. A cylindrical casting, known as the gun slide, to which the trunnions are attached, carries the gun, according to standard navy practice.

The recoil and counter-recoil mechanisms are attached to the gun slide, operating through pistons attached to the yoke. When