hours for placing the cap sills and the five hours for maneuvering the pile driver must be deducted to find the time that the hoist was in operation driving the hammer and lifting the piles from the bottom of ravine into position in the ways. Thus we get seven hours as the actual time the electric hoist was in operation on the piles. During this time four 15 foot, four 20 foot, four 30 foot, and three 35-foot piles were driven on the average 10 feet into the ground. The first eight piles were driven through sand entirely. the next four through a slight depth of loam, the last three through 2 feet of wet mud and sand. The average time taken to drive a pile was then twenty-eight minutes, and this includes hoisting the pile from the ravine bottom, adjusting its foot at the proper place on the ground and adjusting its cap in the ways. The actual time taken to drive each pile in the shore bent was much less than twenty-eight minutes, and the actual time used in maneuvering the big 35-foot piles and driving was correspondingly more than twenty-eight minutes. No record was kept of the number of blows it required to sink the piles the desired depth. It was attempted to drive all piles to a depth of about 10 feet, and the length of the piles in some cases determined this depth, rather than the failure of the hammer to drive farther. In other cases the driving was stopped short of the normal depth when the hammer with a full drop would not drive the pile an inch. On the first day's work the Dock engine operated well and sufficient power was available. On the second day the magneto weakened and the hammer had to be nursed up to the top of the ways, and the engine rested after each drop in order to get it up to speed again. The rest of the time the engine was operated off the battery and sufficient power was available. The average driving time per pile, twenty-eight minutes, may seem long, especially since the two shore bays were driven with small, easily driven piles, and then, too, it was noticed that with the first two or three drops from 3 to 4 feet were gained at the very start. It may, however, be stated that the pile driver detail, from the officer in charge down, were all green hands at such work, and entirely unskilled in the handling of a hoist, which latter was found to require a surprising amount of care and skill in order to check the hammer rope and electric drum at the right instant. Further. three different details of enlisted men, two from B Company and one from C Company, were used as crews, the first B Company detail erecting the pile driver, excavating the approaches, clearing the slopes, cutting, hauling, and carpentering the piles and sills, and driving the piles the first two days. The second B Company crew was taken out at 5.30 one evening and worked until 10.30 by the light of searchlights. The C Company crew put in a half day's work the last day the pile driver was operated. In view of the inexperienced men handling the work, the troubles with the gas engine operating the generator, and the scattered opportunities for operating the pile driver, due to the absence of the officer in charge and the details at the maneuvers nearly every day, the results that were obtained proved that the pile driver, even under most adverse conditions, can handle heavy piles and drive them into sand with fair speed, and show that under favorable conditions in the field with a mobile army the pile driver will prove a vast improvement on the makeshift hand pile drivers, and average up well with the best portable steam plants of the same capacity. A few minor defects will be taken up in detail. It was found difficult to keep the hoisting rope on the base pulley. This should be remedied by designing a suitable guard. The rope ring at the hammer twisted badly and tended to cut the rope. On the hammer ways are four strap iron hooks forming supports for the hammer guards. Unless the hammer is operated by an expert there is apt to be trouble keeping the slack out of the rope. There Deadmas were no experts at hand to operate the hoist, so the hammer many times was followed by some slack, which several times allowed the rope to get twisted around the guard supports with the result that before the hoist could be stopped the rope broke. The necessity for having the hammer guards placed where they are is apparent, but they should be so designed as not to have sharp angles or edges placed so that a rope might catch on them and be cut. It is therefore recommended for the pile driver: 1. That the bottom pulley be fitted with a suitable guard. 2. That the hoisting rope be fitted with a circular rope guard. 3. That U-shaped supports or staples for the hammer guards replace the present sharp-angled L-shaped hooks. A MILITARY SUSPENSION BRIDGE BY Lieut. C. L. STURDEVANT The accompanying photographs show a suspension bridge constructed during engineer drill periods by the mounted section of Company D, First Battalion of Engineers, assisted, during the latter part of the work, by the bridge section of the same company. The bridge was built on level ground, with a span of 100 feet between towers and carried a 9-foot roadway. Fig. 1 shows the arrangement of the cables, the bridge loaded with 104 men, and the construction of the trusses. The anchors consisted of an inclined spar bearing against two horizontal deadmen. The cable passed between the deadmen and around the spar. The cables used were the -inch wire rope issued to engineer companies. Two cables on each side carried wooden blocks to which were attached the suspenders. The blocks insure an equal division of the load between the two cables and are less injurious to the cable than metal clips. The three blocks nearest the towers were held to their proper places on the cables by auxiliary 3-inch cables, wrapped in succession around the blocks, passed over the tower, and attached to independent holdfasts. The other blocks, having less tendency to slide, were held in place by a single No. 10. wire. The slings consisted of four strands of No. 10 wire, wrapped entirely about the blocks and transoms to develop full strength. The transoms, every 5 feet, were in general 8 by 8 inches. Five balk, lashed at each transom, were used to carry the 11-foot chess of the advance guard ponton train. Diagonal wires crossing between transoms, together with floor stringers, formed a horizontal truss. Side-rail balks and lashings and side trusses prevented vertical oscillations. The trusses were of the Pratt tpye, wires being used for the tension members. Since, when one-half of the bridge is loaded, the cable brings an upward load on the other half of the truss, it is possible to get a reversal of the stress in practically any member. Consequently, the truss should be counter-braced throughout. The lack of this arrangement was plainly shown when the men marched on the bridge. The photograph shows 104 men on the bridge. It had previously been loaded with 26 horses and nearly as many men and, later, a load of stone weighing 5,550 pounds was drawn across by a team of mules weighing 1,960 pounds. The first load produced a deflection at middle of about 18 inches. With the load off, the cables returned to their original position, showing that the anchors were holding rigidly. The load of stone, although producing less deflection, broke three of the wire members of the trusses. The wires, however, had been severely strained by twisting with a rack stick a bad construction. It will be seen from the foregoing that the weight of the bridge is excessive and that the cables form the weakest element. This lack of proportion is due to the fact that the most readily available material was used. Nevertheless, the bridge could be used as follows with a factor of safety in the cables approximating 2:5. By infantry in twos; By cavalry in file; By light artillery, one carriage at a time; By siege artillery, teams to cross and guns to be pulled across with ropes. It may occur, in the mountainous country, that a bridge is desired at a place reached by trail only-one, perhaps, to enable mountain artillery to reach a commanding position. Other similar conditions may arise where pack transportation must be relied upon. Under such circumstances, the engineers could readily organize a pack bridge train from the pack train as now prescribed. The mounted section would usually furnish the personnel. The carpenter, pioneer, and demolition packs would not be changed. The supply packs would be altered to suit the particular conditions. and would carry, among other things, lashings, nails, metal clips. thimbles, and staples. Two or more supply packs might be necessary, depending upon the size of the bridge contemplated. To the above would, then, be added cable mules" and "wire mules." A fair-sized mule will carry 300 feet of g-inch cable. Fig. 2 shows a cable mule." The ordinary diamond-hitch was used, and no difficulty was experienced in transporting the pack. |