interior cells, which at the bottom will be air chambers to assist in their manipulation, and at the top, water chambers, to increase their weight as the water rises in the locks. The sheathing is with steel plates riveted on heavy girders. These gates will be opened and shut, to permit the entrance or egress of ships, by electrical apparatus.

As 95 per cent of the vessels in the world are less than 600 feet long, it would be a great waste of water and time to use the full 1,000-foot lock in each passage. So intermediate gates are being constructed which will permit the use of only 400 or 600 feet as the particular vessel may require. There are recesses in the lock walls which allow the gates to be opened and still leave a clear width of 110 feet. At the entrance of the locks, a chain, with links three inches in diameter, stretches from one side to the other to stop vessels which might not obey the signals. In case the first gates should be rammed and broken, a second set of gates especially provided for emergencies have been constructed behind the first set. If both sets of gates should be demolished, the water would rush through with a fearful velocity, but provision has been made against this contingency by having in readiness emergency dams, which would be swung out over the lock and forced down through the inrushing water. This dam, built of steel, is open at the bottom and steel plates are then shoved down it, gradually closing the openings until the flow is stopped. A floating caisson would then be placed in position and sunk, completely shutting out water from

the lock, the emergency dam would be raised, and repairs begun.

It is to prevent such accidents that the plan of towing vessels through the locks with electric locomotives was adopted, as then no misunderstanding of signals from the captain to the engineer of a ship could result. The tracks for these locomotives are on each side wall of the locks, and two will fasten to the rear and two to the front of a ship to effect a passage. If all twelve locks were joined end to end they would make a canyon nearly three miles long, 110 feet wide and 80 feet deep.

THE GATUN Dam

The natural topography of the Isthmus at Panama permitted the Chagres River to escape into the Caribbean Sea through a break in the mountains at Gatun. Engineers logically considered that this was the point at which a dam should be thrown across the Chagres River. Two valleys were formed at Gatun by a hill which rose in the center to an elevation of 110 feet, and the dam that was designed runs from the Gatun locks to this hill and from this hill to the mountains, a total distance of 7,500 feet, or a mile and a half.

As the Chagres River every year discharges enough water to fill the lake, some means of disposing of the surplus water had to be provided. The plan adopted called for a spillway to be constructed in this hill, about third-way in the dam site. This spillway is of concrete, requiring 225,000 cubic yards to complete.

On July 1, 1912, it was more than 90 per cent completed.

The floor of the spillway is 10 feet above sea-level, and 300 feet wide through the hill, which involved excavation through rock for a depth of 100 feet at the highest point of the hill. A concrete dam was built on this floor to a height of 69 feet above sea-level and in shape like a semicircle. On top of the concrete dam, piers were built with an arrangement for steel gates. These steel gates will be electrically operated and regulate the flow of water out of the lake. As much as 140,000 cubic feet of water per second may escape through the spillway when the gates are open.

There will not be a complete loss of this water, as on the east side of the spillway a power plant of the hydro-electric type will be operated. A drop of 75 feet by the water will operate turbine engines which in turn will operate the electric machinery that will generate all the power and illuminating current needed from one end of the canal to the other. But an additional power plant will be maintained at Miraflores for emergencies. The power to operate the lock gates will come from the spillway plant.

The Gatun dam is so stupendous that it almost seems to be a continuation of the hills that enclose the lake. It in fact does complete the natural mountain chains that form the barriers of the Chagres River. It is 105 feet high, or 20 feet above the ordinary level of the lake at 85 feet elevation. The plan of construction has been to build parallel mounds, for the mile and a half, 1,200 feet apart. Between

these mounds, built of rock and earth, a core for the dam has been constructed by pumping mud and sand from the bed of the Chagres River. About 20 per cent of the material pumped is solid matter, and when it has deposited the water is pumped off. This operation has been repeated until an impervious heart has been made in the dam. Even if water from the lake penetrated the outside walls of rock and earth, it would find this core water-tight. The dam is nearly half a mile thick at the base, 398 feet thick where the water surface strikes it at 85 feet, and is 100 feet wide at the top. The outer coverings of rock and earth on the dam close over the hydraulic core at the crest. For about 500 feet the dam will be subjected to the full pressure of 85 feet of water, at other points to a less severe pressure.

Engineers consider the dam excessively safe and the layman has no difficulty in appreciating its strength. This feature was subjected to a storm of criticism throughout the early days of the canal because some engineers believed the earth would not support so heavy a structure, but the present Chief Engineer never has doubted its stability. About half of the material required, 21,994,111 cubic yards, has been brought from the Culebra cut. On July 1, 1912, the dam was more than 90 per cent completed, leaving less than 10 per cent to be done before the passage of the first ship.

On the Pacific side, the first dam encountered is at Pedro Miguel and serves to hold the waters of Gatun Lake at its southern end. It is 1,400 feet long and