Remembering that these gates are nothing more than Brobdingnagian double doors which close in the shape of a flattened V, it follows that they must have hinges. And these hinges are worth going miles to see. That part which fastens to the wall of the lock weighs 36,752 pounds in the case of the operating gates, and 38,476 pounds in the protection gates. These latter are placed in pairs with the operating gates at all danger points--so that if one set of gates are rammed down, another pair will still be in position. The part of the hinge attached to the gate was made according to specifications which required that it should stand a strain of 40,000 pounds before stretching at all, and 70,000 pounds before breaking. Put into a huge testing machine, it actually stood a strain of 3,300,000 pounds before breaking--seven times as great as any stress it will ever be called upon to bear. The gates are all painted a lead gray, to match the ships of the American Navy. Those which come into contact with sea water will be treated with a barnacle-proof preparation. Now that we have described the locks, we may go back and see them in course of construction. The first task was getting the lock building plant designed and built. At Gatun the plant consisted of a series of immense cableways, an electric railroad, and enormous concrete mixers. Great towers were erected on either side of the area excavated for the locks, with giant cables connecting them. These towers were 85 feet high, and were mounted on tracks like steam shovels, so that they could be moved forward as the work progressed. The cables connecting them were of 2-1/2-inch lock steel wire covered with interlocking strands. They were guaranteed to carry 6 tons at a trip, 20 trips an hour, and to carry 60,000 loads before giving way. They actually did better than the specifications called for as far as endurance was concerned. The sand for making the concrete for Gatun came from Nombre de Dios (Spanish for Name of God), and the gravel from Porto Bello. The sand and gravel were towed in great barges, first through the old French Canal, and later through the Atlantic entrance of the present canal. Great clamshell buckets on the Lidgerwood cableways would swoop down upon the barges, get 2 cubic yards of material at a mouthful, lift it up to the cable, carry it across to the storage piles and there dump it. In this way more than 2,000,000 wagon loads of sand and gravel were handled.