How do they close? What unseen power forces those huge gates of steel, shut against the dogged resistance of the water? They are 7 feet thick, 65 feet long and from 47 to 82 feet high. They weigh from 390 to 730 tons each. Add to this weight the resistance of the water and it becomes evident that large power is needed to operate them. At Gatun in the passing of a large ship through the locks, it will be necessary to lower four fender chains, operate six pairs of miter gates and force them to miter, open and close eight pairs of rising stem gate valves for the main supply culverts, and thirty cylindrical valves. In all, no less than 98 motors will be set in motion twice during each lockage of a single ship, and this number may be increased to 143, dependent upon the previous position of the gates, valves and other devices. Down under the surface of the lock wall, packed into a little crypt which seems barely to afford room for its revolving, is a great cogwheel 20 feet in diameter, revolving slowly and operating a ponderous steel arm which thrusts out or pulls back the gate as desired. The bull wheel, they call it, is driven by a 27 horse power motor, while a smaller motor of 7¹⁄₂ horse power locks the gates tight after they are once in position. Two of these bull wheels, and two each of the motors are needed for each pair of gates. [Illustration: THE TANGLED MAZE OF STEEL SKELETONS THAT ARE A LOCK IN THE MAKING] The ship then is in the lowest lock, one pair of gates closed tightly behind her. Another pair confronts her holding back the water in the lock above, which if filled, will be just 28¹⁄₃ feet above the surface of that on which she floats. But the water about her is now slowly rising. Another set of electric motors concealed in the concrete wall have set in motion the valves in the floor of the lock, and the water is flowing in from the tunnels, raising the ship and at the same time lowering the water in the lock above. When the vessel’s keel is higher than the sill of the lock above the upper gates swing slowly back and fold in flat with the wall. The ship is now in a chamber 2000 feet long filled to a level. The locomotives pull her forward a thousand feet or so. Again great gates close behind her. Again the water rises slowly about her lifting her with it. The first process is repeated and she enters the third lock. By the time she has been drawn out into the lake and the locomotives have cast her off, more than 100 electric motors with a horse power ranging from 7¹⁄₂ to 50 each will have contributed to her progress. Altogether over 1000 individual motors will be required for the different locks. Indeed the whole interior of those massive lock walls is penetrated by lighted galleries strung with insulated wires bearing a death-dealing current. Men will be stationed at the various machinery rooms, but the whole line of machinery can be operated from a central operating tower on the lock above. CHAPTER XI