It will be seen that there are two of them side by side--twin locks, they are called, making them like a double-track railway. The lock on the right is nearly filled for an upward passage. The ship will be seen in it, held in position by the four towing engines, which appear only as tiny specks hitched to hawsers from the stem and stern. Behind the ship are the downstream gates. They were first opened to admit the ship, and then closed to impound the water that flows up through the bottom of the lock. Ahead are the upstream gates, closed also until the water in the lock is brought up to the level of the water in the lake. Then the gates will be opened, the big chain fender will be dropped down, and the ship will be towed out into the lake and turned loose. On the side wall of the right lock there is a big bridge set on a pivot so that it can be swung around across the lock and girders let down from it to serve as a foundation upon which to lay a steel dam if anything happens to the locks or gates. On the other lock the bridge has been swung into position, and the steel girders let down. Great steel sheets will be let down on live roller bearings on these girders, and when all are in place they will form a watertight dam of steel. Between this bridge and the reader is a huge floating tank of steel, which may be used to dam all the water out of the locks when that is desired. Referring to the next figure we see a cross section of the twin locks. The side walls are from 45 to 50 feet thick at the floor. At a point 24-1/3 feet above the floor they begin to narrow by a series of 6-foot steps until they are 8 feet wide at the top. The middle wall is 60 feet wide all the way up, although at a point 42-1/2 feet above the lock floor room is made for a filling of earth and for a three-story tunnel, the top story being used as a passageway for the operators, the second story as a conduit for electric wires, and the lower story as a drainage system. [Illustration: A CROSS-SECTION OF LOCKS, GIVING AN IDEA OF THEIR SIZE] In this figure D and G are the big 18-foot culverts through which water is admitted from the lake to the locks. Each of these three big culverts, which are nearly 7,000 feet long, is large enough to accommodate a modern express train, and is about the size of the Pennsylvania tubes under the Hudson and East Rivers. H represents the culverts extending across the lock from the big ones. Each of them is big enough to accommodate a two-horse wagon, and there are 14 in each lock. Every alternate one leads from the side wall culvert and the others from the center wall culvert. F represents the wells that lead up through the floor into the lock, each larger in diameter than a sugar barrel in girth. There are five wells on each cross culvert, or 70 in the floor of each lock. [Illustration: CONCRETE MIXERS, GATUN] [Illustration: A CENTER WALL CULVERT, GATUN LOCKS] [Illustration: THE MACHINERY FOR MOVING A LOCK GATE]