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178 these barges are snatched up by giant unloader-cranes and put into storehouses, out of which, like chicks from a brooder, run intelligent little electric cars that need no motormen, but climb of themselves up into the top story of the dusty mixing-house. Here, eight huge rotary mixers churn the three elements, cement, sand, and stone, into concrete, and drop it wetly into great skips or buckets, two of which sit on each car of a somewhat larger-sized system of electric trains, whose tracks run along one side of the lock-pits. Presently those skips rise in the air and go sailing across the lock-pit in the grip of a carrier traveling on a steel cable stretched between two of the tall skeleton towers that stand on either side of the lock-site. When the skip is squarely above the one of the high steel molds it is to help fill, it is tilted up, and there is so much more concrete in place.

When the last cubic yard has been set, the gates hung, and the water turned in, a ship coming from the Atlantic will stop in the forebay or vestibule of the lowest right-hand locks, and make fast to electric towing-locomotives running along the top of the lock-walls. No vessel will be allowed to enter a lock under her own power, for fear of her ramming a gate and letting the water out, as a steamer did a few years ago in the "Soo" Locks, between Lake Huron and Lake Superior. Every possible precaution has been taken to prevent such an accident at Gatun. Any ship that tried to steam into one of the locks there, for any reason whatsoever, would first have to carry away a heavy steel chain, that will always be raised from the bottom as a vessel approaches, and never lowered until she has come to a full stop. Then the