One of the problems that had to be solved before the Panama Canal could be presented to the American people as a finished waterway, was that of equipping it with adequate and dependable machinery for its operation. Panama canals are not built every year, so it was not a matter of ordering equipment from stock; everything had to be invented and designed for the particular requirement it was necessary to meet. And the first and foremost requirement was safety. When we look over the canal machinery we see that word "safety" written in every bolt, in every wheel, in every casting, in every machine. We see it in the devices designed for protection and in those designed for operation as well. We see it in the giant chain that will stop a vessel before it can ram a gate; we see it in the great cantilever pivot bridges that support the emergency dams; we see it in the double lock gates at all exposed points; we see it in the electric towing apparatus, in the limit switches that will automatically stop a machine when the operator is not attending to his business, in the friction clutches that will slip before the breaking point is reached. Safety, safety, safety, the word is written everywhere. The first thing a ship encounters when it approaches the locks is the giant chain stretched across its path. That chain is made of links of 3 inches in diameter. When in normal position it is stretched across the locks, and the vessel which does not stop as soon as it should will ram its nose into the chain. There is a hydraulic paying-out arrangement at both ends of the chain, and when the pressure against it reaches a hundred gross tons the chain will begin to pay out and gradually bring the offending vessel to a stop. After a ship strikes the chain its momentum will be gradually reduced, its energy being absorbed by the chain mechanism. While the pressure at which the chain will begin to yield is fixed at 100 gross tons, the pressure required to break it is 262 tons. Thus the actual stress it can bear is two and a half times what it will be called upon to meet. The mechanism by which the paying-out of the chain is accomplished is exceedingly ingenious. The principle is practically the reverse of that of a hydraulic jack. The two ends of the 428-foot chain are attached to big plungers in the two walls of the locks. These plungers fit in large cylinders, which contain broad surfaces of water. They are connected with very small openings, which are kept closed until a pressure of 750 pounds to the square inch is exerted against them. By means of a resistance valve these openings are then made available, the water shooting out as through a nozzle under high pressure. This permits the chain plunger to rise gradually, while keeping the tension at 750 pounds to the inch, and the paying-out of the chain proceeds accordingly. Of course not all ships will strike the chain at the same speed, and in some cases the paying-out process will have to be