The machinery for opening and closing the lock gates called for unusual care in its designing. The existing types of gate-operating machinery were all studied, and it was found that none of them could be depended on to prove satisfactory, so special machines had to be designed. A great wheel, resembling a drive wheel of a locomotive, except that a little over half of the rim is cog-geared, is mounted in a horizontal position on a big plate, planted firmly in the concrete of the wall and bolted there with huge bolts 11 feet long and 2-1/4 inches in diameter. This plate weighs over 13,000 pounds, and the wheel, cast in two pieces, weighs 34,000 pounds. As the weight of the rim of the wheel on the eight spokes probably would tax their strength too much when the wheel is under stress, this is obviated by four bearing wheels, perpendicular to the big wheel, which support the rim. Between the crank pin and the point of attachment on the gate leaf there is a long arm, or strut, designed to bear an operating strain of nearly a hundred tons. The wheel will be revolved by a motor geared to the cogged part of the rim. An ingenious arrangement of electric switches is that used to protect the gate-moving machines from harm. The big connecting rod between the master wheel and the gate leaf is attached to the gate leaf by a nest of springs capable of sustaining a pressure of 184,000 pounds, in addition to the fixed pressure of 60,000 pounds. Should any obstruction interfere with the closing of the gate and threaten a dangerous pressure on the connecting rod, the springs, as soon as they reach their full compression, establish an electrical contact and thus stop the motor. Likewise, should any obstruction come against the gate as the connecting rod is pulling it open, the springs again permit the establishment of an electrical contact and stop the motor. All of these precautions are entirely independent of and supplemental to the limit switches, which cut off the power from the gate-moving machine should the strain reach the danger line. These big machines move the huge gate leaves without the slightest noise or vibration. Such a machine is required for each of the 92 leaves used in the 46 gates with which the locks are equipped. The operator can open or close one of these big gates in two minutes. [Illustration: ONE OF THE 92 GATE-LEAF MASTER WHEELS]