In the middle of the dam the level of the lake is controlled by a channel called the "spillway," with walls and floor of concrete, by which the surplus waters will be sluiced off into the old bed of the Chagres River and so passed on to the sea. The entrance to this channel is closed with falling gates or doors. This safety-valve will no doubt be capable of dealing with the biggest and quickest rise of the lake-level that is ever likely to take place. It can pass off 137,000 cubic feet of water a second, the water issuing at a speed of 35 feet a second. But, to complete the security, the big culverts of the mighty Gatun locks close by can be turned open, and 170,000 cubic feet a second carried off there. Indeed, as regards the Gatun Lake the anxiety, if there be any, is that the water-supply will be insufficient rather than dangerously excessive. The level of the lake is to be kept at 85 feet above mean sea-level--that is, the dam, or a considerable length of it, will be exposed to what is called a "head" of water of 85 feet. The lake itself will be 164 square miles in extent. There have been many rational anxieties on the sufficiency of the dam. A certain American senator, however, who visited the works during the construction, worried himself rather unnecessarily on this last figure. Colonel Goethals was showing a congressional delegation round the works, and in the course of the survey they came to the dam with the broad expanse of water behind it. "Colonel," he said, "how is it that so small a body of earth as the Gatun dam can hold in check such a tremendous body of water as the Gatun Lake?" The chief engineer explained that the pressure of a body of water is determined by its height and not by its volume. The inquirer seems not to have been satisfied with the statement of this hydrostatic law. Senator Knox, afterwards Secretary of State, then came to his aid. "Senator," he said, "if your theory were true, how could the dykes of Holland hold in check the Atlantic Ocean?" This was a clincher, and the sceptic joined in the laugh at his own expense.