Fundamentally the spillway is a channel 1,200 feet long and 285 feet wide cut through the solid rock of the island which at this point bisects the now obliterated Chagres Valley. Though cut through rock it is smoothly lined and floored with cement; closed at its upper end by a dam, shaped like the arc of a circle so that, while it bars an opening of only 285 feet, its length is 808 feet. For the benefit of the unprofessional observer it may be noted that by thus curving a dam in the direction of the force employed against it, its resisting power is increased. It resists force exerted horizontally precisely as an arch resists force, or weight, exerted from above. The dam at the spillway extends solidly across the opening to a height of 69 feet. But this is 16 feet below the normal level of the Lake. From the top of the solid dam rise thirteen concrete piers to a height as planned, of 115 feet above sea level, that is the piers will rise 46 feet above the top of the dam. Between each two of these piers will be mounted regulating gates of steel sheathing, made water tight and movable up or down as the state of the Chagres level requires a free or a restricted passage for the water. Nor will those operating the gates await the visual appearance of the flood before throwing wide the passage for its onrush. At divers points along the Chagres, and throughout its water shed are little stations whence observers telephone at regular intervals throughout the day to the office at the spillway the result of their observations of the river’s height. With these figures at hand the controller of the gates can foresee the coming of a flood hours before it begins to beat against the gates. [Illustration: _Photo by Underwood & Underwood_ THE GIANT PENSTOCKS OF THE SPILLWAY] [Illustration: THE SPILLWAY AT HIGH WATER A comparison with the picture on page 179 will show the varying stages of the river] [Illustration: _Photo by Underwood & Underwood_ LOCK GATES APPROACHING COMPLETION]