A system of interlocked levers will prevent him from doing the wrong thing in handling his switches. Before he can open the valves at one end of a lock he must close those at the other end. Before he can open the lock gates, the valves in the culverts must be set so that no harm can result. Before he can start to open a lock gate, he must first have released the miter-forcing machine that latches the gates. Before he can close the gates protected by a fender chain, he must first have thrown the switch to bring the fender chain back to its protecting position, and he can not throw the switch to lower the chain until he first has provided for the opening of the gate it protects. All of this interlocking system makes it next to impossible to err, and taking into consideration the additional safeguard of limit switches, which automatically cut off the power when anything goes wrong, it will be seen that the personal equation is all but removed from the situation. CHAPTER VI CULEBRA CUT Culebra Cut! Here the barrier of the continental divide resisted to the utmost the attacks of the canal army; here disturbed and outraged Nature conspired with gross mountain mass to make the defense stronger and stronger; here was the mountain that must be moved. Here came the French, jauntily confident, to dig a narrow channel that would let their ships go through. The mountain was the victor. And then here came the Americans, confident but not jaunty. They weighed that mass, laid out the lines of a wider ditch, arranged complicated transportation systems to take away the half hundred million cubic yards of earth and rocks that they had measured. Nature came to the aid of the beleaguered mountain. The volcanic rocks were piled helter-skelter and when the ditch deepened the softer strata underneath refused to bear the burden and the slides, slowly and like glaciers, crept out into the ditch, burying shovels and sweeping aside the railway tracks. Even the bottom of the canal bulged up under the added stress of the heavier strata above. Grim, now, but still confident, the attackers fought on. The mountain was defeated. Now stretches a man-made canyon across the backbone of the continent; now lies a channel for ships through the barrier; now is found what Columbus sought in vain--the gate through the west to the east. Men call it Culebra Cut. Nine miles long, its average depth is 120 feet. At places its sides tower nearly 500 feet above its channel bottom, which is nowhere narrower than 300 feet. It is the greatest single trophy of the triumph of man over the terrestrial arrangement of his world. Compared to it, the scooping out of the sand levels of Suez seems but child's play--the tunnels of Hoosac and Simplon but the sport of boys. It is majestic. It is awful. It is the Canal.