In the San Diego work, a huge mass of rocky hillside overhung the cañon which was to be dammed, and at the bottom of which the river flowed.
A cañon is, as you doubtless know, a deep gorge or ravine, formed by the river that flows through it, and which little by little has worn away its bed until it has cut deep down into the heart of the land, hundreds of feet below its original level.
As we have said, the plan for the San Diego work was to build a dam across the cañon, and keep the waters from flowing away.
The engineers thought that if they could only take this huge mass of rocky hill and drop it into the cañon, it would make their work very much easier, as the task of getting enough stone down to the bottom of the gorge to build their dam was by no means an easy one.
They thought the matter out, and finally decided to try and move the mountain-side into the cañon.
They laid their plans, and for two months were busy preparing for the great move.
They tunnelled the hillside in all directions over the section which they desired to move, and in these tunnels laid mines of giant powder or dynamite.
These mines they connected with electric wires, and when all was ready, the various wires were gathered into one bunch, and taken across the gorge that was to be filled up.
When the explosion was about to take place the workmen and engineers moved ever so far away, until they were at a safe distance from the explosion, and one man, the foreman, was sent to the edge of the cañon to touch the wires, and start the firing of the mines.
When he received the signal he touched the wire, and then ran away to safety as fast as he could go, while the others watched the explosion.