"Never mind that.
"The point is: we 'd been having as much trouble putting down that shaft as if it had been another Simplon tunnel. There 'd been an error in the City Engineer's specifications. His blue-prints, furnished us when we were bidding on the contract, showed a bottom of clay and gravel. We found quicksand when we got to work. And that makes all the difference to an engineer that it does to a builder.
"You know what a cofferdam is?—a four-sided dam. You sink your shaft inside it, after you 've pumped out the water enclosed by the dam.
"Well, an ordinary cofferdam, made of wooden piles and timber sheeting, packed with clay, won't hold out water over a quicksand, because it comes in, through the sand, under the piling, as fast as you pump it out. We 'd built an ordinary cofferdam. And when that did n't hold, we strengthened it with another outside of it. Then we put on extra pumps and kept them going until the quicksand shifted under the piling and wrecked our three months' work. After that, we decided to use a caisson.
"A caisson"—he illustrated it with his hands—"is properly a steel tube that 's sunk in sections to make a metal well for the men to dig in. It 's usually fitted with an air-lock and supplied with compressed air. As if the caisson were a diving-bell sunken in the earth—don't you know? The air in it keeps out the water, and the metal holds up the sand.
