Page:St. Nicholas, vol. 40.1 (1912-1913).djvu/769

 1913.] pressure. The lock is just a cylinder with a hinged lid, or door, at the top and bottom. One or other of those lids must be closed all the time, to hold the pneumatic pressure in the caisson. The bottom door is closed when the top door is open to let the men in. After they enter the lock, the lock-tender lifts the upper door shut, and turns a valve to let the compressed air in. All the time, the bottom door is kept closed by the air-pressure in the shaft below, but, as air is let into the lock, its pressure at length equals that in the caisson, and, there being nothing to hold up the bottom door, it swings open of its own weight, so that the men can go on down to the working chamber.”

“Could we go down into one of the caissons, asked Will.

“Oh, no, that is entirely out of the question,” said Mr. Squires. Then, as he saw the disappointment in our faces, he explained: “There is n't anything to see down there, and it is pretty dirty work.”

“We don’t mind the dirt,’ I interrupted.

“Well, I don’t know,” said Mr. Squires, hesitating. “You say Dick Hotchkiss sent you to me? That settles it then, if you really want to go. Come on to the sand-hog house, and I Il see if I can rig you out in boots and oilskins. But hold on. When did you have breakfast? Eight o'clock? You did n’t have a very substantial meal, did you?”

We told him what constituted our usual morning fare.

“That ’s not enough,” he said. “Run over to that restaurant, and fill up with all you can eat.”

That seemed like an odd preliminary to our work. “It ’s like feeding the prisoners just before they are to be executed,” I remarked.

“But,” Mr. Squires explained, “down there you will take in three times the usual amount of oxygen with every breath. Your ‘innards’ are going to work under forced draft, and so you