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went to the bottom of the river and was pumped full of compressed air to keep the water from entering it from below: the top of the caisson is a room called the "material chamber" into which the stuff dug out of the river passes up and is carted away. On the side of the caisson is another room, called the "airlock", into which we were to go to be "compressed". As the compressed air is admitted, the blood keeps absorbing the gasses of the air till the tension of the gasses in the blood becomes equal to that in the air: when this equilibrium has been reached, men can work in the caisson for hours without serious discomfort if sufficient pure air is constantly pumped in. It was the foul air that did the harm, it appeared; "if they'd pump in good air, it would be O. K.: but that would cost a little time and trouble and men's lives are cheaper." I saw that the men wanted to warn me, thinking I was too young, and accordingly I pretended to take little heed.

When we went into the "airlock" and they turned on one aircock after another of compressed air, the men put their hands to their ears and I soon imitated them for the pain was very acute. Indeed, the drums of the ears are often driven in and burst if the compressed air is brought in too quickly. I found that the best way of meeting the pressure was to keep swallowing air and forcing it up into the middle ear where it acted as an air-pad on the inner side of the drum and so lessened the pressure from the outside.

It took about half an hour or so to "compress" us and that half an hour gave me lots to think about. When the air was fully compressed, the door of the airlock opened at a touch and we all went down to work with pick and shovel on the gravelly bottom. My headache soon became acute. The six of us were working naked to the waist in a small iron chamber