Compressed Air Work And Diving 1909

PITCHING. 43 the workers is not great if nothing happens to the caisson, since the water coming in will compress the air in the chamber so that it rarely rises more than about 2 ft. above the cuttino- eclo-e. Sometimes, however, the excavation is carried some way below the cutting eclge, and then the danger will be greater, especially if the workers are unable to swim. When a blow such as this occurs, the escaping air will make a noise like thunder. When the caisson is some way into the ground, blows will only be noticecl by a slight fog and the water trickling in at one or two places. Blows cannot be prevented by regulating or governing the pressure only. The volume of air passeel into the working chamber must also be controlled. The author has founcl the most satisfactory method is to run the air engines at constant speed and to govern the pressure merely by having a governor worked by the air so that when the pressure rises above a certain point the val ves will be lifteel and the air engine run light. In this way the volume of air can never rise above a certain point. If the pressure alone is governed, there is no definite limit to the volume of air pumped into the caisson. The same pressure can be obtained with exceecl- ingly varying quantities of air. For a similar reason no attempt should be made to sink two caissons off one main by means of a reducing valve. A caisson, when it is to be sunk in deep water, is either floated into position, as was done at the Forth Bridge, or else built in situ on staging, as was done at the King Edward VII. Bridge at Newcastle, and then lowered into the water by jacks. When a caisson is floated out, the shafts must be closed at the top, if, as is usually the case, only a few lengths are added at first, in