Compressed Air Work And Diving 1909

42 COMPRESSED AIR WORK. for the bridge pier, wall, or whatever it is that is to come on the top of it. If it is to be a bridge pier, the concrete usually stops at ground level, and then the pier is continued upwards in masonry or brickwork inside the outer skin of the caisson. When the pier is above high water, this outer skin can be taken off by divers clown to ground level. Although concrete is uncloubteclly the most convenient material to use for filling up caissons, other materials are sometimes employecl. For small cylinders, for instance, many engineers prefer blue brick. The regulation of the air supply is an important matter, fhere must be sufficient to keep the pressure at the necessary height, and also to make up for any losses through leakages and for what will be lost every time the bucket chamber or entrance chamber for men is emptied. There must also be sufficient to insure the health of the workers. If there is too much air, what are callecl “ blows ” will be proclucecl. Blows are caused by too large a volume of air escaping at one point. The result will be that the water mixed with the air will be lighter than a column of water of the same height; the solid water will be banked up on each side forming a sort of shaft through which the air will escape; a clense fog will form inside the working chamber due to the drop in tempera- ture caused by the work done by the air in expanding, and eventually, when the water has closed in again and the escape of air stopped, the water will come pouring in at the weak spot and perhaps all round the cutting eclge. A blow like this will be very serious, especially if the cutting eclge of the caisson is only a little way into the ground, and the reduction in pressure may cause the caisson to sink, and perhaps more on one sicle than the other, or even to toppie over. The actual clanger to