Compressed Air Work And Diving 1909

34 COMPRESSED AIR WORK. plates, and is chiefly usecl on large harbour works for the purpose of levelling off the ground to receive the large concrete blocks usecl in harbour construction. The weight when ballasted is about 25 per cent, in excess of the water displaced. Air is delivereel through a receiver from a steam or electrically driven air compressor. In Chapter I., Table III., it has been pointeel out that a minimum air supply will be required by the di ver. For shallow clepths this can be determined with sufficient Fig. 1.—John Smeaton’s Diving Machine, 1778. accuracy by calculating, from the stroke and diameter of cylinder, the capacity of free air per revolution of each cylinder, and then multiplying by the number of cylinders and the number of revolutions per minute. As the depth inereases, losses due to heating, leakage, and cushioning cease to become negligible. Loss from heating will not be serious, provideel the water surrounding the cylinders in the water jacket is changed every half hour or so. Cushioning is a capacity loss chiefly, as about 75 per cent, of the power expencled in compressing will be