These cylinders are sunk by excavating the earth from within and under them in various ways. The water may be kept out of them by pumping in compressed air. The excavation can then be done by men working within the cylinder, or if the water is not forced out the excavation may be carried on by tools, or special excavating machines lowered from above. In cases where the soil is very soft, cylinders have been sunk by exhausting the air from within them, so that they are forced down by the atmospheric pressure acting upon their covers or upper surface. Iron piles and tubes are more in use for the foundations of engineering works than for ordinary buildings; they need not therefore be further noticed in this course.