With ordinary soil, the pipe is easily withdrawn with a chain wrench, and two men will drive one hundred feet in a couple of days. When water is reached, a well point is put on through which water may percolate without carrying too much soil. This type of well is suitable for use in soft ground or sand, up to depths of about one hundred feet, and in places where the water is not abundant. It is most useful for testing the ground to see where water may be found and by pumping from such a well to see what quantity of water may be expected. This type is often used as a shallow well, and the author has seen such wells driven only a dozen feet. Such a well has no protection against pollution, and an ordinary dug well is better for shallow depths. A driven well always has a disadvantage also from the ever present danger that the iron pipe will rust through at the top of the ground water and so admit to the well the most polluted part of the drainage. For larger supplies and for greater depths, a machine like a pile-driver has to be used for forcing down the pipe. This is not usually removed, but driven down as far as possible, and when the limit of the machine has been reached, a smaller size is slipped down inside the driven pipe, to be in turn driven to refusal. In rock, that is, if the well has to penetrate a layer of rock, a drill is used that will work inside of the pipe last driven, and by alternately lifting and dropping the drill, and at the same time twisting it back and forth, a hole through rock may be made many hundred feet below the surface of the ground. Figure 31 shows a cut of a common type of well-drilling machine. In some soils, not rock, it is necessary to keep the drill going in order to churn up or soften the earth so that the pipe may be lowered. The churned-up soil is removed by a sand pump, which is a hollow tube with a flap valve at the lower end opening inwards and a hook on the upper end. By alternately drilling, pipe-driving, and pumping the wet material, length after length of pipe can be forced into the ground until water of a satisfactory quantity is reached. Very often a jet of water is used to wash out the dirt from the interior of the well instead of a sand pump. As shown by Fig. 32 water under pressure is forced down the small pipe _A_ which runs to the bottom of the well. The large pipe _B_ can then, as the sand is loosened by the water, be driven down by the one thousand-pound hammer _M_. The water and sand together flow up in the space outside the small pipe and inside the large pipe, overflowing through the waste pipe _W_. This type of well has been very largely used throughout New York State; on Long Island, in connection with the Brooklyn Water-supply; along the Erie Canal, in connection with the Barge Canal Work, and in New York City, in connection with building foundations. [Illustration: FIG. 32.--Sinking a well by means of a water-jet.]