While Ctesibius is credited with being the first man to make scientific use of compressed air, the use of free air or wind as a power far antedates him. Sailing vessels were probably in use long before windmills were invented. As pointed out elsewhere, windmills possessed an advantage over water wheels in that they could be placed anywhere, instead of being confined to the banks of streams. They were widely used for pumping water, grinding grain, and sawing wood until the steam engine arrived and displaced them. Although wind power is but little used to-day, air is much more widely used in modern machinery than is generally realized. A catalogue of the various devices in which it plays a prominent part is likely to prove astonishing. For instance, we employ it to bore through rock, to stop the speeding express train, and even to quell ocean billows. Being elastic and compressible, it makes an ideal spring for the storage of power and a cushion for absorbing shocks. It is also used to fight back water and permit men to work in the depths of the sea or in water-bearing sand and earth. Despite his extensive use of air Ctesibius knew nothing of the atmospheric pressure. It was not until the seventeenth century that atmospheric pressure was discovered and measured. It was then learned that there is no such thing as suction. RAISING WATER WITH ATMOSPHERIC PRESSURE [Illustration: FIG. 40.--SECTIONAL VIEW OF A SUCTION PUMP] A suction pump should be called an atmospheric pump. The operation of such a pump is illustrated in Figure 40. A piston A slides in a cylinder B. There is an opening in the bottom of the cylinder which is normally closed by a clack valve C that opens upwardly. There is a similar valve D in the piston A. When the piston is pushed down it compresses the air in the cylinder, closing the valve C while valve D opens, permitting the air to escape. Then on the upward stroke the valve D closes and valve C opens permitting air to flow into the cylinder. In this way the air in the pipe and cylinder are exhausted after a few strokes. As the air is rarefied and the pressure is reduced above the column of water in the pipe the greater pressure of the water outside the pipe forces the column to rise until it eventually flows through the valve in the piston and out of the spout of the pump. The maximum height to which water can be raised by a suction pump is thirty-four feet but in actual practice the lift seldom exceeds twenty-six to twenty-eight feet because the fit between the cylinder and piston is not perfect. [Illustration: FIG. 41.--CONVENTIONALIZED SECTIONAL VIEW OF A FORCE PUMP]