The hydraulic jet is also used for general excavating wherever water power is available. Sometimes it is employed under water when clearing a channel to level down piles of stones that are too large to be picked up by a suction dredge. Hollow iron piles are driven into a sandy bottom by means of hydraulic jets. No hammer is needed. Water is pumped into the pile and on issuing from the bottom of the pile it carries sand with it, making a hole into which the pile sinks. Wooden piles are driven in the same way by loosely attaching a water pipe to them so that the pipe may be withdrawn when the pile has been driven far enough. The pile is grooved at the lower end so that the pipe outlet may be centered at the bottom of the pile. COMPRESSING AIR WITH WATER [Illustration: FIG. 35.--HOW THE VENA CONTRACTA RAISES WATER] A very ingenious apparatus for compressing air was invented in the earliest years of the iron age to furnish a continuous blast of air for the Catalan forges. This compressor, known as a “trompe,” can hardly be termed a machine because it contains no moving parts except water, which is the motive power, and the air which it traps and compresses. To understand its operation we must look into a peculiar property of water flowing out of a reservoir into a pipe or nozzle. There is a converging motion that tends to contract the jet of water just after it leaves the pipe. This is known as the _vena contracta_. It produces a partial vacuum in the pipe. If air ports are opened into the pipe at this point, air will be sucked in to fill the vacuum and will be carried out of the pipe by the friction of the water. In Figure 35 a pipe is shown running from the _vena contracta_ to a water tank below. The rise of water in this pipe indicates the degree of vacuum produced by the jet. [Illustration: FIG. 36.--THE “TROMPE” BY WHICH COMPRESSED AIR WAS FURNISHED FOR CATALAN FORGES] This principle is used in the hydraulic-air compressor or “trompe” as it is called. Water flows out of one reservoir through a pipe into another reservoir lower down. (See Figure 36.) Air enters the pipe through ports at the point where the vein of water contracts and is carried down into the second reservoir. This reservoir is sealed so that the air is trapped in it. The water passes out through a pipe which is carried high enough to keep a certain pressure of air in the reservoir and prevent it from blowing out. [Illustration: FIG. 37.--DIAGRAMMATIC SECTION OF A LARGE HYDRAULIC AIR COMPRESSOR IN MICHIGAN]