and form bubbles in the veins. Any sudden release in thepressure on a fluid results in the freeing of some gasesthat are dissolved in the fluid. You have seen this happenwhen you suddenly relieve the pressure on a bottle ofpop by removing the cap. The careful matching ofhydrostatic pressure on the diver by air pressure in thediving suit is essential if diving is to be done at all.Determining Ship’s SpeedDid you ever wonder how the skipper knows thespeed the ship is making through water? The skipper canget this information by using several instruments-thepatent log, the engine revolution counter, and thepitometer (pit) log. The “pit log” operates, in part, byhydrostatic pressure. It really shows the differencebetween hydrostatic pressure and the pressure of thewater flowing past the ship-but this difference can beused to find ship’s speed.Figure 10-4 shows a schematic drawing of apitometer log. It consists of a double-wall tube thatsticks out forward of the ship’s hull into water that is notdisturbed by the ship’s motion. In the tip of the tube isan opening (A). When the ship is moving, two forces orpressures are acting on this opening: (1) the hydrostaticpressure caused by the depth of the water above theopening and (2) a pressure caused by the push of the shipthrough the water. The total pressure from these twoforces transmits through the central tube (shown inwhite on the figure) to the left-hand arm of a manometer.In the side of the tube is a second opening (B) thatdoes not face the direction in which the ship is moving.Opening B passes through the outer wall of thedouble-wall tube, but not through the inner wall. Theonly pressure affecting opening B is the hydrostaticfigure 10-4.-A pitometer log.pressure. This pressure transmits through the outer tube(shaded in the drawing) to the right-hand arm of themanometer.When the ship is dead in the water, the pressurethrough both openings A and B is the same, and themercury in each arm of the manometer stands at thesame level. However, as soon as the ship begins to move,additional pressure develops at opening A, and themercury pushes down in the left-hand arm and up intothe right-hand arm of the tube. The faster the ship goes,the greater this additional pressure becomes, and thegreater the difference will be between the levels of themercury in the two arms of the manometer. You can readthe speed of the ship directly from the calibrated scaleon the manometer.Since air is also a fluid, the airspeed of an aircraftcan be found by a similar device. You have probablyseen the thin tube sticking out from the nose or theleading edge of a wing of the plane. Flyers call this tubea pitot tube. Its basic principle is the same as that of thepitometer log.HYDRAULIC PRESSUREPerhaps your earliest contact with hydraulicpressure was when you got your first haircut. Thehairdresser put a board across the arms of the chair, satyou on it, and began to pump the chair up to a convenientlevel. As you grew older, you probably discovered thatthe gas station attendant could put a car on the greasingrack and-by some mysterious arrangement-jack ithead high. The attendant may have told you that oilunder pressure below the piston was doing the job.Come to think about it, you’ve probably knownsomething about hydraulics for a long time.Automobiles and airplanes use hydraulic brakes. As asailor, you’ll have to operate many hydraulic machines.You’ll want to understand the basic principles on whichthey work.Primitive man used simple machines such as thelever, the inclined plane, the pulley, the wedge, and thewheel and axle. It was considerably later beforesomeone discovered that you could use liquids andgases to exert forces at a distance. Then, a vast numberof new machines appeared. A machine that transmitsforces by a liquid is a hydraulic machine. A variation ofthe hydraulic machine is the type that operates with acompressed gas. This type is known as the pneumaticmachine. This chapter deals only with basic hydraulicmachines.10-4
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