Perhaps your earliest contact with hydraulic pressure was when you got your first haircut. The hairdresser put a board across the arms of the chair, sat you on it, and began to pump the chair up to a convenient level. As you grew older, you probably discovered that the gas station attendant could put a car on the greasing rack and-by some mysterious arrangement-jack it head high. The attendant may have told you that oil under pressure below the piston was doing the job.

Come to think about it, youve probably known something about hydraulics for a long time. Automobiles and airplanes use hydraulic brakes. As a sailor, youll have to operate many hydraulic machines. Youll want to understand the basic principles on which they work.

Primitive man used simple machines such as the lever, the inclined plane, the pulley, the wedge, and the wheel and axle. It was considerably later before someone discovered that you could use liquids and gases to exert forces at a distance. Then, a vast number of new machines appeared. A machine that transmits forces by a liquid is a hydraulic machine. A variation of the hydraulic machine is the type that operates with a compressed gas. This type is known as the pneumatic machine. This chapter deals only with basic hydraulic machines.

A Frenchman named Pascal discovered that a pressure applied to any part of a confined fluid transmits to every other part with no loss. The pressure acts with equal force on all equal areas of the confining walls and perpendicular to the walls.

Remember when you are talking about the hydraulic machine, you are talking about the way a liquid acts in a closed system of pipes and cylinders. The action of a liquid under such conditions is somewhat different from its behavior in open containers or in lakes, rivers, or oceans. You also should keep in mind that you cannot compress most liquids into a smaller space. Liquids dont "give" the way air does when you apply pressure, nor do liquids expand when you remove pressure.

Punch a hole in a tube of toothpaste. If you push down at any point on the tube, the toothpaste comes out of the hole. Your force has transmitted from one place to another through the toothpaste, which is a thick, liquid fluid. Figure 10-5 shows what would happen if you punched four holes in the tube. If you were to press on the tube at one point, the toothpaste would come out of all four holes. You have illustrated a basic principle of hydraulic machines. That is, a force applied on a liquid transmits equally in every direction to all parts of the container.

We use this principle in the operation of four-wheel hydraulic automobile brakes. Figure 10-6 is a simplified drawing of this brake system. You push down on the brake pedal and force the piston in the master cylinder against the fluid in that cylinder. This push sets up a pressure on the fluid as your finger did on the toothpaste in the tube. The pressure on the fluid in the master cylinder transmits through the lines to the brake cylinders in each wheel. This fluid under pressure

pushes against the pistons in each of the brake cylinders and forces the brake shoes out against the drums.