stomach, kidneys, etc. It here imparts oxygen to the body, and takes in impurities. It then enters the veins, and through them flows back to the right auricle; is driven into the right ventricle; is expelled into the pulmonary (lung) arteries; enters the lungs, and is purified. It returns to the left auricle through the pulmonary veins; enters the left auricle, passes to left ventricle, and so on.
A healthy heart beats from 120 times per minute in a one-year-old infant to 60 per minute in a very aged person. The normal rate for a middle-aged adult is from 80 to 70 beats.
Heart disease signifies the failure of the heart valves to close properly. Blood passes back when the heart contracts, and the circulation is much enfeebled. By listening through a stethoscope the doctor is able to tell whether the valves are in good order. A hissing sound during the beat indicates a leakage past the valves; a thump, or "clack," that they shut completely.
THE HYDRAULIC PRESS.
It is a characteristic of fluids and gases that if pressure be brought to bear on any part of a mass of either class of bodies it is transmitted equally and undiminished in all directions, and acts with the same force on all equal surfaces, at right angles to those surfaces. The great natural philosopher Pascal first formulated this remarkable fact, of which a simple illustration is given in Fig. 179. Two cylinders, A and B, having a bore of one and two inches respectively, are connected by a pipe. Water is poured in, and pistons fitting the cylinders accurately and of equal weight are inserted. On piston B is placed a load of 10 lbs. To prevent A rising above the level of B, it must be loaded proportionately. The area of piston A is four times that of B, so that if we lay on it a 40-lb. weight, neither piston will move. The walls of the cylinders and connecting pipe are also pressed outwards in the ratio of 10 lbs. for every part of their interior surface which has an area equal to that of piston B.