part 10

90 The body has many different ways of keeping itself warm enough and not too warm. It uses up more food, more fuel for the cells, in cold weather. It shivers—that is, its muscles go on making quick little motions to keep it warm. We cannot stop ourselves from shivering. The body's heating system is not under our conscious control; it is automatic. To stop shivering we have to warm ourselves some other way: we may run or walk quickly, or take hot drinks, or cover ourselves up warmly, or use hot water bottles to warm our beds.

91 Our bodies give off some of their heat all the time by sending a little liquid out through the skin. Sometimes the amount gets greater and our skin becomes wet. This cools the body quickly. Here the man on the left is shivering because he has been waiting in line for a long time in the cold wind. The man on the right is wet all over because he got too hot in the sun. He is drinking water because he feels the need of it. He is thirsty.

92 This man is in training. He is going to run a distance of one mile in the shortest possible time. The fastest runners before him have taken a little under four minutes and he wants to make a new record. When a man runs a mile in four minutes he is going at a rate of fifteen miles an hour, but it is not possible to keep running at this rate very long. The runner is standing now in front of his doctor who has an instrument in his hand. Through this the doctor can hear the sounds made by the runner's heart.

93 Now the doctor is taking the man's pulse. He has his finger on the man's wrist and is counting the times his heart pumps the blood in one minute. The doctor can feel the motion or pulsing of the blood every time the man's heart pumps. In most people the pulse rate is between seventy and eighty times a minute. In children it is higher. The runner's pulse rate is 72. It will be higher when he is running.

94 There is a good story about the use the great Italian scientist Galileo once made of his knowledge of the pulse rate. Galileo lived three hundred years ago before people had watches for measuring time. In the sixteenth century there were very few clocks in the world and no watches. One day when Galileo was a young man of eighteen, he was in the great church of the city of Pisa where he lived. Watching the motion of a light hanging down on a long chain from the roof high above his head, he saw that as the light moved forward and back on its chain, it seemed to take the same time between turns, however far it went.

95 To see if he was right about the motion of the light, Galileo put his fingers on his wrist. He timed the motion of the light as it went forward and back on its chain, measuring it by his pulse rate. He was right. He had the proof. He could prove that the time was the same, however short or long the journey of the light through the air was. Galileo had made a great discovery about the motion of a pendulum. Among other things this discovery made possible a better instrument for measuring time—the pendulum clock.