FIG. 26.—THE BALANCE LEVER, PREDECESSOR OF THE PENDULUM
The pendulum had an interesting origin. Galileo, while a student in Pisa in 1581, was attending a service in the cathedral one day when his attention was drawn to the swinging of a large hanging lamp. One of the attendants had drawn the lamp toward him, so that he could reach it more readily to light it. When he let go the lamp began to swing slowly back and forth, and the observant young student noticed that although the oscillations gradually slowed down the period of oscillation was constant. He had no watch with which to measure the length of the period, but being a medical student he knew that he had a fair timepiece in his own heartbeats, and so by counting his pulse he proved that it took the lamp just as long to complete each oscillation when it swept through a long arc as when it died down to but a few inches. He made note of this peculiar action and began experimenting with pendulums of different lengths. Then it occurred to him that if he could time the pendulum with his pulse he could time his pulse with a pendulum. So he devised a pendulum whose length could be adjusted until its oscillation would coincide with the throb of a patient’s pulse, and then the length of the pendulum would give him the rate of the pulse beat. The invention was seized upon by the medical profession of that day, and the pulsilogia, as it was called, became an indispensable instrument for physicians.
BUCKET OF A GIANT DREDGE WITH ITS JAWS SPREAD WIDE OPEN
GOLD DREDGER EATING ITS WAY THROUGH RIVER SANDS
A SUCTION DREDGE EXCAVATING A CHANNEL AND BUILDING NEW LAND