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Make Your Own Working Paper Clock

Descriere: Clocksby Isaac AsimovThrough most of history, people hardly felt the need of clocks. It seemed sufficient to consult one's own physiology to tell when one was hungry or sleepy, or to observe the general position of the sun in the sky during the day or that of the Big Dipper at night.Those who were meticulous enough to want something better searched for some regular motion that existed in nature or that could be contrived. In ancient times, the sundial was invented so that the passage of the shadow of a rod could be followed as the sun crossed the sky.Or else one could observe the extent to which a candle burned downward, or wait till a certain amount of sand had sifted through a small opening. Such devices could be used on cloudy days or at night, when the sun could not be seen and shadows were not observed.In ancient times, the best timepiece was the clepsydra, or water clock, which measured time by the regular dripping of water through a narrow opening. As water accumulated in the lower reservoir, a float carrying a pointer rose and marked the hours. The best water clocks were quite elaborate but few in number and fragile. They could not be relied on to tell time more closely than a fairly large fraction of an hour.In medieval Europe, the mechanical clock was invented. Clever arrangements of gears and wheels were devised that could be made to turn by weights attached to them. As the weights were pulled downward by the force of gravity, the wheels were forced to turn in a slow, regular manner. A pointer, properly attached to the wheels, marked the hours.These mechanical clocks were less delicate than water clocks and required less maintenance. They became commonin churches and monasteries and could be relied on to tell when to toll the bells for regular prayers or church attendance. (The very word "clock" is from the French cloche, meaning "bell.")Eventually, mechanical clocks were designed to strike the hour and even to chime the quarter-hour. However, they had only an hour hand and were not enclosed. Even the best such clocks would gain or lose up to half an hour a day.No clock in existence, up through 1656, could measure short intervals of time accurately, or could possibly be relied on to tell time to the minute. This meant that advances in physical science were scarcely possible. Almost all of physics and much of chemistry (and even biology) depend on rates, on the rapidity with which processes take place, on the amount of change that takes place per unit of time. In order to measure such rates with the precision required for the development of the laws of nature, intervals of time must be marked off with far greater exactness than was possible for the crude clocks of ancient and medieval times.In the 1590s, for instance, the Italian scientist Galileo measured the speed of falling bodies. This was the crucial beginning of modern physics and, therefore, of modern science. His experiments disproved the physics of Aristotle that had held sway for eighteen centuries and laid the foundation for Isaac Newton's later laws of motion and theory of universal gravitation, on which (allowing for Einstein's improvements, and for the addition of electromagnetism and the two nuclear forces) science is still based.Yet Galileo had no device for measuring the time it took for balls to slide down the groove of an inclined plane. it took them anumber of seconds to do so, and there existed no clock that could mark off seconds. He had to stand there taking his own pulse and counting his heartbeats as the balls moved downward. That sufficed for the purpose-but just barely. It made his conclusions little more than an intelligent approximation. To go further, more was needed.Yet in 1582, Galileo (then a teenager) had noticed the swaying chandeliers in a cathedral. it seemed to him that the movement back and forth was always the same whether the swing was a large one or a small one. He timed that with his pulse and then experimented with swinging weights when he got home. He found that the "pendulum" was a way of marking off small intervals of time more regularly than the pulse beat, although he himself never used it for the purpose.Once Galileo had made the discovery, it was inevitable that the regular beat of the pendulum would someday be used to regulate the movement of the wheels and gears of a clock so that they would be made to go, as the common phrase has it, "as regular as clockwork."It wasn't easy. The pendulum swings through the arc of a circle, and when that is so, the time of the swing does vary slightly with its size. To make the pendulum keep truly accurate time, it must be made to swing through a curve known as the "cycloid." One must also figure out a way of hitching it to clockwork so that the falling weights keep the pendulum swinging, and the pendulum then forces the clockwork to move more regularly than with the weights alone.In 1656 the Dutch astronomer Christian Huygens first devised a successful pendulum clock. (Astronomy could not advance further without knowledge of just how quickly the heavenlyobjects moved across the sky and how they shifted position relative to one another.) He used short pendulums that beat several times a second, encased the works in wood, and hung the clock on the wall.In 1670 an English clockmaker, William Clement, made use of a pendulum about a yard long; it took a full second to move back and forth, allowing greater accuracy. He encased the pendulum and weights in wood also, in order to diminish the effect of air currents. Thus was born the "grandfather's clock." For the first time, it made sense to add a minute hand to the dial, since it was now possible to measure time to the nearest second.There have been numerous improvements to time -- keeping devices ever since. In place of pendulums we now use atomic vibrations that will keep clocks accurate to within a second or less for thousands of years. Nothi

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Autori: James Smith Rudolph | Editura: Harper Paperbacks | Anul aparitiei: 1983 | ISBN: 9780060910662 | Numar de pagini: 40 | Categorie: Crafts  


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