Sabtu, 03 April 2010

The History of Clocks

The word clock is actually related to words that mean "bell" in other languages, for example the Latin clocca, the Middle Dutch clocke, and the Middle German glocke. The earliest, medieval clock towers had no hands to mark the hours. Instead, a bell rang out passing hours.

The earliest, most ancient clocks simply reflected the sky's cycles for people to see. They were giant circles made of stones or other material that marked the passing of seasons and the movement of stars. Many scientist believe that Stonehenge, England's ancient grouping of huge stones, once served as such a "clock". At certain times of the year, the sun and the moon would line up with certain stones. When this happened, the ancient people knew a new season had begun.

No one ever thought about dividing days into equal parts until about 4,000 years ago. It was then that the idea of a 24-hour day was invented, probably by the ancient Babylonians. To measure the hours, they invented the sundial. A sundial is a circle with marks that show the hours between sunrise and sunset. A stem in the middle of the dial throws a shadow on the marks. As the sun travels across the sky, the shadow moves and shows time. But sundials were not always useful. For one thing, they only worked on sunny days.

Then about 3,400 years ago, the Egyptians learned to create cycles that they could use to tell time. They figured out that water flows from a hole in a container at a steady rate. This idea led to the invention of the water clock. In early water clocks, water seeped out a small hole in the bottom of a stone container. Markings on the sides showed the hours. A person could tell time by the amount of water left in the container.

People were using sand clocks by the 1300's. They worked in much the same way as water clocks. The simplest sand clocks were two glasses bulbs connected by a small "neck". Sand poured from the upper bulb to the lower. When all the sand had flowed from the upper bulb, a timekeeper knew that a certain amount of time had passed. Sand clocks that took an hour to empty were called hourglasses.

Then, about 700 years ago, mechanical clocks first appeared in Europe. To mark the passing time, these clocks used rising and falling weights to turn a gear. The gear turned until it triggered the ringing of a bell. Mechanical clocks, like water clocks and sand clocks., measured hours as they passed. But the earliest mechanical clocks really didn't "tell the time". They had no dial or hands-they only gave a signal when an hour had passed. And they didn't even do that very well. The chiming was off by up to fifteen minutes a day.

Time-telling as we know it began in 1656, when Dutch inventor, Christiaan Huygens, developed a pendulum clock. He used the steady motion of a freely swinging pendulum to mark the time. The swinging of the pendulum is a more dependable cycle than dripping water or hanging weights. As a result, the pendulum clock was accurate to within about fifteen seconds a day!

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