Chemical Change A change that produces one or more new substances and may release energy. Chemical Property A second set of properties in all kinds of matter. Circuit A path that is made from an electrical current. Claws A sharp, usually slender, and curved nail on the toe of an animal.
Originally, sandglasses were used as a measure for periods of time like the lamps or candles, but as clocks became more accurate they were used to calibrate sandglasses to measure specific periods of time, and to determine the duration of sermons, university lectures, and even periods of torture.
The clock is one of the oldest human inventions, meeting the need to consistently measure intervals of time shorter than the natural units: the day, the lunar month, and the year. Devices operating on several physical processes have been used over the millennia.