Methods for keeping approximate track of time date from antiquity. Sundials, for example, were used by the ancient Egyptians. In the cloudier climates of Europe, however, sundials proved (1)___. Another ancient Egyptian invention was the water clock, in which water dripped from one reservoir into another at a relatively constant (2) ___. The second reservoir contained a float attached to a pointer arm. As the float rose over the course of a day, the pointer indicated the approximate hour.

The first true mechanical clocks emerged in the 14th century in Europe. History gives no single person credit for the key invention that made these clocks (3)___. The name clock, which originally meant "bell," was first applied in the present sense to the huge, mechanical time indicators 4)___ in bell towers in the late Middle Ages.

Clockworks were (5)___ heavy, bulky devices. The apparatuses for controlling their rates were crude and the clocks inaccurate. Early mechanical clocks had only hour hands—understandable considering that these timepieces were often (6)___ by as much as two hours a day. Subsequent refinements, such as the development of the pendulum, made it possible to (7)___ reasonably accurate track of minutes and seconds as well. Minute and second hands, and crystals to protect both the dial and hands, first appeared in the 17th century.

The achievement of artificial timekeeping has had a great impact. It became an important part of navigation, as mariners (8)___ on accurate time measurements to calculate longitude. It was a (9)___to science, as scientific observations often require accurate measurements of time. The same is true for many of the operations of business and industry, for which coordination of events and human activities is of great importance. Today, an increasingly industrialized world is highly structured by time: our clocks (10)___ when we work, play, eat, and sleep.