ANAXIMANDER—Greek astronomer to whom the Greeks ascribed the invention of the sun-dial in the sixth century B. C. ARBOR—The axle or axis on which a wheel of a watch or clock turns. Also applied to a spindle used by watchmakers. ARC—Any section of the circumference of a circle. ARCHIMEDES—A famous Greek philosopher and scientist sometimes credited with the invention of the clock. About 200 B. C. he made a machine with wheel work and a maintaining power but having no regulator it was no better as a time teller than a planetarium turned by a handle. It may have furnished the suggestion for later time-keeping machines. ARNOLD, JOHN—Born 1736. An English watchmaker of note. He invented the helical form of the balance spring and a form of chronometer escapement much like Earnshaw's. Died 1799. Arnold's devices have been most useful and permanent. ASSEMBLING—The putting together of the finished parts of a watch. In a three-quarter plate watch this is done on the lower plate. In a full plate movement it is easier and more satisfactory to assemble on the top plate. ASTROLABE—1. An instrument of various forms formerly used especially in navigation to measure the altitudes of planets and stars. 2. A projection of a sphere upon any of its great circles. ASTRONOMICAL TIME—Means solar time, as computed from observing the passage of the sun across the meridian from noon of one day to noon of the following day. It is counted continuously up to 24—not in two 12-hour divisions. ASTRONOMY—The science which treats of the motions, real and apparent, of the heavenly bodies. Upon this science, through its determination of the length of the year, is founded the science of horology—or time-keeping. AUTOMATA—FOR STRIKING—Very common on old clocks and very complicated, such as: Indian King hunting with elephants, Adam and Eve, Christ's flagellation, and many others. See _Clocks, Interesting Old_. AUTOMATIC MACHINERY—The second great contribution of America to watchmaking after the establishment of the principle of interchangeability of parts, and making possible the effective execution of that principle. AUXILIARY—A device attached to a compensation balance to reduce what is known as the "middle temperature error." Some are constructed to act in high temperatures only—as Molyneux's; and some in low temperatures only—as Poole's. BALANCE—The vibrating wheel in a watch or chronometer which with the aid of the balance spring (hair-spring) regulates the rate of travel of the hands. The balance is kept in vibration by means of the escape wheel. See _Compensation Balance_. BALANCE ARC—In detached escapements, that part of the vibration of the balance in which it is connected with the train. The remainder is called the drop. BALANCE-CLOCK—A form of clock built before the pendulum came into use. The regulating medium was a balance on the top of the clock made with a verge escapement. See _Foliot_. [Illustration: BALANCE COCK]