514 BELL SYSTEM TECHNICAL JOURNAL 



In fact, it was not until the tenth century A.D. that any really novel effort 

 was made to improve upon them as timekeepers. The first efforts to 

 improve upon them, making use of falling weights for motive power and 

 various frictional devices to control the rate of fall, were not very successful 

 because no satisfactory means were known to keep a friction-controlled 

 device sufficiently constant for the job. Clocks so constructed were no 

 better timekeepers on the whole than the traditional clepsydra. They had, 

 however, the hope of compactness, and much ingenuity was exercised in 

 their design over several centuries. 



Also in the category of continuous flow devices should be mentioned the 

 methods depending on the rate of burning, such as in time candles, time 

 lamps and their numerous variations. Such timekeepers are not very 

 accurate but are thoroughly reliable in dry, quiet places, even providing 

 their own illumination at night. Such timekeepers are known to have been 

 used before the tenth century A.D. and certain variations still are used by a 

 few isolated tribes, especially in the tropics. 



The Epoch of Aperiodic Control 



In or about the year 1360 the invention of an escapement mechanism for 

 controlling an alternating motion from a steady motive power, such as a 

 suspended weight, was the first really important step in the history of pre- 

 cision clock development, and marks the beginning of the second major 

 epoch in timekeeping evolution. The escapement in one form or another 

 was soon applied in practically all timekeepers, the most outstanding example 

 of an early application being a clock constructed by Henry De Vick for 

 Charles V of France in or about the year 1360 A.D. and still in use — with 

 extensive modifications — in the Palais de Justice in Paris. 



This invention was important, not because De Vick's clock, or any of its 

 immediate successors, were good timekeepers, but because this was the 

 first time that vibratory motion in a mechanism was used deliberately to 

 control the rate of a time-measuring device. All precision clocks depend in 

 one way or another on using energy to produce vibratory motion, and on 

 using the rate of that motion to regulate suitable dials and other mechanisms. 



No simple improvement on De Vick's clock could ever have produced a 

 precision clock in the modern sense, however, because the essential rate- 

 controlling feature was still lacking. His invention consisted of the use of 

 a verge escapement which produced oscillatory motion in a dynamically 

 balanced member, known as a foliot balance, having essentially only mo- 

 ment of inertia and friction. The rate of oscillation, therefore, depended to 

 a large extent on the applied force exerted by the falling weight through a 

 train of wheels, and upon the friction of the escapement parts and of the 

 oscillating member itself. 



