EVOLUTION OF QUARTZ CRYSTAL CLOCK 511 



days when the sun shines clearly enough to cast a shadow. These short- 

 comings became more and more important with advances in society and, 

 for measuring duration, man soon began inventing timekeeping means that 

 would work without benefit of the sun. 



The evolution of timekeeping devices may be divided into three main 

 periods, each employing a specific type of method, although overlapping to 

 some degree in their applications, and characterized by increasing orders of 

 accuracy. 



A graphical representation of this evolution, indicating these three periods 

 of development, and showing the relation between some of the major contri- 

 butions to time keeping and the resulting accuracy of time measurement, is 

 shown in Fig. 1. The methods employed chiefly during these three periods 

 may be classified broadly as CONTINUOUS FLOW from the beginning up 

 until about 1000 A.D., as APERIODIC CONTROL from then until about 

 1675 A.D. and as RESONANCE CONTROL from that time up to the 

 present. Keeping in mind the logarithmic nature of the time and accuracy 

 scales used in this graph, it can be seen readily that most of the advance- 

 ment has been made in a very small part of the total time, corresponding to 

 the resonance control epoch. 



The Epoch of Continuous Flow 



Perhaps due to a feeling that the passage of time was like the flow of some 

 medium, the first time measuring devices were those depending on the flow 

 of water into or out of suitable basins. It was recognized that, with an 

 orifice properly chosen, the time required to fill or empty a given basin should 

 be about the same on repetition, and hence was born the first reliable means 

 for measuring time at night or on overcast days. A great variety of devices 

 operating on this principle were constructed and used, some of the earliest 

 having been made by the Babylonians and the Egyptians 3500 years ago. 



Some of these water clocks, or clepsydra as they were called, had floats 

 or other indicators which were intended to subdivide a unit of time into 

 substantially ^uniform divisions. Others were constructed so that successive 

 fillings of the basin would be counted or would operate a stepping device, 

 associated with a dial or other indicator. Through the centuries great 

 numbers of such devices were constructed, with some of the later ones having 

 elaborate mechanisms for striking the hours or for animating figures of 

 people or animals. 



For use in places where water was not readily available and where sand 

 was plentiful, clepsydra were developed that would operate with the flow 

 of sand in much the same way as with the flow of water. The basic ideas 

 were not greatly diff^erent, the substitution being merely one of expedience. 



