EVOLUTION OF QUARTZ CRYSTAL CLOCK 



513 



The hour glass, and its smaller counterparts, is one of the most convenient 

 forms of this device and until quite recent times served a useful purpose 

 where accuracy was of no great importance. The hour glass shown in Fig. 2 

 was used by a pastor in the early eighteen hundreds to determine the length 

 of his sermons. The average variation among a set of ten one-hour de- 

 terminations made recently with this glass was 3 minutes, or about 5 per cent. 

 The clepsydra that were designed to repeat and totalize an endless succes- 

 sion of cycles were especially adaptable to the measurement of extended 

 intervals of time, although with very poor accuracy as we now think of it. 



Fig. 2 — Hour glass. 



By suitable design any desired number of cycles could be made equal to the 

 natural large unit, the day, so that any fraction of a day within the accuracy 

 of a given instrument could be determined simply by counting off the 

 number of cycles from a particular starting point such as sunrise, sunset, or 

 high noon. It was possible with these devices to operate without calibra- 

 tion over periods of several days, although the cumulative error inevitably 

 was very large. 



An error of a few hours was of small importance in the days when the 

 speed of communication and travel alike depended on pack animals or the 

 caprices of the wind. And so, in spite of the inaccuracies of the water clocks 

 and sand clocks, they served their purpose well through many centuries. 



