SOME EEFINEMENTS OF MECHANICAL SCIENCE. 143 



true meridian, with an accuracy never before attained, was consid- 

 ered one of the greatest scientific achievements of that age. 



Since an unknown time the day has been divided into twenty-four 

 liours, and as civilization has advanced the greater has been the ne- 

 cessity for the utmost precision in the measurement of each hour with 

 its subdivisions. 



The sun dial is not only the earliest, but the most interesting of all 

 the numerous arrangements that have been devised for measuring 

 the divisions of the day. Notwithstanding its limitations, it has 

 been a subject which has attracted the brightest minds for ages. 

 Within these later years there has been a renewed interest in this an- 

 cient timekeeper, not only in copying the types of dials, which are 

 valuable because of their antiquity, but in working out new forms. 

 Recently a new dial has been invented by which the rays of the sun 

 w^ll indicate the true mean time for each day of the year with an 

 error not to exceed one minute. 



The hour glass, which came later, was considered a much more 

 practical method, inasmuch as it could be used either day or night, 

 and because its use was not confined to a ])articular location ; how- 

 ever, as a timekeeper it was not satisfactory, even in those early 

 days. 



The clepsydra, or water clock, which is supposed to have been in- 

 vented by the Greeks, was found to be a nnicli better timekeeper than 

 either the sun dial or hour glass, and it was a great step in advance 

 toward the accurate measurement of time. 



These water clocks are to this day used extensively in the East, 

 more especially in China. Those first used by the Greeks consisted 

 of two water jars so arranged that the water from the upper ran into 

 the lower, and the time of day was determined by measuring the 

 depth of water in the upper jar, and at sunrise each day the water 

 was returned to the upper jar. In the city of Canton there is a water 

 clock which has been running for eight hundred years, and at the 

 present time it is the standard clock of that city. This clock consists 

 of four water jars, each having a capacity of 8 or 10 gallons. The 

 jars are placed one above the other in the form of a terrace, the 

 three upper ones being provided with a small orifice near the bottom 

 through which the water drops into the jar next below, and so on 

 down from one to the other until the water reaches the lowest or 

 registering jar. In this there is a float, to which is attached an up- 

 right, having graduations for the hours and parts of hours, and as 

 the water rises the time can be determined l)v noting the height of 

 the float in relation to the crossbar at the top of the jar. 



In this improved form of water clock the variation in the flow of 

 water due to the difference in height is overcome by having a series 

 of jars, the outlet of the upper l)eing so graduated that there is but 



