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T.ECTURE XVII. 



ON TIMEKEEPERS. 



X HE measurement of time by clocks and watches is a very important and in- 

 teresting department of practical mechanics. The subject is intimately con- 

 nected with the consideration of astronomical instruments, but it is not essen- 

 tially dependent on astronomical principles. 



Time is measured by motion ; but in order that motion may be a true 

 measure of time, it must be equable. Now a motion perfectly free and un- 

 disturbed, and consequently uniform, is rendered unattainable by the resist- 

 ances inseparable from the actual constitution of material substances. It 

 becomes therefore necessary to inquire for some mode of approximating to such 

 a motion. Astronomical determinations of time, which are the most accurate, 

 can only be made under particular circumstances, and even then they as- 

 sist us but little in dividing time into small portions. 



The first timekeepers somewhat resembled the hour glasses which are still 

 occasionally employed ; they measured the escape of a certain quantity, not of 

 sand, but of water, through a small aperture. In these clepsydrae, it ap- 

 pears from Vitruvius's account that wheelwork was employed, and the hour 

 was shown on a graduated scale; the graduations were also probably so ad- 

 justed as to correct the error arising from the inequality of the velocity oc- 

 casioned by the variation of the height of the water in the reservoir. This 

 inconvenience was however sometimes wholly avoided, by means of a con- 

 stant steam, which kept the vessel full, or still more elegantly, by the siphon 

 of Hero, which was a bent tube supported by a float, so that its lower orifice, 

 at which the water was discharged, was always at a certain distance below the 

 surface. Dr. Hooke proposed to keep the reservoir full, by means of a 



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