360 THE PRINCIPLES OF SCIENCE. 



wonderful advances which have been made in the practical 

 measurement of its efflux. The rude sun-dial or the 

 rising of a conspicuous star, gave points of reference, while 

 the flow of water from the clepsydra, the burning of a 

 candle, or, in the monastic ages, even the continuous 

 equable chanting of psalms, gave the means of roughly 

 subdividing periods, and marking the hours of the day and 

 night 6 . The sun and stars still furnish the standard of 

 time, but means of accurate subdivision have become 

 requisite, and this has been furnished by the pendulum 

 and the chronoscope. By the pendulum we can accurately 

 divide the day into seconds of time. By the chronograph 

 we can subdivide the second into a hundred, a thousand, 

 or even a million parts. Wheatstone measured the dura- 

 tion of an electric spark, and found it to be no more than 



zoo part of a second, while more recently Captain Noble 



has been able to appreciate intervals of time, not exceed- 

 ing the millionth part of a second. 



When we come to inquire precisely what phenomenon 

 it is that we thus so minutely measure, we meet insur- 

 mountable difficulties. Newton distinguished time accord- 

 ing as it was absolute or apparent time, in the following 

 words : 



' Absolute, true, and mathematical time of itself and 

 from its own nature, flows equably without regard to any- 

 thing external, and by another name is called duration ; 

 relative, apparent and common time, is some sensible and 

 external measure of duration by the means of motion''. 

 Though we are perhaps obliged to assume the existence 

 of a uniformly increasing quantity which we call time, 



e Sir G. C. Lewis gives many curious particulars concerning the mea- 

 surement of time, 'Astronomy of the Ancients,' pp. 241, &c. 



' Principia/ bk. i. Scholium to Definitions.' Translated by Motte, 

 vol. i. p. ix. See also, p. 1 1. 



