308 THE PRINCIPLES OF SCIENCE. [CHAP. 



and the chronograph. 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 

 one ii5,2OOth part of a second, while more recently 

 Captain Noble has been able to appreciate intervals of 

 time not exceeding 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 

 anything external, and by another name is called duration; 

 relative, apparent and common time, is s'ome sensible and 

 external measure of duration by the means of motion." 1 

 Though we are perhaps obliged to assume the existence 

 of a uniformly increasing quantity which we call time, 

 yet we cannot feel or know abstract and absolute time. 

 Duration must be made manifest to us by the recurrence 

 of some phenomenon. The succession of our own thoughts 

 is no doubt the first and simplest measure of time, but a 

 very rude one, because in some persons and circumstances 

 the thoughts evidently flow with much greater rapidity 

 than in other persons and circumstances. In the absence 

 of all other phenomena, the interval between one thought 

 and another would necessarily become the unit of time, 

 but the most cursory observations show that there are 

 changes in the outward world much better fitted by their 

 constancy to measure time than the change of thoughts 

 within us. 



The earth, as I have already said, is the real clock of the 

 astronomer, and is practically assumed as invariable in 

 its movements. But on what ground is it so assumed ? 

 According to the first law of motion, every body perseveres 

 in its state of rest or of uniform motion in a right line, 

 unless it is compelled to change that state by forces im- 

 pressed thereon. Eotatory motion is subject to a like 



1 Principia, bk. i. Scholium to Definitions. Translated by Motte, 

 vol. i. p. 9. See also p. 1 1. 



