UNITS AND STANDARDS OF MEASUREMENT. 361 



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 and change 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 be- 

 tween one thought and another, would necessarily become 

 the unit of time. 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 impressed thereon. Rotatory motion is subject 

 to a like condition, namely, that it perseveres uniformly 

 unless disturbed by extrinsic forces. Now uniform mo- 

 tion means motion through equal spaces in equal times, 

 so that if we have a body entirely free from all resistance 

 or perturbation, and can measure equal spaces of its path, 

 we have a perfect measure of time. But let it be remem- 

 bered at the same time, that this law has never been 

 absolutely proved by experience ; for we cannot point to 

 any body, and say that it is wholly unresisted or undis- 

 turbed ; and even if we had such a body, we should need 

 some entirely independent standard of time to ascertain 

 whether its motion was really uniform. As it is in moving- 

 bodies that we find the best standard of time, we cannot 

 theoretically speaking use them to prove the uniformity 

 of their own movements, which would amount to a petitio 

 principii. Our experience amounts to this, that when 

 we examine and compare the movements of bodies which 

 seem to us nearly free from disturbance, we find them 

 give nearly harmonious measures of time. If any one 



