22 LFXTURE ir. 



come to consider its uniformity. Here the idea of time enters into our sub- 

 ject. To define time in general is neither easy nor necessary; but we must 

 have some measure of equal times. Our abstract idea of time depends on the 

 memory of past sensations ; but it is obvious that the results of an intellectual 

 measure of the duration of time would be liable to the greatest uncertainty. 

 We may observe, that, on a journey, the perpetual succession of various ob- 

 jects will often make a week appear, upon retrospection, as long as a month 

 spent in a continuation of such employments as are uniform, without being la- 

 borious ; the multitude of new impressions not only serving to increase the ap- 

 parent magnitude of the interval, by filling up its vacuities, but tending also 

 to diminish the vivacity of the ideas which they have superseded, and to give 

 them the character of the fainter recollections of an earlier date. We are 

 therefore obliged to estimate the lapse of time by the changes in external ob- 

 jects: of these changes, the simplest and most convenient is the apparent mo- 

 tion of the sun, or rather of the stars, derived from the actual rotation of the 

 earth on its axis, which is not indeed an undisturbed rectilinear motion, but 

 which is equally applicable to every practical purpose. Hence we obtain, by 

 astronomical observations, the well known measures of the duration of time, 

 implied by the terms day, hour, minute, and second. 



Now the equality of times being thus estimated from any one motion, all 

 other bodies moving without disturbance, will describe equal successive parts 

 of their lines of direction in equal times. And this is the second law of mo- 

 tion, which, with the former law, constitutes Newton's first axiom or law of 

 motiort: " that every body perseveres in its state of rest or uniform rectilineaf 

 motion, except so far as it is compelled by some force to change it." It ap- 

 pears that this second law is strictly deducible from the axioms and definitions 

 which have been premised, and principally from the consideration of the re- 

 lative nature of motion, and the total deficiency of a criterion of absolute mo- 

 tion. For, since the velocity of a body, moving without resistance or disturb- 

 ance, is only a relation to another body, if the second body has no mechanical 

 connexion with the first, its state with respect to motion can have no efi^ect 

 on the velocity of the first body, however great its comparative magnitude 

 may be : and if a body is at rest, there is nothing to determine it to begin to 

 move either to the right hand or to the left; if it is at rest with respect to 

 any other bodies, it will remain in the same condition, whatever the relative 



