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$. 16. Whether these several ideas in a Ideas, towa man's mind be made by certain motions,

ever made, I will not here dispute : but this I am sure,

include no that they include no idea of motion in their sense of mo.

tion, appearance ; and if a man had not the idea of motion otherwise, I think he would have none at all : which is enough to my present purpose, and suthiciently shows, that the notice we take of the ideas of our own minds, appearing there one after another, is that which gives us the idea of succession and duratio!), without which we should have no such ideas at all. It is not then notion, but the constant train of ideas in our minds, whilst we are waking, that furnishes us with the idea of duration : whereof motion no otherwise gives us any perception, than as it causes in our minds a constant succession of ideas, as I have before showed : And we have as clear an idea of succesion and duration, by the train of other ideas succeeding one another in our minds, without the idea of any motion, as by the train of ideas caused by the uninterrupted sensible change of distance between two bodies, which we have fron motion; and therefore we should as well liave the idea of duration, were there no sense of motion at all.

j. 17. Having thus got the idea of duration, the next thing natural for the mind Time is du

ration set out to do, is to get some neasure of this com- by measures. mon duration, whereby it might judge of its different lengths, and consider the distinct order wherein several things exist, without which a great part of our knowledge would be confused, and a great part of history be rendered very useless. This consideration of duration, as set out by certain periods, and marked by certain measures or epochs, is that, I think, which most properly we call time,

18. In the measuring of extension, A good mea. there is nothing inore required, but the ap- sure of time plication of the standard or measure we must divide make use of to the thing; of whose exten- its whole due sion we would be inforined.

equal periods. measuring of duration, this cannot be done, because no two different parts of succession can be put

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sun and moon, the

together to measure one another : and nothing being a
measure of duration but duration, as nothing is of ex-
tension but extension, we cannot keep by us any stand-
ing unvarying measure of duration, which consists in a
constant fieeting succession, as we can of certain lengths
of extension, as inches, feet, yards, &c. marked out in
permanent parcels of matter. Nothing then could serve
well for a convenient measure of time, but what has di-
vided the whole length of its duration into apparently
equal portions, by constantly repeated periods. What
portions of duration are not distinguished, or considered
as distinguished and measured by such periods, come
not so properly under the notion of time, as appears by
such phrases as these, viz. before all time, and when time
shall be no more.
The revolu. $. 19. The diurnal and annual revolu-
tions of the tions of the sun, as having been, from the

beginning of nature, constant, regular, and

universally observable by all mankind, and properest measures of supposed equal to one another, have been time.

with reason made use of for the measure of duration. But the distinction of days and years having depended on the motion of the sun, it has brought this mistake with it, that it has been thought that motion and duration were the measure one of another: for men, in the measuring of the length of time, having been accustomed to the ideas of minutes, hours, days, inonths, years, &c. which they found themselves upon any mention of time or duration presently to think on, all which portions of time were measured out by the motion of those heavenly bodies: they were apt to confound time and motion, or at least to think that they had a necessary connexion one with another : whereas any constant periodical appearance, or alteration of ideas in seemingly equidistant spaces of duration, if constant and universally observable, would have as well distinguished the intervals of time, as those that have been made use of. For supposing the sun, which some have taken to be a fire, had been lighted up at the same distance of time that it now every day comes about to the same meridian, and then gone out again

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about twelve hours after, and that in the space of an annual revolution, it had sensibly increased in brightness and heat, and so decreased again ; would not such regular appearances serve to measure out the distances of duration to all that could observe it, as well without as with motion. For if the appearances were constant, universally observable, and in equidistant periods, they would serve mankind for measure of time as well, were the motion away.

$. 20. For the freezing of water, or the But not by blowing of a plant, returning at equidis- their motion, tant periods in all parts of the earth, would but periodias well serve men to reckon their

years by, as the motions of the sun: and in effect we see, that some people in America counted their years by the coming of certain birds amongst them at their certain seasons, and leaving thein at others. For a tit of an ague, the sense of hunger or thirst, a smell or a taste, or any other idea returning constantly at equidistant periods, and making itself universally be taken notice of, would not fail to measure out the course of succession, and distinguish the distances of time. Thus we see that men boru. blind count time well enough by years, whose revolutions yet they cannot distinguish by motions, that they perceive not: and I ask whether a blind man, who distinguished his years either by the heat of summer, or cold of winter; by the smell of any flower of the spring, or taste of any fruit of the autumn; would not have a better measure of time than the Romans had before the reformation of their calendar by Julius Cæsar, or many other people, whose years, notwithstanding the motion of the sun, which they pretend to make use of, are very irregular? And it adds no small difficulty to chronology, that the exact lengths of the years that several nations counted by, are hard to be known, they differing very much one from another, and I think I may say all of them from the precise motion of the sun, And if the sun moved from the creation to the flood constantly in the equator, and so equally dispersed its light and heat to all the habitable parts of the earth, in days all of the same length, with

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out its annual variations to the tropicks, as a late in-
genious author supposes *; I do not think it very easy
to imagine, that (notwithstanding the motion of the
sun) men should in the antediluvian world from the be-
ginning, count by years, or measure their time by pe-
riods, that had no sensible marks very obvious to dis-
tinguish them by.
No two parts

$. 21. But perhaps it will be said, withof duration out a regular motion, such as of the sun, can be cer- or some other, how could it ever be known tainly known that such periods were equal? To which I to be equal.

answer, the equality of any other returning appearances might be known by the same way that that of days was known, or presumed to be so at first ; which was only by judging of them by the train of ideas which had passed in nien's minds in the intervals : - by which train of ideas discovering inequality in the natural days, but none in the artificial days, the artificial days or rugbúuspe were guessed to be equal, which was sufficient to make them serve for a measure : though exacter search has since discovered inequality in the diurnal revolutions of the sun, and we know not whether the annual also be not unequal. These yet; by their presumed and apparent equality, serve as well to reckon time by (though not to measure the parts of duration exactly) as if they could be proved to be exactly equal. We must therefore carefully distinguish betwixt duration itself, and the measures we make use of to judge of its length. Duration in itself is to be considered as going on in one constant, equal, uniform course : but none of the measures of it, which we make use of, can be known to do so ; nor can we be assured, that their assigned parts or periods are equal in duration one to another; for two successive lengths of duration, however measured, can never be demonstrated to be equal. The motion of the sun, which the world used so long and so confidently for an exaet measuro ot duration, has, as I said, been found in its several parts unequal : And though inen have of late made use of a

# Dr. Burner's Theory of the Earth,

pendulum,

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pendulum, as a more steady and regular motion than
that of the sun, or (to speak more truly) of the earth;
yet if any one should be asked how he certainly knows
that the two successive swings of a penduluin are equal,
it would be very hard to satisfy him that they are intal-
libly so: since we cannot be sure, that the cause of
that motion, which is unknown to us, shall always
operate equally; and we are sure that the medium in
which the pendulum moves, is not constantly the same :
Either of which varying, may alter the equality of such
periods, and thereby destroy the certainty and exactness
of the measure by motion, as well as any other pe-
riods of other appearances; the notion of duration still
remaining clear, though our measures of it cannot any
of them be demonstrated to be exact. Since then no
two portions of succession can be brought together, it
is impossible ever certainly to know their equality.
All that we can do for a measure of time is to take
such as have continual successive appearances at seem-
ingly equidistant periods; of which seeming equality
we have no other measure, but such as the train of our
own ideas have lodged in our memories, with tlie con-
Currence of other probable reasons to persuade us of
their equality.
5. 92. One thing seems strange to me,

Time not the that whilst all men manifestly measured

measure of tiine by the motion of the great and visible motion. bodies of the world, time yet should be detined to be the measure of motion;" whereas it is obvious to every one who reflects ever so little on it, that to measure motion, space is as necessary to be considered as time : and those who look a little farther, will find also the bulk of the thing moved necessary to be taken into the computation, by any one who will estimate or measure motion, so as to judge right of it. Nor indeed does motion any otherwise conduce to the measuring of duration, than as it constantly brings about the return of certain sensible ideas, in seeîning equidistant periods. For if the motion of the. siin were as unequal as of a ship driven by unsteady winds, sometilges very slow, and at others irregularly very swift;

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