treme bars, and allow the other feven to move freely.

In all this it is fuppofed, that the expansion of the bra's is to that of the steel exactly as five to three, and that all the dimenfions are accurately laid down. If the contrary appears, the gridiron may be adjusted by fhifting the cross-piece F upwards or downwards, as occafion may require.

Before we proceed to a fresh fubject, I fhall beg leave to fay a few words on the nature of time; as, unless you are able to form abftract ideas of time and fpace, you will never truly apprehend fpiritual fubjects; whereas juft notions thereof will throw very important light on religious and metaphyfical fpeculations. "If nobody afk me what time is," fays St. Auguftine, I think I know; but if any body afk me, I cannot tell :" and indeed it is not easy to give a clear idea thereof. Are days, and months, and years, defcriptive of time? Days, and months, and years, are measured by the laws of motion; but the measure is different from, and only relative to time. Time would be, were days, and years, and months, to be confounded. Lord Monboddo defines time as the measure of the duration of things that exift in fucceffion, by the motion of the celestial bodies.

All the fubjects to which time is applicable, are faid to be in time; and are all, fome way or other affected by time. But it is evident, that beings which fuffer no change in substance, qualities, nor energies, cannot, without fome reftriction, be faid to be in time. Of this kind we conceive the Divine Being to be.

For time cannot be applied to any being of ftable and permanent duration, without change or variation of any kind; but only to beings that are in a conftant flux, and always changing, either as to their fubftance, or their qualities and energies.

The

Dr. Clarke, by confounding the ideas of fucceffive and unfucceffive duration, advanced a strange paradox, which he thus expreffes: "How any thing can have exifted eternally, that is, how an eternal duration can be actually paft, is utterly unconceivable; and yet to deny that an eternal duration is now actually paft, is an exprcfs contradiction." Now he fhould have faid quite the contrary; namely, that to maintain that an eternal duration is actually paft, is altogether repugnant and contradictory. It is indeed not only unconceivable, but abfolutely impoffible. Wherever there is a laft, there must be a firft: now in eternity there can be no firft, and therefore no last, Whatever is fucceffive cannot be eternal: no addition of finite fucceffions can make abfolute infinite.

This fubject cannot be confidered too closely, if you wish to avoid the errors of modern materialifts; for you will find one of them declaring, that he can cafier admit the non-existence of God himself, than the non-existence of space or duration thus conceiving, as Mr. Hindmarth has juftly inferred, time and space, which in themfelves are unfubftantial, inanimate, and deftitute of intelligence, to be more neceffary and independent in their existence, than him whom we call GOD.*

I will conclude this part of the prefent Lecture, with a fhort extract from that part of Mr. Tucker's " Light of Nature" which he terms "The Vifion."--" And there ftood before him an angel; his countenance was mild and lively, and his raiment white and fhining. He had spangled wings growing from his fhoulders, his fides, and his legs. And he faid, 'O man, come along with me: I will 'teach

*For a confutation of Dr. Frieftley's notions on this and other fubje&s, lee Hindmarsh's Letters to Priestley,"

teach thee what to think of glories thou canst not 'comprehend, and make thee more fenfible of the 'infinite distance between the creature and the 'Creator.' So faying, he took me up, and carried me to the utmoft bounds of the univerfe. And he faid, Try now if thou canft create a new world 'beyond this.' And I faid, 'Far be it from me to attempt incroaching upon the divine prerogative. Thou knoweft I am poor and weak, unable to act 'without corporeal inftruments; and the little power I have, is given me.' And he faid, ‹ I 'know thy weaknefs; that the power is not thine; ' nor do I expect that thou fhouldeft operate. I 'am commiffioned for thine inftruction. Know, 'then, that creative power awaits thy direction, for a trial of what thou canft perform by it.' 'Alas!' faid I, I know not where to begin, nor how to proceed!'

And he faid, Stretch forth thy right-arm, and 'thruft it ftrait from thy fide.' And I effayed, but could not thruft it out. Not that I found any thing to refift me; but when my arm came to the utmoft verge of the univerfe, it feemed as if I had loft the use of it, fo that I could not move it further. And I asked the angel, Wherefore cannot 'I move my arm this way? I can thruft it above or 'below, before or behind me; but I cannot stretch it out from my fide.' And he faid, Because there is no space to receive it.' And I faid, Since it hath fo pleased the creative power, and this for mine inftruction, may there be fpace." And he faid, Put forth thine arm again.' And I put it forth with eafe ftrait from my fide, as I could have done any other way.

And he faid, Wave now thy fingers to and fro; moving them in order, one after another.' Accordingly, I endeavoured to do as I was bidden, but could not perceive whether my fingers moved VOL. III.

or

direction of the motion), to be interpofed between the two bodies; then in whatever degree the spring is compreffed, it will exert the fame elaftic force on each of the bodies, (the inertia of the spring not being confidered.) When the spring first begins to be compreffed, the force by which it retards the ftriking body, and accelerates the body ftruck, is the leaft of all; it afterwards will continually increase during each fucceffive inftant, till it's compreffion is the greateft poffible. The intensity of this force may vary according to any imaginable law; but whatever be the variation, it will act equally on both fides.

When the two bodies have acquired a common velocity, the fpring can acquire no further compreffion, and may then begin to restore itself with various degrees of force, from o to the force by which it was compreffed. When the reftitutive force is equal to that of compreffion, the action of the fpring will be fimilar to that of perfectly elaftic bodies.

Motion cannot be communicated to any body inftantaneously, but must be produced by gradual acceleration; it not being conceivable, that any really exifting body fhould pafs from quiefcence into finite motion, or from one degree of finite motion to another, without having poffeffed all the intermediate degrees of velocity.

Suppose a spherical body to impinge on another body of the fame form, moving in the fame direction with the line which joins their centers, and let both of them be perfectly non-elaftic.

When the furfaces of thefe fpheres are just in contact, the distance of their centers will just be half the fum of their diameters; but as the figures are gradually changed by the impact, their centers will become nearer than before. During the time of their approach, the center of the first ball will

move with a greater velocity than that of the ball ftruck, and the refiftance which the spheres oppofe to the change of their figures, will act equally in both spheres, but in contrary directions; that is, the force by which the ftriking body is refifted, will urge forward the body ftruck; which, therefore, will be gradually accelerated, and the striking ball retarded, until the centers are at their greatest distance, the change of the bodies figures being then the greateft; at which inftant all acceleration of the ball ftruck, and retardation of the ftriking ball, ceases, and the two centers begin to go on with a common velocity.

This reafoning, concerning collifion of bodies, is equally applicable to the cafe in which both bodies are non-elaftic, as when one is perfectly non-elaftic, and the other perfectly hard. It is likewife equally applicable to the collifion of bodies which are claftic in any degree, fo far as regards the velocity communicated to the body ftruck, during that fmall but finite portion of time in which the figures of the spheres receive their greatest change, their centers then beginning to go on with a common velocity in bodies of every kind of texture.*

OF ELASTICITY.

The action whereby bodies, whose figures are changed, restore themselves to their former figure, is termed elafticity. An elaftic body is therefore one whofe figure being changed, it recovers, or has a tendency to recover, it's figure.

There are in nature, as you have already feen, varieties of activities; in fome of which the causes are rendered manifeft by experimental inquiries; in others, and among thefe we may reckon clastiQ 3

*See Atwood on Re&tilinear Motion.

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