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the enquiry into nature, either in the way ofcertain calculation, estimation, or comparison, as the case will admit. 46. In the twenty-second place, among our prerogative instances come instances of the course, or stage, which we also sometimes call hydrometrical instances", deriving the term from the hour-glasses of the ancients, wherein they used water instead of sand. These instances measure nature by moments of timet, as the instances of the stafff measure them by degrees of space. For all motions, or natural actions, are performed in time; one indeed swifter, and another slower, but all in certain moments well known to nature. Even those actions which seem suddenly performed, or in the twinkling of an eye, as we phrase it, are yet found to differ in time, as to more or less.
And, first, we see that the revolutions or returns of the celestial bodies are performed in certain times or periods, so likewise is the flux and reflux of the sea. The descent of heavy bodies towards the earth, and the ascent of light bodies towards the heavens, is performed in certain moments, according to the nature of the body, and the medium it movesin. The motions of a ship, in sailing; of a horse, or other creature, in running; of a projectile, in flying, &c. are all, in like manner, performed in certain times, measurable in the amount or result. And, with regard to heat, we see that boys, in the winter, will wash their hands in the flame of a common fire, without burning themselves; and, in the way of sport, others will, by a nimble and equable motion, turn glasses of wine, or water, upside down, and recover them again, without spilling; and there are many particulars of the same kind. So likewise some compressions, dilatations, and eruptions, or explosions of bodies, happen swifter, and others slower, according to the nature of the body and the motion, but they happen in certain moments of time. Thus, in the joint explosion of several large cannons, which may be heard sometimes to the distance of thirty miles, the report is first audible to those near the place, where the discharge is made, and afterwards to those who are farther off”. And in vision, where the action is exceeding swift, it is plain that certain moments of time are required to its performance, as is plain from hence, that bodies are rendered invisible through too great a velocity of motion, as in the discharge of a bullet from a gun, where the ball flies too swift to have its impression received by the eye. And, upon comparing this with the like cases, we have sometimes entertained a strange suspicion, viz. whether the stars of a clear sky be seen by us at the precise time they really exist, or rather somewhat later; and whether there be not, with regard to the sight of the heavens, a true and apparent time, as well as a true place, and apparent place, which is observed by the astronomers in the parallaxes. For it seems incredible that the rays of the celestial bodies should instantly travel such an immense distance to the sight, and not rather take up some considerable time in the journey". But this suspicion, as to any great interval betwixt the real and apparent time, afterwards vanished, upon considering that infinite loss and diminution of quantity, as to sight, between the real body of a star, and the apparent object, which difference is caused by the distance; and, at the same time, considering to what a distance objects that are barely white may, of a sudden,
* As if it were instances of the time-keeper, or hourglass
* See above, Aph.44. # See above, Aph. 45.
* See Mr. Whiston's second edition of his Essay upon the Longitude; and Dr. Derham's Paper upon the Motion of Sounds, in the Philosophical Transactions, No. 313.
* See this time computed by M. Huygens de la Lumiere, p. 8 and 9, See also Sir Isaac Newton upon the subject; and compare them both with the Papers of M. Maraldi in the French Memoirs, Ann. 1707.
be seen here below, amounting to sixty miles at the least”; for there is no question but that the light of the celestial bodies has not only the vivid strength of whiteness, but also vastly exceeds the light of flame, as we find flame here in power and strength of radiancy. Nay, that immense velocity wherewith gross matter moves, in the diurnal rotation, renders this wonderfully swift motion of the rays of light, from the fixed stars, more probable. But what has the greatest weight with me, is this, that if there should here be any considerable space of time between reality and sight, or the existence of the object, and its being seen, it must then happen, that the sight would be frequently intercepted and confounded by clouds arising in the mean time, or by the like disturbances in the medium. And thus much for the simple mensuration of time. The measure of motions and actions is not, however, to be sought only simply, but much rather comparatively, this being a thing of excel
lent use, and having regard to very many parti
culars. We find, that the flash of a great gun is seen before the sound is heard, although it is certain that the bullet must strike the air before the flame, which was behind it, could get out; and that this must happen from a greater velocity in
* See again, Mr. whiston's Essay upon the Longitude.
the motion of light, than in the motion of sound. We find also, that visible objects are sooner received than let go by the sight, whence it is that the strings of a musical instrument, struck with the finger, appear double, or treble, in, the vibration, viz. because a new object is received before the other is discharged; and for the same reason, rings twirling upon an axis seem spheres; and a lighted flambeau, carried hastily by night, appears tailed, like a comet. And, from this foundation of the inequality of motion in point of velocity, Galilaeo imagined the cause of the flux and reflux of the sea to be from the earth's revolving with a greater velocity than the waters; whence the waters gathering into a heap upwards, afterwards sunk down by degrees, as we see in a vessel of water briskly revolved. But this solution he invented barely upon supposition, and not upon proof, of the earth's motion, and also without being well informed of the sexhorary motion of the sea. But we have an eminent example of the comparative measure of motion, and at the same time of its remarkable use in the business of powder-mines, wherein vast masses of earth, piles of buildings, &c. are overturned, and tossed into the air with a small quantity of gunpowder. The cause whereof is doubtless this, that the motion of dilatation in the powder, which is theim