Page:Mathematical collections and translations, in two tomes - Salusbury (1661).djvu/228

 roof, if we remove it far from perpendicularity, and then let it go, have you not observed that, it declining, will pass freely, and well near as far to the other side of the perpendicular?

I have observed it very well, and find (especially if the plummet be of any considerable weight) that it riseth so little less than it descended, so that I have sometimes thought, that the ascending arch is equal to that descending, and thereupon made it a question whether the vibrations might not perpetuate themselves; and I believe that they might, if that it were possible to remove the impediment of the Air, which resisting penetration, doth some small matter retard and impede the motion of the pendulum, though indeed that impediment is but small: in favour of which opinion the great number of vibrations that are made before the moveable wholly ceaseth to move, seems to plead.

The motion would not be perpetual, Sagredus, although the impediment of the Air were totally removed, because there is another much more abstruse.

And what is that? as for my part I can think of no other?

You will be pleased when you hear it, but I shall not tell it you till anon: in the mean time, let us proceed. I have proposed the observation of this Pendulum, to the intent, that you should understand, that the impetus acquired in the descending arch, where the motion is natural, is of it self able to drive the said ball with a violent motion, as far on the other side in the like ascending arch; if so, I say, of it self, all external impediments being removed: I believe also that every one takes it for granted, that as in the descending arch the velocity all the way increaseth, till it come to the lowest point, or its perpendicularity; so from this point, by the other ascending arch, it all the way diminisheth, untill it come to its extreme and highest point: and diminishing with the same proportions, wherewith it did before increase, so that the dgrees of the velocities in the points equidistant from the point of perpendicularity, are equal to each other. Hence it seemeth to me (arguing with all due modesty) that I might easily be induced to believe, that if the Terrestrial Globe were bored thorow the centre, a Canon bullet descending through that Well, would acquire by that time it came to the centre, such an impulse of velocity, that, it having passed beyond the centre, would spring it upwards the other way, as great a space, as that was wherewith it had descended, all the way beyond the centre diminishing the velocity with decreasements like to the increasements acquired in the descent: and the time spent in this second motion of ascent, I believe, would be equal to the time of descent. Now if the moveable by diminishing that its greatest degree of velocity which it