Page:Mathematical collections and translations, in two tomes - Salusbury (1661).djvu/440

 derate the time, the Artificers that make them accomodate a certain voluble staffe horozontally, and at each end of it they fasten two Weights of Lead, and when the time goeth too slow, by the onely removing those Leads a little nearer to the centre of the staffe, they render its vibrations more frequent; and on the contrary to retard it, it is but drawing those Weights more towards the ends; for so the vibrations are made more seldome, and consequently the intervals of the hours are prolonged.

Here the movent vertue is the same, namely the counterpoise, the moveables are those same Weights of lead, and their vibrations are more frequent when they are neerer to the centre, that is, when they move by lesser circles. Hanging equal Weights at unequal cords, and being removed from their perpendicularity, letting them go; we shall see those that are pendent at the shorter cords, to make their vibrations under shorter times, as those that move by lesser circles. Again, let such a kind of Weight be fastened to a cord, which cord let play upon a staple fastened in the Seeling, and do you hold the other end of the cord in your hand, and having given the motion to the pendent Weight, whilst it is making its vibrations, pull the end of the cord that you hold in your hand, so that the Weight may rise higher and higher: In its rising you shall see the frequency of its vibrations encrease, in regard that they are made successively by lesser and lesser circies. And here I desire you to take notice of two particulars worthy to be observed. One is that the vibrations of one of those plummets are made with such a necessity under such determinate times, that it is altogether impossible to cause them to be made under other times, unlesse it be by prolonging, or abreviating the cord; of which you may also at this very instant ascertain your selves by experience, tying a stone to a pack-threed, and holding the other end in your hand, trying whether you can ever by any artifice be able to swing it this way and that way in other than one determinate time, unlesse by lengthening of shortening the string, which you will find to be absolutely impossible. The other particular truly admirable is, that the self same pendulum makes its vibrations with one and the same frequency, or very little, and as it were insensibly different, whether they be made by very great, or very small arches of the self-same circumference. I mean that whether we remove the pendulum from perpendicularity one, two, or three degrees onely, or whether we remove it 70. 80. nay to an entire quadrant, it being let go, will in the one case and in the other make its vibrations with the same frequency, as well the former where it is to move by an arch of but four or six degrees, as the second, where it is to passe arches of 160. or more