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LECTURE XVII.

observations, by the vibrations of pendulums; but they never connectedthem with machinery. The equality of the times occupied by these vibra-tions, whether larger or smaller, was known to Galileo in 1600, and sometime before 1635, he proposed that they should be applied to the regulationof clocks. But Sanctorius , in his commentary on Avicenna , describes an in-strument to which he had himself applied the pendulum in 1612. Huygens made the same application only in 1658, which is the date of his work onthe subject. In the same year, Hooke applied a spring to the balance of awatch; and soon after, lie conceived the idea of improving timekeepers suffi-ciently for ascertaining the longitude at sea, but he was interrupted in thepursuit of his plan. Hooke was also probably the first that employed for aclock a heavy weight vibrating in a small arc; an arrangement from whichthe peculiar advantages of a pendulum are principally derived.

The objects which require the greatest attention in the construction oftimekeepers, are these; to preserve the moving power, or sustaining force,as equable as possible, to apply this force to the pendulum or balance in themost eligible manner, and to employ a pendulum or balance of which the vi-brations are in their nature as nearly isochronous as possible. In clocks, thesustaining force, being generally derived from a weight, is already sufficientlyequable, provided that care be taken that the line by which it is suspended maybe of equal thickness throughout, and may act on a perfect cylinder. Butin some docks, and in all watches, the moving power is a spring. One of thefirst clock springs is said to have been an old sword blade; a clock with such

a spring was lately preserved at Brussels : the spring which is at present used,

is a thin elastic plate of steel, coiled into a spiral form. Every spring exertsthe more force as it is more bent; in order to correct this inequality, thechain or cord by which it acts on the work is wound on a spiral fusee; so

that, in proportion as the force is lessened, it is applied to a larger cylinder,

or a longer lever. The general outline of the fusee must be nearly such,that its thickness at any part may diminish in the same proportion as itbecomes more distant, from the point at which the force would cease alto-gether, the curve being that which is denominated a hyperbola; but theworkmen have in general no other rule than a habitual estimation, (PlateXV. Fig. 199.)