192 LECTURE XVII. 



obsert^ations, by the vibrations of pendulums; but they never connected 

 them with machinery. The equaUty of the times occupied by these vibra- 

 tions, whether larger or smaller, was known to Galileo in 1600, and some 

 time before 1633, he proposed that they should be applied to the regulation 

 of 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 on 

 the subject. In the same year, Hooke applied a spring to the balance of a 

 watch; and soon after, he conceived the idea of improving timekeepers suffi- 

 ciently for ascertaining the longitude at sea, but he was interrupted in the 

 pursuit of his plan. Hooke was also probably the first that employed for a 

 clock a heavy weight vibrating in a small arc; an arrangement from which 

 the peculiar advantages of a pendulum are principally derived. 



The objects which require the greatest attention in the construction of 

 timekeepers, are these; to preserve the moving power, or sustaining force, 

 as equable as possible, to apply this force to the pendulum or balance in the 

 most 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, the 

 sustaining force, being generally derived from a weight, is already sufficiently 

 equable, provided that care be taken that the line by which it is suspended may 

 be of equal thickness throughout, and may act on a perfect cylinder. But 

 in some clocks, and in all watches, the moving power is a spring. One of the 

 first 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 exerts 

 the more force as it is more bent; in order to correct this inequality, the 

 chain 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 it 

 becomes more distant from the point at which the force would cease alto- 

 gether, the curve being that which is denominated a hyperbola; but the 

 workmen have in general no other rule than a habitual estimation. (Plate 

 XV. Fig. 199.) 



