ON TIMEKEEPERS. 147 



rel a foot in circumference to revolve in two seconds, each hundredth of an 

 inch would correspond to the six hundredth part of a second ; and the scale 

 might be still further enlarged if it were necessary. (Plate XV. Fig. 198.) 



By means of this instrument we may measure, without difficulty, the 

 frequency of the vibrations of sounding bodies, by connecting them with a 

 point, which will describe an undulated path on the roller. These vi- 

 brations may also serve in a very simple manner for the measurement of 

 the minutest intervals of time ; for if a body, of which the vibrations are 

 of a certain degree of frequency, be caused to vibrate during the revolution 

 of an axis, and to mark its vibrations on a roller, the traces will serve as a 

 correct index of the time occupied by any part of a revolution, and the 

 motion of any other body may be very accurately compared with the 

 number of alternations marked in the same time, by the vibrating body. 

 For many purposes, the machine, if heavy enough, might be turned by a 

 handle only, care being taken to keep the balls in a proper position, and it 

 would be convenient to have the descent of the barrel regulated by the 

 action of a screw, and capable of being suspended at pleasure. 



But for the general purposes of timekeepers, all other inventions have 

 been almost universally superseded by the pendulum and the balance 

 spring, or pendulum spring. About the year 1000, Ibn Junis, and 

 the other Arabian astronomers were in the habit of measuring time, 

 during their observations, by the vibrations of pendulums; but they 

 never connected them with machinery. The equality of the times 

 occupied by these vibrations, 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 instrument to which he had him- 

 self 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 ; t and soon after, 

 he conceived the idea of improving timekeepers sufficiently for ascertaining 

 the longitude at sea, J 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 vibrations are in their nature as nearly isochronous as possible. In 

 clocks, the sustaining force, being generally derived from a weight, is al- 

 ready sufficiently equable, provided that care be taken that the line by 

 which it is suspended may be of equal thickness throughout, and may act 



* Mem. dell' Acad. del Cimento. The date is there stated as 1583. 



t Cassini laid claim to this invention, in behalf of Huygens, but Hooke proves 

 that he had not only conceived it but sent it to Huygens fifteen years before, who 

 wrote a letter against it as impracticable. Philosoph. Exp. &c. by Hooke, p. 388. 

 The main merit of the application of the pendulum to clocks probably belongs to 

 Huygens. + Ibid. p. 4. 



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