146 LECTURE XVII. 



by a fly ; the index is stopped by the machinery, and points out the time 

 elapsed without an error of the hundredth part of a second. 



The alternate motion of a balance, thrown backwards and forwards by 

 the successive actions of a wheel impelling its pallets, is also capable of 

 producing a degree of uniformity in the motion of the wheel ; for the force 

 operating on the pallet is consumed in destroying a velocity in one direc- 

 tion, and in generating a velocity in the contrary direction ; and the space 

 in which it acts being nearly the same in all cases, the velocity generated 

 will also be nearly the same at all times, as long as the force remains the 

 same. The addition of a balance to a clock was made soon after the year 

 1400, for Arnault* who died in 1465, describes a planisphere constructed 

 by his master De Fondeur, which had a balance with a scapement like that 

 of a common watch, but without a spring. Such a balance vibrates much 

 more slowly than a balance provided with a spring ; if the balance spring 

 of a common watch be removed, the hands will pass over the space of about 

 twenty eight minutes in an hour. 



It is said that before the pendulum was used, a balance wheel was some- 

 times suspended in a horizontal position by a thread passing through its 

 axis, which coiled round it and caused it to rise and fall as it oscillated 

 backwards and forwards. This mode of regulation differed but little in 

 principle from the modern pendulums, but it was more complicated and 

 less accurate. Huygens, in somewhat later times, constructed a clock with 

 a revolving weight, which rose higher, and increased the resistance, when- 

 ever an augmentation of the force increased the velocity ; and he caused 

 the thread which supported the weight, to bend round a curve of such a 

 form as to preserve the equality of the revolutions. 



A chronometer may be constructed on this principle for measuring small 

 portions of time which appears to be capable of greater accuracy than 

 Mr. Whitehurst's apparatus, and by means of which an interval of a 

 thousandth part of a second may possibly be rendered sensible. If two 

 revolving pendulums be connected with a vertical axis, in such a manner as 

 to move two weights backwards and forwards accordingly as they fly off 

 to a greater or smaller distance, the weights sliding, during their revolution, 

 on a fixed surface, a small increase of velocity will considerably increase 

 the distance of the weights from the axis, and consequently the effect of 

 their friction, so that the machine will be immediately retarded, and 

 its motion may thus be made extremely regular. It may be turned by 

 a string coiled round the upper part, and this string may serve as a 

 support to a barrel, sliding on a square part of the axis, which will conse- 

 quently descend as it revolves. Its surface, being smooth, may be covered 

 either with paper or with wax, and a pencil or a point of metal may be 

 pressed against it by a fine spring, so as to describe always a spiral line on 

 the barrel, except when the spring is forced a little on one side by touching 

 it slightly, either with the hand, or by means of any body of which the 

 motion is to be examined, whether it be a falling weight, a vibrating cord 

 or rod, or any other moving substance. In this manner, supposing a bar- 



* Venturi, Essai sur les Ouvrages de L. da Vinci, p. 28, quoting MS. No. 7295, in 

 the National Library of Paris. 



