VOL. XtVII.J PHILOSOPHICAL TRANSACTIONS. 275 



lengthen or shorten a pendnlum of a common construction, in such a manner, 

 as sufficiently to correct the irregularities arising from the influence of cold or 

 heat upon it. The manner of applying them is represented in fig. 3, 



In which, aaaa represent the back plate of the clock, bbb a triangular piece 

 of brass, screwed by two screws, through the slits cc, to the plate, yet so that 

 it may be drawn backward or forward by means of the screw at d ; ef is a brass 

 bar, about 2 feet in length, made fast at the bottom, by a screw and two pins at 

 f, to an iron one of equal dimensions, to which it is likewise screwed by the 

 screws l , 2, 3, &c. after the same manner as the rod of the pendulum already 

 described. The iron bar is fastened at the upper end of the triangular piece of 

 brass, nearly under that part of the brass bar marked e; gh is a strong brass or 

 iron lever, moveable on a centre at g, and is supported by the upper end of the 

 brass bar; ii is the cock, on which, in a common clock, the pendulum is hung; 

 kk, part of the rod of the pendulum, whose spring passing through a fine slit in 

 the cock ii, is fastened to a stud rivetted into the lever at 1. The slit in the 

 cock must be made so close, as to prevent the spring from having any lateral 

 motion in it. 



From this description it is evident, that if the brass bar expand more than the 

 iron one, it will raise up the lever, and consequently the pendulum, which is 

 fastened to it; and as the length of the pendulum is only from the centre of 

 oscillation to the under part of the slit, through which the spring passes, the 

 pendulum will be thereby shortened; and by making the point of the brass bar 

 to act on a proper part of the lever (to which it is capable of being adjusted by 

 means of the screw d) the pendulum may be shortened to whatever degree shall 

 be necessary. 



To prevent the pendulum from bending the bars, which it would be liable to 

 do, if the ball of the pendulum was of any considerable weight, the end of the 

 lever, farthest from its centre of motion, is hooked to the end of a chain, which 

 is wound about and fastened to a small pulley at m. On the same arbor, to 

 which this pulley is fixed, is fastened another pulley, of a much larger diameter, 

 to which is hung, by a silk line, the weight or counterpoise n. By means of 

 this counterpoise, any part of the weight of the pendulum may be taken off 

 from pressing against the brass bar. And if, on the end of the arbor to which 

 the pulleys are fixed, an index be placed, so as to point to a graduated circle, the 

 least motion of the lever will not only be easily perceived, but also whether that 

 motion is uniform and regular, or not. And on having, some time after, made 

 a clock with this contrivance added to it, he found the index not only to move 

 very sensibly, but very regularly, and never, that he could perceive, by jerks. 

 And he doubts not, but, when the [xiint of bearing of the brass bar on the lever 



NN 2 



