390 PHILOSOPHICAL TRANSACTIONS. [ANNO 17Q8. 



gg and gg are the end walls of the building, w and w are the leaden weights; 

 which are suspended by the copper rods RrprR, and the wooden bar rr, from the 

 centre pin pjb. This pin passes through a hole in the beam hh, perpendicularly 

 over the centre of the instrument, and turns round in it, being prevented from 

 falling by the plate/), mm is a pulley, fastened to this pin; and Mm a cord wound 

 round the pulley, and passing through the end wall ; by which the observer may 

 turn it round, and so move the weights from one situation to the other. 



Fig. 1 is a plan of the instrument, aaaa is the case ; ssss the 4 screws for 

 supporting it ; h h the arm and balls , w and w the weights; mn, the pulley for 

 moving them. When the weights are in this position, both conspire in drawing 

 the arm in the direction hw ; but, when they are removed to the situation w and 

 w, represented by the dotted lines, both conspire in drawing the arm in the con- 

 trary direction hw. These weights are prevented from striking the instrument, by 

 pieces of wood, which stop them as soon as they come within ± of an inch of the 

 case. The pieces of wood are fastened to the wall of the building ; and I find 

 that the weights may strike against them with considerable force, without sensibly 

 shaking the instrument. 



In order to determine the situation of the arm, slips of ivory are placed within 

 the case, as near to each end of the arm as can be done without danger of touch- 

 ing it, and are divided to 20ths of an inch. Another small slip of ivory is placed 

 at each end of the arm, serving as a vernier, and subdividing these divisions into 

 5 parts ; so that the position of the arm may be observed with ease to lOOths of an 

 inch, and may be estimated to less. These divisions are viewed, by means of the 

 short telescopes t and t, fig. 1, through slits cut in the end of the case, and 

 stopped with glass ; they are enlightened by the lamps l and l, with convex 

 glasses, placed so as to throw the light on the divisions ; no other light being 

 admitted into the room. The divisions on the slips of ivory run in the direction 

 ww, fig. 2, so that, when the weights are placed in the positions w and w, repre- 

 sented by the dotted circles, the arm is drawn aside, in such direction as to make 

 the index point to a higher number on the slips of ivory ; for which reason, I call 

 this the positive position of the weights. 



fk, fig. 1, is a wooden rod, which, by means of an endless screw, turns round 

 the support to which the wire gl is fastened, and so enables the observer to turn 

 round the wire, till the arm settles in the middle of the case, without danger of 

 touching either side. The wire gl is fastened to its support at top, and to the 

 centre of the arm at bottom, by brass clips, in which it is pinched by screws. In 

 these 2 figures, the different parts are drawn nearly in the proper proportion to 

 each other. 



Before proceeding to the account of the experiments, it will be proper to say 

 something of the manner of observing. Suppose the arm to be at rest, and its 

 position to be observed, let the weights be then moved, the arm will not only be 

 thus drawn aside, but it will be made to vibrate, and its vibrations will continue a 



