228 



NEW PENDULUM. 



Dcfcription of 

 the tubular 

 pendulum by 

 the drawing. 



Difficulty of 

 preventing flex 

 ure and darting 

 motion in the 

 gridiron^ 



clofes the upper end of the external tube, the, large circle in 

 the centre is the place where the brafs covering for the upper 

 part of the middle wire is inferted, and the two fmall circles 

 denote the faftening for the wires of ihe laft expanfion. Fig. 7 

 reprefents the bottom of the exterior tube, in which the fmall 

 circles (hew the faftening places for the wires of the fecond 

 expanfion, and the larger ones the holes for the other pair of 

 wires to pafs through. Fig. 8. is a cylindrical piece of brafs, 

 which fliews how the lower ends of the wires of the laft ex- 

 panfion are faftened to it, and the hole in the middle is that 

 whereby it is pinned to the centre of the bob. The faftening 

 of the upper ends of the two pair of wires is done by fcrew- 

 ing them into the pieces which ftop up the ends of the tubes, 

 but at the lower ends, they are all fixed as reprefented in Fig. 8. 

 I have only to add to this defcription, that the pieces repre- 

 fented by Fig, 7 and 8, have each a jointed motion, by means 

 of which the fellow wires of each pair would be equally 

 ftretched, although they were not exactly of the fame length. 



In the apparatus thus conne(5ted, the middle wire will be 

 flretched by the weight of the whole; the interior tube will 

 fupport at its top the whole except the middle wire: the 

 fecond pair of wires will be ftretched by all except the mid- 

 dle wire and interior tube; the exterior tube fupports at its top 

 the weight of the fecond pair of wires and the bob, and the 

 fecond pair of wires are ftretched by the weight of the bob 

 only, 



Thefirft pendulum which I made of the tubular kind, had 

 only three fteel wires, and one tube above the bob; that is two 

 expanfions down and one up; and the quantity which one 

 of brafs falls fliort to corredl two of fteel, was compenfated for, 

 by extending tbofe branches of the rod below the bob, and bring- 

 ing up an external tiibe to which the bob was affixed. There 

 is an aukwardnefs in this conftruclion, owing to the rod reach- 

 ing about 13 inches below the lower edge of the bob, other- 

 wife, it is not inferior to the one firft dc fcribed. 



The principles of the gridiron pendulum, I believe, have 

 'never been queftioned, indeed they cannot be; the difficulty 

 of conftrufting it ftrong enough to prevent lateral flexure 

 in the lifting bars, and confequent friction in the holes of the 

 different crolfing pieces neceftary to bind it together, which 

 occafion it to act by ftarts, has been the only obftacle to pre- 

 vent its gen«ral application to the beft clocks. 



To 



