238 



NOTE upon the Pendulum Experiment fvide page 232). 



The Committee appointed to carry out the wishes of the 

 Society in the performance of Eoucault's experiment, after 

 several meetings, at last succeeded in devising an apparatus 

 and mode of suspension which has proved most satisfactory. 

 A cast-iron ball, weighing twenty-nine pounds avoirdupoise, 

 turned and bored through its axis, was suspended by an iron 

 wire, l-20th of an inch in diameter, from delicately construct- 

 ed jimbals fixed to a beam of wood so strong as to resist any 

 vibration arising from the motion of the pendulum. The 

 jimbals consisted of two brass cradles and two cross-bars of 

 steel at right angles. These bars were most carefully formed 

 at the extremities into knife edges, working in the same 

 horizontal plane within steel bushes inserted into the cradles. 

 To the under cradle the suspending wire was attached, and 

 the upper cradle was firmly fixed to the beam. A universal 

 motion was thus obtained, with exceedingly little fnction and 

 great strength. On the latter ground, this mode of suspen- 

 sion is superior to that at Paris ; and the result of the experi- 

 ments, which have been very numerous, goes to shew that 

 pe^^fect confidence may be imposed in the apparatus. Below 

 the ball a circular and concave table was placed, the centre of 

 which coincided with the vertical, when the pendulum was at 

 rest. This table, formed of cement upon a wooden frame, was 

 ten feet in diameter and carefully divided into degrees, so that 

 the motion might be readily and accurately noted; and a 

 mechanical contrivance enabled the observer to drop the ball 

 without its receiving at the first an elliptical motion. 



The pendulum when made to vibrate, invariably conformed 

 with the theory, not only in respect to the direction of its 

 apparent motion, but in point of time, supposing the motion to 

 be unity at the pole and infinity at the equator. Each degree 

 was passed over in five minutes, and this equally at every 

 angle ; thus giving to a complete revolution a period of thirty 

 hours, which for this latitude is slightly more than the time 



