318 PHILOSOPHICAL TRANSACTIONS. [aNNO 1739. 



arm that held it, though at no other time has he ever been sensible of any 

 such thing. 



He several times after repeated this experiment with the thread and ball both 

 wet, and found it succeed much better than when they were both dry ; and 

 once he had 220 revolutions before he rested his arm. He tried too with the 

 ball dry, and the string only made wet ; but the virtue did not continue so 

 long, as when both were wet. 



He now flattered himself with hopes of success, if the thread was suspended 

 from an undoubted fixed point, which therefore he proceeded again to try with 

 the greatest care and caution, but in vain; the revolutions were uncertain. 



This ditFerence naturally led him to reflect on the cause of it. The tremor 

 of the hand would not account for it ; for this being both ways backward as 

 well as forward, must as often hinder as promote a continual motion one way: 

 and though in two opposite parts of a circle, the motion is really in contrary 

 directions, and therefore the contrary impulses of a tremor may promote a re- 

 volution applied at opposite places of the orbit ; yet as these tremors are irre- 

 gular, and succeed much quicker than the revolutions are performed, they seem 

 insufficient to account for the motions of the pendulous body, performed with 

 any degree of regularity. 



A stream of air in the room might impel along the tangent the pendulous 

 body, kept at a distance from the ball by its repulsive force ; and then gravity, 

 taking place, might with the first motion compound a curve: but still the re- 

 sistance of the air would soon destroy the original impulse, could a few revo- 

 lutions be performed; and besides, one revolution could not be performed, 

 because the same stream of air that began the motion, must be contrary to it 

 in its return. 



A finger held on the right hand near the pendulous body, when suspended 

 from a fixed point, will make it revolve from west to east; but then it must 

 be applied and removed alternately : the repulsive force therefore which the arm 

 may acquire, by being held in the sphere of the effluvia, is insufficient ; for, 

 as it is in one place, it must impel only one way, and constantly the same way; 

 and therefore, like a stream of air in the room, though it might create the 

 beginning, it must binder the completion of a revolution. 



Sometimes he doubted, whether the pulse of the arm might not be assisting 

 in giving a projectile motion. When one leg is laid over the knee of the 

 other, a motion and heaving of the leg that lies over, answering to every stroke 

 of the pulse, is very apparent at a distance : the arm therefore in some posi- 

 tions, in which its great artery meets with a proportionable pressure or resist- 

 ance, may have a constant motion, though less perceivable. 



