Obstructions to the Vihratiofi of a Mercurial Pendulum. 157 



the weight was above the ball, it gained about as much. The 

 time of winding is Saturday evening, and the bottom of the 

 weight reaches the top of the jar of mercury on Wednesday 

 morning, and descends about four inches, or half its length, be- 

 low it, before the time of winding ; passing within about five 

 eighths of an inch of the side of the jar and in a line with it. 

 I found that on winding up the weight, the vibrations would at- 

 tain their usual length (1° 20') in about thirty minutes. I found 

 also, that by opening the door of the clock when the weight and 

 pendulum ball were in conjunction, the vibrations would imme- 

 diately increase and soon extend to l'^ 30'. 



Further confirmation of the evidence that these changes were 

 caused by the obstruction in the passage of the air, was furnished 

 when I had occasion to take out the glass door of the clock and 

 put in one of wood, (temporarily,) which diminished the distance 

 from the ball of the pendulum from three fourths to one fourth 

 of an inch. This change caused a decrease in the arcs of vibra- 

 tion, of 0^ 12' on each side of the centre, and a loss in time of 

 three seconds daily. On replacing the glass door, the usual vi- 

 brations of 1° 20' were resumed, and the time corrected. 



It is well known that the velocity of steam vessels and canal 

 boats is much diminished, by shoal water in narrow passages, 

 where the escape of the water displaced by the boat, is obstruct- 

 ed on the same principle by which the weight of a clock ob- 

 structs the escape of the air displaced by the motion of the pen- 

 dulum, when confined in a small space. 



It may be well to state, that the jar of my pendulum is eight 

 inches in depth, and two inches in diameter inside. The col- 

 umn of mercury is seven inches, and the weight of the whole, 

 with the rod and jar frame, is twelve pounds. The weight is of 

 lead, cased in brass ; length, nine inches and five tenths ; diame- 

 ter, one inch and four tenths ; shape, round ; weighs five pounds 

 and three ounces, and is suspended by a double line and pulley. 

 Depth of the clock case in the clear, five and a half inches ,* 

 width at the centra of oscillation, fourteen inches ; and extends 

 below the bottom of the jar three inches. 



Since recording these observations, I have read a chapter in 

 Read's Treatise on clock and watch making, on pendulums, 

 (which I had overlooked,) where I find an account of similar 

 experiments, although the influence of the weight on the motion 



