THE FORCE OF GRAVITATION II 



very beautiful although very common (most beautiful things 

 are common), and I am going to put the weight upon it, to 

 give you a sort of illustration of the downward pressure of 

 the iron, and of the power which the air possesses of re- 

 sisting that pressure ; it may burst, but we must try to avoid 

 that. [During the last few observations the lecturer had 

 succeeded in placing the half cwt. in a state of quiescence 

 upon the inflated India-rubber ball, which consequently as- 

 sumed a shape very much resembling a flat cheese with 

 round edges.] There you see a bubble of air bearing half 

 a hundred weight, and you must conceive for yourselves 

 what a wonderful power there must be to pull this weight 

 downward, to sink it thus in the ball of air. 



Let me now give you another illustration of this power. 

 You know what a pendulum is. I have one here (Fie. i), 

 arid if I set it swinging, it will continue to swing to and fro. 

 Now I wonder whether you can tell 

 me why that body oscillates to and 

 fro that pendulum bob, as it is 

 sometimes called. Observe, if I hold 

 the straight stick horizontally, as high 

 as the position of the ball at the two 

 ends of its journey, you see that the 

 ball is in a higher position at the two 

 extremities than it is when in the ^ ___ 



middle. Starting iroin one end of \J 



the stick, the ball falls toward the FIG. i 



centre, and then rising again to the opposite end, it con- 

 stantly tries to fall to the lowest point, swinging and vi- 

 brating most beautifully, and with wonderful properties in 

 other respects the time of its vibration, and so on but 

 concerning which we will not now trouble ourselves. 



If a gold leaf, or piece of thread, or any other substance 

 were hung where this ball is, it would swing to and fro in 

 the same manner, and in the same time too. Do not be 

 startled at this statement; I repeat, in the same manner and 

 in the same time, and you will see by-and-by how this is. 

 Now that power which caused the water to descend in the 

 balance which made the iron weight press upon and flatten 

 the bubble of air which caused the swinging to and fro of 



