12 GRAVITATION. 



a weight, because in it the exercise of that 

 power of pressing downwards is especially used 

 for the purposes of weighing ; and I have also 

 one of these little inflated india-rubber bladders, 

 which are 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 pos- 

 sesses of resisting that pressure ; it may burst, 

 but we must try to avoid that. [During the 

 last few observations the Lecturer had suc- 

 ceeded in placing the half cwt. in a state of 

 quiescence upon the inflated india-rubber ball, 

 which consequently assumed 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 downwards, 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 (fig.\\ and if I set it swinging, 

 it will continue to swing to and fro. Now, I 

 wonder whether you can tell me why that body 



