284 



THE POPULAR SCIENCE MONTHLY. 



As soon as I blow gently on the upper surface of the gravel, yovi see 

 the liquid in tlie manometer moving. The motion of the air which I 

 produce acts in the first instance on the surface of the gravel, jDropa- 

 gates itself through the same to the bottom of the cylinder, enters 



Fig. 2. 



the lower end of the tube, rises through it and through the tubing 

 into the manometer, where it presses on the column of liquid, and sets 

 it in motion. 



Wliy does the liquid move in the manometer ? Because the air, 

 after the migration just described, presses with greater weight on the 



