PRESSURE IN LIQUIDS 25 



the openings are all at the same depth. The mercury is thus forced 

 up into the long arm of each tube (which must reach above water) 

 by the force of the water at the end. Compare the height of the 

 three mercury columns. What does this show regarding liquid 

 pressure at a given depth? 



34. Pressure depends upon Depth. Now, since pres- 

 sure in liquids is caused by the action of gravity upon 

 their molecules, and gravity acts downward only, it is 

 clear that the pressure upon any point will depend only 

 upon the weight of the molecules above it. It is also 

 clear that the weight will depend upon the number of 

 particles above the point, .and that their number will 

 in turn depend upon the depth of that point below the 

 surface. From this we may state the general principle : 

 In any liquid, pressure upon a point increases with its 

 depth below the surface. 







Experiment 23. Push an empty can into water slowly, tak- 

 ing care not to get it entirely below the surface. Do you notice 

 any difference in the force that you have to exert as the can goes 

 farther down? 



35. Surface Level. Since gravity pulls all particles 

 of a liquid as low as possible, and the particles are all 

 free to move, no part of a liquid surface can be higher 

 than another unless acted upon by some force that is 

 stronger than gravity ; that is, the surface of a liquid 

 at rest is always level. 



36. Water Supply. If a vessel a (Fig. 11) be filled 

 with water to a height cc' and then connected with an 

 empty vessel b by a tube at the bottom, the water will 

 flow out of a and rise in b until it stands at the same 



