124 



HYDROSTATICS. 



that the weight of the counterpoise in the scale-pan, the 

 weight of the liquid contained in P, and the downward 

 pressure exerted on the plate by the liquid contained in 

 M, P, or Q are equal. We therefore deduce the following 

 rule: 



To find the downward pressure on a horizontal 

 surface, find the weight of an imaginary column 

 of the given liquid, ivhose base is the same as the 

 given surface, and whose altitude is the same as 

 the depth of the given surface below the surface 

 of the liquid. 



Note.^A cubic foot of water weighs about 1000 ounces, 624 

 pounds (more exactly 62.42 Ibs.). 



. Upward Pressure. Some persons have dif- 

 ficulty in understanding that liquids have upward pres- 



sure. This upward pressure may 

 be illustrated as follows : Take a 

 glass tube open at both ends, hav- 

 ing at its lower end a glass or mica 

 disc supported from its centre 

 by a thread. If this apparatus 

 be placed in water, the tube 

 being vertical, the upward pres- 

 sure of the water will hold tbe 

 disc- in its place. If the disc does 

 not accurately fit the end of the 

 tube, water will be forced into the 

 tube, and gradually fill it from 

 below. If the disc does fit accu- 

 rately, as is desirable", pour water 

 carefully into the tube. In either case, the disc will be 



