Fluid Pressure 85 



below the water-surface. The magnitude of the upward 

 pressure usually called the " upthrust " at this depth 

 is found by obtaining the quotient of the weight of 

 the disc, cork, and string previously found by the area 

 of the disc. Calculate this and express your result as 

 a pressure in grams per sq. cm. 



Exercise for Student. 



What evidence did you obtain while performing this experiment 

 of the existence of this upward pressure ? 



Remove the chimney and disc from the water, dry 

 them as completely as possible, and replace the disc 

 on the end of the chimney. On the upper surface of 

 the disc place weights equivalent to the weight of the 

 disc, etc., as already determined, and again lower the 

 arrangement to the bottom of the vessel. Loosen the 

 string and slowly raise the chimney and its contents 

 till the plate again drops off. Measure the depth of 

 the end of the chimney below the surface of the water 

 when this happens. Calculate the pressure at this 

 depth. 



On comparing the second depth with that first 

 obtained, it will probably be found that the second 

 is practically double the first. Since the pressure in 

 the second case is also double that in the first, we are 

 led to the conclusion that the upthrust varies directly 

 with the depth of immersion. This conclusion should 

 be tested by repeating the experiment, placing different 

 weights on the upper surface of the brass disc, and 

 noting whether the depth at which the disc falls off 

 agrees with that required by the above statement. 



50. To obtain some further information with respect 

 to fluid pressure, measure the external diameter of the 



