1 92 PHYSIOLOGICAL PHYSICS. [Chap. xix. 



were compared in the scales of a balance, the weights, 

 that is, the pressure communicated to the scale, would 

 be different, the former being heavier than the other 

 by the amount of fluid enclosed by the walls of the 

 vessel represented by the dotted lines. This is called 

 the HYDROSTATIC PARADOX. It is to be noted that 

 the pressure communicated to the pan of the balance 

 is not merely the pressure on the bottom of the 

 vessel, but is the resultant pressure, for the whole 

 vessel. Thus in the vessel AB'C' there is exerted 

 on the upper surface of the wide portion of the 

 vessel, outside of AD, an upward pressure every- 

 where equal to the height of the column AA'. 

 This iipward pressure is equal to the downward 

 pressure that would be exerted by the additional 

 quantity of liquid contained if the vessel were of the 

 shape BB'CC'. Therefore the pressure transmitted 

 to the scale by a vessel of the latter shape would 

 exceed that of a vessel shaped like AB'C' by the amount 

 of this downward pressure, which a vesssl shaped like 

 AB'C' transmits upwards. 



In estimating the pressure on a given surface on 

 the side of a vessel, the centre of gravity of the given 

 portion of the surface is obtained, and the height of 

 the column of liquid is taken from this point. 



The upward pressure spoken of is also in ac- 

 cordance with, and, indeed, offers another proof of, 

 Pascal's law. It is illustrated by a very simple 

 experiment. A glass vessel is nearly filled with 

 water. A glass cylinder, open at both ends, is taken, 

 and a disc of cardboard is cut of sufficient size just 

 to close one end. A thread is attached to the middle 

 of the cardboard, by means of which the disc is held 

 in position against one end of the tube. This end is 

 now immersed in the vessel of water, and the up- 

 ward pressure of the water retains the disc in position 

 without further use of the thread. Push the cylinder 



