126 



HYDROSTATICS. 



torn, it will be seen that the weight may be the resultant 

 of several forces, compounded according to the first and 

 second cases specified in 80. 



230. Lateral Pressure. We have already seen 

 that downward and upward pressure are proportional to 

 the depth of the liquid. Owing to the principle of equal 

 transmission of pressure in all directions, the same holds 



true for lateral pressure, the 

 effects of which are some- 

 times disastrously shown by 

 the giving way of flood-gates, 

 dams, and reservoirs. 



(a.) These effects of lateral 

 pressure may be safely illus- 

 trated by a tall vessel provided 

 with a stop-cock near its base, 

 and arranged to float upon the 

 water. When this vessel is filled 

 with water, the lateral pressure 

 at any two points at the same 

 depth and opposite each other 

 FIG. 76. will be equal. Being equal and 



opposite they will neutralize each other and produce no motion. If 

 now the stop-cock be opened, the pressure at that point tending to 

 drive the apparatus in a certain direction, say toward the left, is re- 

 moved ; the pressure at the opposite point tending to drive the 

 vessel toward the right, being no longer opposed by its equal, will 

 now produce motion and the vessel will float in a direction opposite 

 to that of the spouting water. Instead of being floated upon water, 

 the vessel may be supported by a long thread. The same principle 

 is illustrated in Barker's Mill. (Fig. 91.) 



231. Rule for Lateral Pressure. To find the 

 pressure upon any vertical surface, find the weight 

 of an imaginary column of the liquid whose base 

 is equal to the given surface and whose altitude 

 is the same as the depth of the centre of the given 

 surface Tjelow the surface of the liquid. 



