Pressure of Fluids. 291 



simply as a medium of communication between the columns AE 

 and DF ; so that EBCF transmits to the column DF all the 

 pressure it receives from AE ; and reciprocally it transmits to 

 AE all it receives from DF. It is also evident that we should 

 arrive at the same conclusions, if instead of the columns AE, DF, 

 we substitute two forces of the same value ; hence, as the result 

 is not affected by any inclination of the branch BC, we conclude 

 that, if a fluid, destitute of gravity, be contained in any vessel, cmc/ Fig.198. 

 if, having made an opening in the vessel, we apply to this opening any 

 given pressure, the force thus exerted will diffuse itself equally in all 

 directions. 



405. Now it will be readily seen, not only that the pressure 

 transmits itself equally in all directions, but also that it acts at 

 each point perpendicularly to the surface of the vessel contain- 

 ing the fluid ; for if, acting on the surface, it did not act perpen- 

 dicularly, its effect could not be entirely destroyed by the resist- 

 ance of this surface; there would result therefore an action up- 

 on the parts of the fluid itself, which, as it could not but trans- 404 - 

 mit itself in all directions, would necessarily occasion a motion 



in the fluid ; it would be impossible, therefore, on such a suppo- 

 sition, for a fluid to remain at rest in a vessel, which is contrary 

 to experience. 



406. We hence conclude, that if the parts of a fluid contain- FiJ91 

 ed in any vessel A.BCD, open toward AD, are urged by any 

 forces whatever, and are notwithstanding preserved in a state of 

 equilibrium, these forces must be perpendicular to the surface 

 AD; for if there be an equilibrium, this equilibrium would 



still obtain, if a covering were applied of the same figure with 

 the surface AD ; but we have just seen, that in this case the 

 forces acting at the surface AD must be perpendicular to this 

 surface. 



407. Accordingly, let us suppose that the forces acting on 

 the parts of the fluid are gravity itself; we shall infer that the 

 direction of gravity is necessarily perpendicular to the surface 

 of tranquil fluids ; and that consequently the parts of the same, 

 heavy fluid must bt on a level, in order to be in equilibrium > what- 

 ever be the figure of the vessel. 



