INTRODUCTION TO HYDROSTATICS, Ivii 



This deficiency of cohesion is the reason why fluids can never be formed 

 into figures or maintained in heaps ; for though it is true the wind raises 

 water into waves, they are immediately afterwards destroyed by gravity. 

 Thus liquids always find their level. The definition of the equilibrium of 

 a fluid is, that every part of the surface is equally distant from the point to 

 which gravity tends ; that is to say, from the centre of the earth. Hence 

 the surface of all fluids must partake of the spherical form of the globe and 

 be bulging. This is evident in large bodies of water, such as the ocean ; 

 but the sphericity of small bodies of water is so trifling as to render their 

 surfaces apparently flat. 



The equilibrium of fluids is the natural result of their particles gravitating 

 independently of each other ; for when any particle of a fluid accidentally 

 finds itself elevated above the rest, it is attracted down to the level of the 

 surface of the fluid, and the readiness with which fluids yield to the slightest 

 pressure, will enable the particle by its weight to penetrate the surface of 

 the fluid and mix with it. But this is the case only with fluids of equal 

 density, for a light fluid will float on the surface of a heavy one, as oil on 

 water; and air will rise to the surface of any liquid whatever, being forced 

 up by the superior gravity of the liquid. Fig. 1. represents an instrument 

 Fig. 1. called a water-level, which is constructed 



A B. upon the principle of the equilibrium of 



QJT" - ' 7 |Q fluids. It consists of a short tube, AB, 



closed at both ends, and containing 



water and a bubble of air ; when the tube is not perfectly horizontal the 

 water runs to the lower end, which makes the bubble of air rise to the 

 upper end, and it remains in the centre only when the tube does not 

 incline on either side. It is by this means that the level of any situation, 

 to which we apply the instrument, is ascertained. 



Solid bodies, therefore, gravitate in masses, the strong cohesion of their 

 particles making "them weigh altogether, while every particle of a fluid 

 may be considered as a separate mass, gravitating independently. Hence 

 the resistance of a fluid is considerably less than that of a solid body. 

 The particles of fluids acting thus independently, press against each other 

 in every direction, not only downwards but upwards, and laterally or 

 sideways ; and in consequence of this equality of pressure, every particle 

 remains at rest in the fluid. If you agitate the fluid, you disturb this 

 equality, and the fluid will not rest till its equilibrium be restored. 



Were there no lateral pressure, water would not flow from an opening 

 on the side of a vessel ; sand will not run out of such an opening, because 

 there is scarcely any lateral pressure among the particles. Were the 

 particles of fluids arranged in regular columns, as in Jig. 2, 

 there would be no lateral pressure, for when one particle is per- 

 pendicularly above the other, it can only press it downwards ; 

 but as it must continually happen that a particle presses between 

 two particles beneath (fig. 3), these last suffer a lateral pressure ; 

 just as a wedge driven into a piece of wood separates the parts 

 The lateral pressure is the result therefore of the pressure 

 downwards, or the weight of the liquid above ; and consequently 

 the lower the orifice is made in the vessel, the greater will be 

 ri the velocity of the water rushing out of it. Fig. 4 represents 

 OO the different degrees of velocity with which a liquid flows from 

 a vessel furnished with three stop-cocks at different heights. Since the 

 lateral pressure is entirely owing to the pressure downwards, it is not 

 affected by the horizontal dimensions of the vessel, which contains the 

 liquid, but merely by its depth ; for as every particle acts independently of 



