[ 385 ] 

 CHAP. XXXIII. 



OUTLINE OF HYDROSTATICS. 

 SECTION I. 



General Propositions. 



A FLUID is a body, the particles of which, when tfcey are left 

 to themselves, are all in equilibrio, or are ready to move upon the 

 smallest force being applied to any of them. All the particles of 

 fluids gravitate as well as those of solids. A perfect fluid ought to 

 have no viscosity ; but among liquid bodies we are acquainted with 

 none that are entirely free from this property. The following pro- 

 positions comprehend the principles of hydrostatics. The word 

 fluid is used in most of them, instead of liquid, because several of 

 these propositions apply to aerial fluids as well as liquids ; and stat- 

 ing such, generally, will save us some repetition hereafter. 



1. The surface of every fluid when at rest is horizontal, or per- 

 pendicular to the direction of gravity. It will at once be perceiv- 

 ed, that when the extent of surface of the fluid is considerable, 

 instead of being flat it will assume perceptibly the form of the seg. 

 ment of a sphere. For example, if a pond extends two miles 

 every way, it can be shewn that the centre is eight inches higher 

 than the sides. The quantity of curvature increases as the square 

 of the arches described. Hence in levelling, a correction is ne- 

 cessary for this curvature, and it is usually made in this way : If D 

 be the distance in miles, two-thirds of D a is equal to the correction 

 in feet. 



2. The fluid in a vessel being at rest, and subjected to the sole 

 action of gravity, every particle of it is subjected to the same pres- 

 sure every way : and the pressure is equal to the perpendicular co- 

 lumn of water above the particle. 



3. The pressure of water upon the sides of the vessel is equal to 

 the greatest height of the water, without any regard to the extent 

 of its upper surface. This is what is usually called the liydrostatical 



TOL. in. 2 C 



