HYDROSTATICS 169 



called Hydrostatics, and that which explains the 

 laws of their motion is called Hydraulics. When 

 they are compressible and elastic, like air, the sci- 

 ence which treats of the laws of their equilibrium 

 is called Aerostatics, and that which treats of their 

 motion is Pneumatics. 



The fluids here treated of, are supposed to gravitate 

 in lines, which are either parallel, or directed to one 

 centre, or to several centres. 



248. The surface of every fluid when at rest, is 

 horizontal, or perpendicular to the direction of 

 gravity. 



If the directions of gravity are all parallel, the surface 

 of the fluid is a plane : If they converge to a point, 

 the surface of the fluid is a portion of a spherical 

 surface, having that point for a centre. 



BOSSUT, Hydrod. torn. i. 19. CAVALLO, vol. n. 

 chap. ii. prop. &. 



Levelling is the art of drawing a line at the surface 

 of the earth, to cut the directions of gravity every 

 where at right angles. 



This line is nearly an arch of a circle, and if L be the 

 length of any such arch in English miles, and D 

 the depression, in feet, of one extremity of it, be- 

 low a tangent drawn to it at the other extremi- 



tv D- 



3 



If a communication, by means of a tube or pipe, ei- 

 ther straight or crooked, be made between the wa- 



ter 



