10 



HYDROSTATICS. 



ascertaining whether any surface is 

 level, or any line horizontal ; for finding 

 what point is on the same level with any 

 given point, and how much any point 

 is above or below the level of any other 

 point. A B (fg. 1 1.) is a tube, with its 

 two ends, d, c, turned up, and open ; it is 



filled with water or mercury : upon the 

 fluid at d and at c are placed small 

 floats, each carrying an upright sight, 

 or square with a wire or hair across 

 it ; and the sights are placed across the 

 direction AB. If the instrument is 

 placed on any surface, or held in the 

 hand, whether level or not, on looking 

 through the sight c you find the cross 

 wire of c cover the cross wire of d, be- 

 cause the fluid stands equally high at 

 both ends ; and if the two cross wires 

 cover any object towards which the 

 sights are pointed, that object must ne- 

 cessarily be on the same level with the 

 instrument. In the use of this instru- 

 ment it is quite immaterial how you 

 place or hold it, whether level or not. 

 The common spirit-level is a tube filled 

 with spirit of wine, excepting a small 

 space, in which there is a bubble 

 of air left ; and then the tube is her- 

 metically sealed, or closed by being 

 melted with a blow-pipe. When the 

 tube is laid on a. level surface, the 

 air-bubble stands in the very middle 

 of the tube ; when the surface slopes, 

 the bubble rises to the higher end. 

 AB (Jig. 12.) is the spirit-level, and 



O covered by the two cross wires of the 

 sights A and B, is on the same level 

 with the surface where A B is placed. 

 Then there is a limb A F, with sights at 

 A and F, and moving round A, on a 

 quadrant or quarter of a circle D E di- 

 vided into ninety equal parts. "When 

 you desire to know how many degrees 

 any object P is below the level of AB, 

 or of 6, you turn A F until P is covered 

 by the cross wires of A and F, and ob- 

 serve the number of divisions in E G. 

 If P is above the level of AB, you 

 turn up the instrument, and raise the 

 arm A F above A B, by turning it round 

 on A, until the cross wires cover P. 



CHAPTER IV. 



Pressure on Oblique Surfaces Centre 

 of Pressure. 



HITHERTO we have treated of the 

 pressure of any fluid upon a horizontal 

 or level surface ; and it is always easily 

 found. You have only to multiply the 

 height or depth of the fluid by the ex- 

 tent of the surface, and the product 

 gives the bulk of fluid, the weight, of 

 which is equal to the pressure upon the 

 surface. Thus if the surface is three 

 feet by two, or six square feet, and the 

 height of the fluid three feet, the pres- 

 sure is equal to the weight of eighteen 

 cubic or solid feet of the fluid. If it is 

 water, a cubic foot of which weighs 

 62| pounds, the pressure is equal to 

 1125 pounds. 



But if the surface is not horizontal, a 

 different rule must be applied ; for then 

 the pressure is equal to the weight of a 

 bulk of fluid found by multiplying the 

 extent of the surface "into the depth of 

 the centre of gravity of the surface ; that 



A is, of the point, which, being supported, 

 the whole surface remains balanced or at 

 rest. If AB (Jig. 13.) is the surface of the 

 13. 



. 12. 



sights being fixed at A and B, when 

 AB is so placed that the bubble C 

 stands at the middle point, any object 



