HYDRAULICS. 



force, and by pressing downwards will 

 remove such particles as are opposed 

 to its descent until an equilibrium of 

 pressure is produced; and this equili- 

 brium can never exist until the. whole 

 mass of fluid is operated upon by 

 the same force, an effect that occurs 

 only when the surface is truly level. 

 In the same manner, if one perpen- 

 dicular column of fluid is conceived 

 to be shorter than the others that sur- 

 round it, it will contain fewer particles, 

 and hence will be lighter than them ; the 

 consequence of which will be that the 

 heavier surrounding columns will press 

 upon and buoy up that which is lighter, 

 until an equilibrium of pressure is pro- 

 duced by their becoming equal or level. 



As the particles of all fluids gravitate, 

 so will any vessel whatever, that con- 

 tains a quantity of water or other fluid, 

 be drawn towards the earth with a 

 power equivalent to the weight of the 

 fluid it contains ; and if the quantity 

 of the fluid be double, triple, or in- 

 creased in any other proportion, so 

 will the weight or gravitating influence 

 be doubled, tripled, or increased in like 

 proportion ; from which we learn an 

 important hydraulic corollary, that the 

 weight or pressure of fluids at rest is 

 simply as their quantities or heights : so 

 that if a perpendicular pipe three inches 

 diameter, and three feet high, contains 

 nine pounds of water, that pressure of 

 nine pounds will be exerted upon a 

 valve or stopper of any description, and 

 three inches diameter, placed in the bot- 

 tom of the pipe ; and if the pipe is 

 made twice as high, or six feet long, the 

 pressure will be eighteen pounds, if 

 three times as high, twenty- seven 

 pounds the pressure increasing in the 

 same ratio as the altitude of the column, 

 while the valve or orifice below remains 

 the same, a circumstance which is of 

 great consequence to be known in the 

 construction of pumps and engines for 

 raising water. 



Water not only gravitates with the 

 vessel that contains it as in the last case, 

 but independently of it ; and thus, if the 

 containing vessel is supposed stationary 

 and a hole is bored in its bottom, the 

 contained water will flow . out and de- 

 scend through the air for the purpose of 

 obtaining a lower situation than it be- 

 fore occupied; and, in so flowing out, 

 those particles of fluid which were over, 

 or in immediate contact with the hole, 

 will be discharged first. Their motion 

 will of course cause a momentary va- 



cuum or void space above the hole, and 

 from the ease with which the particles 

 of fluids slide over each other, and 

 thereby press in all directions alike, it is 

 not a perpendicular column of particles, 

 equal in their base to the area of the 

 hole that will be set in motion, but par- 

 ticles will flow in all directions towards 

 the hole, and thus put the whole mass 

 of fluid into motion ; and if the water so 

 flowing out falls perpendicularly, its 

 descent will be accelerated in the same 

 proportion, and its motion will be regu- 

 lated by the same laws as apply to the 

 falling of solid bodies. When water 

 flows in a current or stream, as in rivers 

 or channels, it does so in consequence 

 of the end of the channel towards which 

 it is flowing being lower than that from 

 which it proceeds, in which case its mo- 

 tion is referrible to that of solids de- 

 scending inclined planes ; but, from the 

 want of cohesion among the particles of 

 fluids, their motions are much more 

 irregular than those of solids, and they 

 involve a number of intricacies of very 

 difficult solution, and which are ren- 

 dered still more uncertain in their in- 

 vestigation, by the few experiments that 

 have accurately been tried on a large 

 scale to furnish data for calculation. 

 The friction that occurs between a solid 

 and the surface upon which it moves 

 can be accurately ascertained, but not so 

 with a fluid ; for in this, while one part 

 may be moving rapidly, another may be 

 quite stationary, moving slowly, or even 

 moving in a contrary direction. This 

 is particularly observable in rivers, where 

 the central part or main current will 

 always be found flowing with much 

 greater rapidity than either side ; and 

 experiment proves that the same effect 

 occurs when water flows through pipes, 

 for that water which is in contact with 

 the side of the pipe moves with much 

 more resistance than that at the centre, 

 whereby the calculated discharge of any 

 given pipe of considerable length be- 

 comes much less than is due to its mag- 

 nitude. The term ' friction' is applied to 

 this obstruction to the passage of fluids, 

 in the same manner as it is to solids, 

 and it exists to such an extent as to be- 

 come an object of considerable incon- 

 venience in practice. It can only be 

 obviated by making the conveying-pipe 

 of much larger dimensions than would 

 otherwise be necessary, so as to allow 

 the free passage of a sufficient quantity 

 of fluid through the centre of the pipe, 

 while a ring or hollow cylinder of water 



