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LECTURE XXVn. 

 I 



ON THE REGULATION OF HYDRAULIC FORCES. 



.L HOSIi motlificatioHs of the motions of fluids which are employed either for 

 conducting them from place to place, or for applying their powers to the pro- 

 duction of mechanical eft"ects,may be considered as constituting a separate di- 

 vision of practical hydrauHcs, which is analogous to the subject of general 

 machinery in practical mechanics. 



A supply of water may be obtained from a reservoir, situated above the level 

 at which it is w^anted, whatever its distance may be, either by means of open 

 canals, or aqueducts, or of closed pipes. Where an uninterrupted declivity 

 cannot be obtained, it is necessary to employ pipes, which may be bent up- 

 wards or downwards at pleasure, provided that no part of them be more than 

 thirty feet above the reservoir, and when the pipe is once filled, the water will 

 continue to flow from the lower orifice ; but it is best in all such cases to 

 avoid unnecessary angles ; for when the pipe rises and falls again, a portion of 

 the air, which is always contained in water, is frequently collected in the 

 angle, and very materially impedes the progress of the water through the pipe. 

 When the bent part is wholly below the orifices of the pipe, this air may be 

 discharged by various methods. The ancients used small upright pipes, called 

 columnaria, rising from the convexity of the principal pipe, to the level of the 

 reservoir, and suffering tlie air to escape without wasting any of the water. 

 It may however frequently be inconvenient or impossible to apply a pipe of 

 this kind ; and the same purpose may be answered, by fixing on the pipe a 

 box containing a small valve, which opens downwards, and is supported by 

 a float, so as to remain shut while the box is full of water, and to fall open 

 when any air is collected in it. (Plate XXI. Fig. 288.) 



Ifthejiipe were formed into a syjhon, having its flexure above both orifices, 



