PNEUMATICS. 



27 



so rarefying it, fy. 28. 

 that the pressure 

 of the atmos- 

 phere on the sur- 

 face D will force 

 the liquid up the 

 tube beyond the 

 point B, and until 

 it descends in the 

 leg BC below the 

 level D' of the 

 water D in the 

 vessel. It will then descend by its weight, 

 and flow continually from the tube at C. 



The principle of the syphon is easily 

 understood. Suppose the suction of the 

 mouth or a syrinsre applied at C so as to 

 produce a considerable rarefaction of 

 the air in A B C. The elasticity of the 

 rarefied air in the syphon pressing on the 

 surface of the water in the leg B A will 

 then be unable to balance the atmos- 

 pheric pressure acting on the surface D. 

 The liquid will therefore be forced up 

 the leg B A. After passing B into the 

 leg B C, its descent is still opposed by 

 the resistance of the air, and it will be 

 necessary to keep up the rarefaction 

 until the liquid passes the level D'. For, 

 suppose that at any point above D', as 

 E, the rarefaction were discontinued, 

 the atmosphere would then press the 

 .c? E upwards with its whole force. 

 This pressure would, however, be re- 

 i "by the weight of the liquid B E ; 

 the atmospheric pressure diminished 

 by the weight of B E would then be 

 the effective force which presses the 

 surface E upwards. The pressure of the 

 atmosphere on D is transmitted through 

 the liquid to E (see Hydrostatics) ; but 

 this pressure is diminished by the weight 

 of the column D B which it sustains. 

 Hence the effective force which urges 

 the surface E downwards is the atmos- 

 pheric pressure diminished by the weight 

 of the column D E. So long, therefore, 

 as B E is less than D E, the force which 

 urges E upwards will be greater than 

 that which presses it downwards, and 

 it will therefore return into the ves- 

 sel D. 



The rarefaction must therefore be con- 

 tinued until the liquid has been drawn 

 below the level of D. After that the 

 force downward will exceed the force 

 upward by the weight of the liquid in 

 B C below the level D'. 



Since the liquid is raised in D B by 

 the atmospheric pressure, the leg B D 

 must be shorter than a column of the 

 liquid whose pressure is equal to that of 



the atmosphere ; that is, less than thirty- 

 four feet for water, thirty inches for 

 mercury, &c. 



It is evident, that the power of the 

 syphon is limited to merely decanting a 

 liquid, but it will not raise it above the 

 level of the liquid in the original vessel. 

 Neither will it continue to act after the 

 level of the vessel into which it is de- 

 canted becomes equal to that from 

 which it is drawn off. 



Instead of exhausting the syphon, 

 which is sometimes a difficult process, it 

 may be inverted and filled with water ; 

 then stopping each end, and placing it 

 with the shorter leg immersed in the 

 water to be drawn off, remove the stops, 

 and it will immediately begin to flow 

 from the longer leg. 



When the syphon is large, this pro- 

 cess is, however, not easy. In this case, 

 an aperture may be made in the highest 

 point B of the inflected leg of the sy- 

 phon, and, each end being plugged, the 

 syphon may be filled through the aper- 

 ture. This aperture being then plugged, 

 and the plugs removed from the ends, 

 the liquid will flow through it*-*In cases 

 where the syphon is used to can*}' water 

 over an elevation or a hill, this method 

 is often adopted. 



(50.) A syphon, in which the extre- 

 mities of the legs are turned upwards, 

 called the Wirtemburg syphon, may be 

 kept constantly filled. The open ends 

 D and E (Jig. 29.), are at the same level, 



and the height of the highest point B, 

 above this level should be less than the 

 height of a column of the liquid, whose 

 pressure is equal to that of the atmos- 

 phere. If the leg D be immersed in a 

 liquid, it will flow out of E, until its level 

 is reduced to D : for the pressure of 

 the liquid above the level D, exerted on 

 the surface of the liquid in the syphon, 

 is transferred by it to the end E, where, 



