fig. 26. 



26 PNEUMATICS. 



will be forced out in a 

 continued stream. 



The force with which 

 the water issues from 

 the tube T T is easily G 

 determined. Let us 

 suppose that one-half 

 of the air-vessel is fill - 

 ed with water. The 

 air it contains is there- 

 fore reduced to half 

 its original bulk, and 

 therefore (26) exerts 

 twice its original pres- 

 sure. It is therefore 

 forced from the tube 

 TT', with a force equal 

 to the weight of thirty- 

 four feet of water, for 

 the atmospheric pres- 

 sure balances one-half 

 of the force with which 

 it is pressed up the 

 tube. Again, suppose 

 the vessel is three- 

 fourths filled, the air is 

 then reduced to one- 

 fourth of its original 

 force, and therefore 

 exerts four times its 

 original pressure. Once 

 its pressure being ba- 

 lanced by the atmos- 

 phere, an effective force 

 is obtained equal to 

 three times the pres- 

 sure of the atmosphere, or to 132 feet 

 of water, and so on. 



It is proved in Hydrostatics, that water 

 pressed out of a vertical tube will ascend 

 to nearly the height of a column of water 

 of equivalent pressure, setting aside the 

 resistance of the air. Hence we may 

 easily infer, that in the cases just stated, 

 except so far as the resistance of the 

 air is stated, a jet or fountain would 

 rise to the heights already mentioned. 



By screwing on the tube TT', jets 

 pierced with apertures in various direc- 

 tions, ornamental fountains may be con- 

 structed. 



It should be observed, that by thus 

 introducing the elastic force of the air 

 no additional force is gained, nor is the 

 mechanical efficacy of the apparatus, 

 properly speaking, increased; for the 

 force used in the depression of the pis- 

 ton in compelling the water to enter the 

 air-vessel is exactly equal to the elasti- 

 city of the compressed air. This elastic 

 force is as it were a number of accumu- 

 lated strokes of the piston, stored up or 





forestalled, and then exerted in continual 

 pressure. The air-vessel may there- 

 fore be considered as a kind of magazine 

 of power. There will be a constant 

 stream, provided that as much water is 

 forced in by pumping as is ejected by 

 the pressure of the confined air ; if less 

 be pumped in, the air-vessel will at 

 length be emptied and the stream stop. 

 If more be forced in the air-vessel it 

 must at last burst. 



(48.) The fire-engine is a modification 

 of the forcing-pump. 



A B {fig. 27.) are two forcing pumps, 

 whose pistons P are wrought by a beam 

 whose fulcrum is at F. V V are valves 

 which open upwards from a suction- 

 tube T, which descends to a reservoir ; 

 t are force-pipes which communicate by 

 valves V V ; opening into an air-vessel 

 M. A tube L is inserted in the top of this 

 vessel, terminating in a leathern tube or 

 hose, through which the water is forced 

 by the pressure of the air confined in M, 

 as described in (47.) 

 fig- 27. 



By the double pump wrought by the 

 same lever, the process is expedited and 

 the power economised. It is not neces- 

 sary to enter into further particulars 

 respecting this machine, after what has 

 been said on the forcing-pump. There 

 are many varieties in iire-engines, but 

 most of them are governed by the same 

 principles. 



IV. The Syphon. 



(49.) The syphon is a bent tube with 

 one leg, A B {fig. 28.) shorter than the 

 other, used for transfening a liquid 

 from one vessel to another. 



This is effected by exhausting the 

 syphon of the air it contains, or at least 



