48 HYDROSTATICS AND HYDRAULICS. 



a valve in the piston which will allow it to go through 

 but not to return, it is thus made to issue from the spout 

 of the pump. 



In the Forcing Pump, the water is made to rise in a 

 manner similar to what it does in the sucking pump, 

 and by means of a fixed valve it is also prevented from 

 returning ; but the piston being solid, unlike that of the 

 sucking pump, by the action of the piston the water is 

 forced through a tube contrived for that purpose, and 

 may be thus thrown to a very considerable height. It is 

 by means of a forcing pump, worked by a steam-engine, 

 that the water is raised from the New River into the 

 reservoir at Pentonville, above named. Fire-engines are 

 on the principle of the Forcing Pump. 



There is a small hydraulic instrument called a Siphon, 

 which requires to be noticed ; it is merely a bent tube, 

 having one leg shorter than the other: its chief use is for 

 drawing off liquors from one vessel to another. The 

 shorter leg is immersed in the liquor to be drawn off, and 

 by exhausting the air from the tube, which, if small, may 

 be done by the breath, the liquor will then rise in the 

 tube, and flow through the longer leg ; or the tube may 

 be filled with the liquor, and immersed in the vessel with 

 both ends stopped, on removing the stoppage the liquor 

 will flow off as before. The principle on which the Siphon 

 acts is as follows : When the tube is exhausted of air, 

 the pressure of the atmosphere forces the liquor up the 

 shorter pipe, and as the upward atmospheric pressure 

 on the outside is somewhat less than that on the liquor, 

 it flows down, and will continue to do so until the vessel 

 is emptied. If the legs of the Siphon, were equal, or if 

 the longer leg were immersed in the liquor, the upward 

 pressure of the atmosphere would prevent the liquor 

 from flowing down the tube, by overcoming the perpen- 

 dicular pressure. 





