50 Experimental Philosophy. [Lecture 5, 



where they otherwise would not. The syphon or ' 

 crane, is a bent tube, of which one leg is longer 

 than the other (fig. 1). With this instrument 

 we want to draw off the fluid contained in the 

 vessel D, which we will suppose immoveable, as 

 a well or a heavy cistern. We know that if the 

 instrument is put into the vessel, without some 

 particular management the fluid can never be made 

 to flow over the bent part B ; for the air which 

 presses on the surface of the fluid will also press 

 through the bore of the tube, and prevent its 

 pursuing that course. In order to use it, there- 

 fore, we fill the syphon with water or some other 

 fluid, and stopping both ends, immerse the 

 shorter leg in the vessel D. The stoppage be- 

 ing removed, the water will flow out at the leg C 

 by its own gravity, and, by the pressure of the 

 atmosphere on the surface, will continue to flow 

 while there remains any fluid in the vessel. If a 

 vacuum is made in the syphon, by drawing out 

 the air with one's mouth, or in any other way, the 

 same effect will take place. 



The syphon fountain is a beautiful example of 

 the effect from the pressure of the atmosphere. 

 In fig. 20, a is the long or outer leg of the 

 syphon, which is inserted by a brass or wooden 

 cap in the glass vessel c; the inner leg b also 

 passes through the cap,and terminates in a spout- 

 ing pipe of an extremely small bore. To make 

 it act, we must first put it in a position the reverse 

 of what it stands in at present, and through the 



