108 Mr Stewart on the Causes of Obstruction in 



The movements of the valves may also be managed by cranks 

 fixed on their axes, which are for that purpose prolonged, 

 passing through leathern collars, in the side of the air-vessel ; 

 and to obtain a simultaneous action, the cranks may be connect- 

 ed by a working rod ; only, that in this case, as the axes must 

 be in the same perpendicular line, the position of the hinges must 

 be reversed. Stop-cocks may also be substituted for valves in 

 the case of water-pipes ; — in which case, there are two air-vessels 

 connected by a short pipe, which contains a stop-cock, and this 

 stop-cock being fixed in the pipe which connects the lower air- 

 vessel with the water-pipe. 



Fig. 3. 



The air-extractor, thus constructed, is applied to use in the 

 following manner. The pipe C, fig. 1., the upper end of which 

 is connected with the lower valve j^ has its lower end inserted in 

 that part of the syphon or water-pipe in which the air is found 

 to lodge ; and the cylinder being filled with water, the upper 

 valve ^, is shut, and the lower valve ^ is opened. Under this 

 arrangement, it is plain, that whenever any air accumulates at 

 the bottom of the pipe C, it will immediately rise through it into 

 the lower cylinder B, displacing an equal bulk of the water, 

 which will descend through the pipe C, and be carried off by 

 the moving water, which, relieved of the air, is flowing without 

 obstruction. When the air has again accumulated, so as to fill 

 the cylinder B, and pipe C, the obstruction is found to recur ; 

 and in order to get rid of it, the communication between the cy- 

 linder, and the pipe or syphon, must first be cut off, by shutting 



