142 



Hydraulic Illusions. 



ferable. Whatever may be the liquid employed it is prevented 

 flowing out of the small aperture in the bottom of the tube by 

 a valve opening upwards. This valve is contained in and con- 

 cealed by the small metal cap placed on the lower end of the 



glass tube. 



The cistern d is filled with water through the aperture e, 

 which is then closed. On opening the small stopcock at the 

 bottom of the tube e, the water will flow out, owing to the 

 greater pressure in the longer tube f than in the shorter tube a. 

 In order to supply the vacancy created in the cistern, the exter- 



nal air will enter by the opening at the bottom of the glass 

 tube and ascend in a regular series of bubbles through the 

 liquid it contains. This action will continue as long as any 

 water remains in the cistern, and may be renewed by replen- 

 ishing it from time to time as may be requisite or desirable. 



Such is the simple apparatus that has puzzled the wits of 

 some thousands of spectators during a period of several years, 

 and has tended to give greater notoriety and reputation to the 

 warehouse in which it is placed than many more elaborate con- 

 trivances would have done. 



The aspirator, as the contrivance is termed on which the 

 action of this deception depends, has long been known to 





