Hydraulic Illusions. 



141 



E 



E==S 



Gik 



If our readers will look at the apparatus, in the Strand they 

 may observe that the g'lass 

 tube a, up which these 

 bubbles ascend in appa- 

 rently such a mysterious 

 manner, is perfectly free 

 and unattached below ; this 

 want of communication with 

 any other portion of the 

 apparatus rendering the 

 action less easily compre- 

 hended. If, however, they 

 look to the upper part of 

 the glass tube they will find 

 that it is enclosed in a metal 

 tube b, and that from this 

 a smaller tube c ascends to 

 and passes along the ceiling 

 of the shop. No other por- 

 tion of apparatus is visible, 

 and it is the extreme simpli- 

 city of the arrangement and 

 the apparent want of any 

 adequate cause that renders 

 the whole so incomprehen- 

 sible and attractive. 



If we wish to understand 

 how the effect is produced, 

 we must imagine that the 

 small tube c, after passing along the ceiling to a convenient 

 locality, is made to descend and enter into a perfectly air-tight 

 cistern d. This is furnished with an aperture E, for the pur- 

 pose of filling it when required. The aperture is capable of being 

 closed by a screw with a leather washer, that enables the open- 

 ing to be shut in a perfectly air-tight manner. From the cistern 

 descends a long tube f, having a stopcock at its lower extre- 

 mity. No particular proportions are requisite except one which 

 is absolutely indispensable, that is, that the length of the column 

 of water in the cistern d and tube f should be greater than 

 that in the glass tube a ; and should this be filled with syrup or 

 any liquid heavier than water, the difference must be propor- 

 tionately greater. 



Having described the apparatus, let us now explain its 

 action. We will suppose the glass tube a, which is alone pre- 

 sented to the gaze of the public, to be filled with liquid. Water 

 would answer, but as the object is to cause a slow ascent of the 

 globes of air, a thicker liquid, such as clear syrup, would be pre- 



