The Right Hon. Lord Rayleigh 



[March 20, 



board, each side being merely hent in one dimension. A model is 

 before you, and a study of it forms a simple and useful exercise in 

 solid geometry. 



Another form of the experiment is perhaps better known, though 



Fig. 2. 



rather more difficult to think about. A tube (Fig. 8) ends in a 

 flange. If I blow through the tube, a card presented to the flange 

 is drawn up pretty closely, instead of being blown away as might be 

 expected. When we consider the matter, we recognize that the 

 channel between the flange and the card through which the air 

 flows after leaving the tube is really an expanding one, and thus 

 that the inner part may fairly be considered as a contracted place. 

 The suction here developed holds the card up. 



Fig. 3. 



Fig. 4 



A shght modification enhances the effect. It is obvious that 

 immediately opposite the tube there will be pressure upon the card 

 and not suction. To neutralize this a sort of cap is provided, 

 attached to the flange, upon which the objectionable pressure is 

 taken (Fig. 4). By blowing smartly from the mouth through this 

 little apparatus it is easy to hft and hold up a penny for a short 

 time. 



