1914] 



on Fluid Motions 



71 



for the moment I will call your attention to simple phenomena of 

 which theory can give a satisfactory account. 



Considerable simplification attends the supposition that the 

 motion is always the same at the same place— is steady, as we say — 

 and fortunately this covers many problems of importance. Consider 

 the flow of water along a pipe whose section varies. If the section 

 were uniform, the pressure would vary along the length only in 

 consequence of friction, which now we are neglecting. In the 

 proposed pipe how will the pressure vary 't I will not prophesy as 

 to a Royal Institution audience, but I believe that most unsophis- 

 ticated people suppose that a contracted place would give rise to an 

 increased pressure. As was known to the initiated long ago, nothing 

 can be further from this fact. The experiment is easily tried, either 

 with air or water, so soon as we are provided with the right sort of 

 tube. A suitable shape is shown in Fig. 1, but it is rather trouble- 



FlG. 1. 



some to construct in metal. W. Froude found paraffin-wax the 

 most convenient material for ship models, and I have followed him 

 in the experiment now shown. A brass tube is filled with candle- 

 wax and bored out to the desired shape, as is easily done with 

 templates of tin plate. When I blow through, a suction is developed 

 at the narrows, as is witnessed by the rise of liquid in a manometer 

 connected laterally. 



In the laboratory, where dry air from an acoustic bellows or a 

 gas-holder is available, I have employed successfully tubes built up of 

 cardboard, for a circular cross-section is not necessary. Three or 

 more precisely similar pieces, cut for example to the shape shown in 

 Fig. 2 and joined together closely along the edges, give the right 

 kind of tube, and may be made air-tight with pasted paper or with 

 sealing-wax. Perhaps a square section requiring four pieces is best. 

 It is worth while to remark that there is no stretching- of the card- 



