102 MECHANICAL RECREATIONS [CH. V 



moving in a tube, the pressure on the sides of the tube is less 

 than when the air is at rest— and the quicker the air moves 

 the smaller is the pressure. This fact was noticed by Hauksbee 

 nearly two centuries ago. In an elastic perfect fluid in which 

 the pressure is proportional to the density, the law connecting 

 the pressure, p, and the steady velocity, v, is p = ITcr" 2 , where 

 II and a are constants : the establishment of the corresponding 

 formula for gases where the pressure is proportional to a power 

 of the density presents no difficulty. 



The principle is illustrated by a twopenny toy, on sale in 

 most toy-shops, called the pneumatic mystery. This consists 

 of a tube, with a cup-shaped end in which rests a wooden ball. 

 If the tube is held in a vertical position, with the mouthpiece 

 at the upper end and the cup at the lower end, then, if anyone 

 blows hard through the tube and places the ball against the 

 cup, the ball will remain suspended there. The explanation is 

 that the pressure of the air below the ball is so much greater 

 than the pressure of the air in the cup that the ball is 

 held up. 



The same effect may be produced by fastening to one end of 

 a tube a piece of cardboard having a small hole in it. If a piece 

 of paper is placed over the hole and the experimenter blows 

 through the tube, the paper will not be detached from the card 

 but will bend so as to allow the egress of the air. 



An exactly similar experiment, described in many text- 

 books on hydromechanics, is made as follows. To one end of a 

 straight tube a plane disc is fitted which is capable of sliding 

 on wires projecting from the end of the tube. If the disc is 

 placed at a small distance from the end, and anyone blows 

 steadily into the tube, the disc will be drawn towards the tube 

 instead of being blown off the wires, and will oscillate about a 

 position near the end of the tube. 



In the same way we may make a tube by placing two books 

 on a table with their backs parallel and an inch or so apart and 

 laying a sheet of newspaper over them. If anyone blows 

 steadily through the tube so formed, the paper will be sucked in 

 instead of being blown out. 



