370 Professor Airy on certain Conditions 



would be more rapid every time than at the time previous. In 

 this case the velocity would go on perpetually increasing: or the 

 velocity might be made uniform if the machine were retarded 

 by some constantly acting resistance : or, in other words, the 

 machine might move with uniform velocity, and might at the 

 same time do work : which is commonly understood to be the 

 meaning of the term perpetual motion. If the machine had no 

 work to do, the increasing friction, &c. would operate as an in- 

 creasing work, and the velocity would be accelerated till the ac- 

 celeration caused by the forces was equal to the retardation caused 

 by the friction: after which it would remain unaltered. 



For this idea I am indebted to the admirable account of the 

 organs of voice given by Mr. Willis. The phenomenon to be 

 explained was this. When two plates are inclined at an angle 

 greater than a certain angle, it is found that the effect of a cur- 

 rent of air passing between them is to give a tendency to open 

 wider. When they are inclined at any angle smaller than that 

 certain angle, the effect of the current is to make them collapse. 

 If then the plates be supposed to vibrate through the position 

 corresponding to that angle, the tendency of the forces at all 

 times is to bring them to that position. Each plate is in the 

 state of a vibrating ijendulum : and whatever be the law of force 

 which acts on it it is certain that if the force be the same when 

 the plate is in the same position, this force will have no tendency 

 to increase the velocity. The retardation arising from friction, 

 &c. will therefore soon destroy the motion. But it is found, in 

 fact, that the motion is not destroyed. What then is the accele- 

 rating force which keeps up the motion? Mr. Willis explains 

 this by supposing that time is necessary for the air to assume 

 the state and exert the force corresponding to any position of the 

 plate : which is nearly the same as saying that the force depends 

 on the position of the plate at some previous time. In this 



