458 POPULAR LECTURES AND ADDRESSES. 



about ii miles per hour. If water were a per- 

 fect fluid, this would be the state of the case : 

 a boat dragged along a canal at any velocity 

 less than the natural speed of the long wave 

 in the canal would leave a train of waves behind 

 it of so much shorter length that their velocity 

 of propagation would be equal to the velocity of 

 the boat; and it is mathematically proved that 

 the boat would take such a position as is shown 

 in Scott Russell's diagram referred to, namely 

 just on the rear slope of the wave. It was not 

 by mathematicians that this was found out ; but 

 it was Scott Russell's accurate observation and 

 well devised experiments that first gave us these 

 beautiful conclusions. 



To go back : a wave is the progression through 

 matter of a state of motion. The motion cannot 

 take place without the displacement of particles. 

 Vary the definition by saying that a wave is the 

 progression of displacement. Look at a field of 

 corn on a windy day. You see that there is 

 something travelling over it. That something is 

 not the ears of corn carried from one side of the 



