152 LECTURES TO SCIENCE TEACHERS. 



box, whilst the lower one is sounding ; you will hear that the 

 sound rises and falls according as the phase of the vibration 

 on the top disc is similar to the lower one and reinforcing it, 

 or opposite to it and antagonizing it. In each rotation you 

 thus get eight spaces, four of the loud, and four of the 

 feeble sound. I can also show you the beats which arise 

 from slightly altering the pitch of the upper tone by slowly 

 rotating the upper box by means of the handle, the tone 

 becoming natter when the direction of revolution is the same 

 as the disc, and sharper when it is opposite. You hear the 

 beats very distinctly. These beats involve a most important 

 question of acoustics which must be reserved to the next 

 lecture. We thus reach the second part of the subject, 

 namely, reinforcement and distribution. 



Now reinforcement is common to all kinds of vibrations, 

 it even occurs in the child's swing. When a child swings 

 itself it reinforces the action of the swing by slight muscular 

 exertion. It occurs when peals of bells are ringing. Any 

 one who has been at the top of Magdalen Tower at Oxford 

 on the 1st of May, will have noticed that when the bells 

 begin to ring the top of the tower rocks like a ship in a storm, 

 people have even felt as if sea-sick. Although there is this 

 pendulum motion, like that of a rod or tuning-fork, the 

 tower is particularly safe, for it shows the foundation to 

 be solid, the vibration occurring in the limestone which is 

 elastic. The same reinforcement occurs when soldiers cross 

 a suspension bridge. The rule is that when a regiment has to 

 go over a light bridge they break step. If they march 

 together the effect has been to injure or throw down the 

 bridge. Reinforcement may be denned essentially as a gentle 

 force, acting periodically, as when one pulls a heavy bell; 

 each individual pull will not raise the mass or anything like 

 it, even with your whole weight ; but you keep on, reinforcing 

 the vibrations of the bell, taking the period of the swing 

 and giving each time a gentle impulse, and this gentle force 

 acting periodically becomes magnified into a very great one. 

 Generally speaking reinforcement in sound is correlative with 

 the power of producing sound. All sounding bodies also 

 reinforce, but some have been divided off by Clerk-Maxwell 

 into what he terms distributors. Others again have the 

 power of singling out particular sounds for reinforcement. 

 You may hear this going on in many places. If you take up 



