270 POPULAR LECTURES AND ADDRESSES. 



means of perceiving as the means by which, in 

 John Bunyan's language, the " soul in its citadel " 

 acquires a knowledge of external matter. The 

 physicist has to think of the organs of sense, 

 merely as he thinks of the microscope ; he has 

 nothing to do with physiology. He has a great 

 deal to do with his own eyes and hands, however, 

 and must think of them, if he would under- 

 stand what he is doing, and wishes to get a 

 reasonable view of the subject, whatever it may 

 be, which is before him in his own department. 



Now, what is the external object of this internal 

 action of hearing and perceiving sound ? The 

 external object is a change of pressure of air. 

 Well, but how are we to define a sound simply ? 

 It looks a little like a vicious circle, but is not really 

 so, to say it is sound if we call it a sound if 

 we perceive it as sound, it is sound. Any change 

 of pressure, which is so sudden as to let us perceive 

 it as sound is a sound. There [giving a sudden 

 clap of the hands] that is a sound. There is no 

 question about it nobody will ever ask : Is it 

 a sound or not? It is sound if you hear it. If 



