498 STUDIES IN ADVANCED PHYSIOLOGY. 



that the sound, like autumn leaves, has been blown along 

 by the wind. This is, however, a mere figure of speech. 

 The real explanation consists in the fact that the air being 

 blown over the ground and meeting with resistance there is 

 somewhat condensed, and being therefore denser on the 

 ground than it is further up where there is no resistance to 

 be overcome, the sound waves are deflected towards the 

 ground, by this denser air near it acting like an ordinary 

 lens. In this way much of the sound which would other- 

 wise have radiated upwards into the sky is refracted to the 

 ground and so the perception of the sound made more 

 distinct. The analogy in the case of light occurs, for 

 instance, at the rising or setting of the sun when by means 

 of the refraction of the atmosphere the sun becomes visible 

 in the morning really before it comes above the horizon, 

 and remains visible in the evening some time after it has 

 really set, due to the fact that the rays of light which would 

 have gone off into space are by the atmosphere, like a lens, 

 bent down to the ground. 



THE PHYSICAL PEOPEETIES OF SOUND. 



1. The Intensity or Loudness. Sounds are easily dis- 

 tinguished as louder or softer, and this distinction in loud- 

 ness is described as the intensity of the sound. This in- 

 tensity is produced by the intensity of the vibration, not by 

 the frequency or number of vibrations. This must remain 

 the same. The intensity is in the added distance through 

 which a single vibration moves. Thus, in the case of a 

 piano string if it be struck very lightly, it vibrates up and 

 down through a very small distance, if it be struck harder 

 the number of its vibrations is not increased, but the string 

 at every vibration passes through a greater amplitude. The 

 explanation of intensity may be easily shown to the eye by 

 the pendulum. One can easily satisfy himself that a pendu- 

 lum of a definite length makes no more vibrations in a given 

 time when it swings out far to the right and left at each 

 beat, than when it moves but little out of its vertical posi- 



