136 THEORY OF SOUND. SECT. XVI 



should steer to the S. E. ; but if the gale begins from 

 the N. E., and changes through N. to N. W., the ves- 

 sel should go to the N. W. In the northern part of the 

 torrid zone, if the storm begin from the N. E. and veer 

 through E. to S. E., the ship should steer to the N. E. ; 

 but if it begin from the N. W. and veer by W. to S. W., 

 the ship should steer to the S. W., because she is in the 

 south-western side of the storm. Since the laws of 

 storms are reversed in the southern hemisphere, the 

 rules for steering vessels are necessarily reversed also. 

 A heavy swell is peculiarly characteristic of these 

 storms. In the open sea the swell often extends many 

 leagues beyond the range of the gale which produced it. 

 Waterspouts are occasioned by small whirlwinds, 

 which always have their origin at a great distance from 

 that part of the sea from which the spout begins to rise, 

 where it is generally calm. The whirl of the air be- 

 gins in the clouds, and extending downward to the sea, 

 causes the water to ascend in a spiral by the impulse of 

 the centrifugal force. When waterspouts have a pro- 

 gressive motion, the vortex of air in the cloud above 

 must move with the same velocity, otherwise the spouts 

 break, which frequently happens. 



SECTION XVI. 



Sound Propagation of Sound illustrated by a Field of Standing Corn 

 Nature of Waves Propagation of Sound through the Atmosphere 

 Intensity Noises A Musical Sound Quality Pitch Extent of 

 Human Hearing Velocity of Sound in Air, Water, and Solids Causes 

 of the Obstruction of Sound Law of its Intensity Reflection of Sound 

 Echoes Thunder Refraction of Sound Interference of Sounds. 



ONE of the most important uses of the atmosphere is 

 the conveyance of sound. Without the air deathlike 

 silence would prevail through nature, for in common 

 with all substances it has a tendency to impart vibrations 

 to bodies in contact with it. Therefore undulations re- 

 ceived by the air, whether it be from a sudden impulse 

 such as an explosion or the vibrations of a musical chord, 

 are propagated in every direction, and produce the sen- 

 sation of sound upon the auditory nerves. A bell rung 

 under the exhausted receiver of an air-pump is inaudi- 



