394 WELLS'S NATURAL PHILOSOPHY. 



a vacuum, and that the air rushing in to fill it produces 

 the sound. Every explanation that has yet been ofiered 

 is somewhat unsatisfactory. 



The rolling of tho thunder has been ascribed to the effect of echo, but this 

 undoubtedly is not the only cause. The rolling of thunder is heard as per- 

 fectly at sea as upon land, but there none of the causes which are generally 

 supposed to produce echo, as mountains, hills, buildings, etc., etc., are present. 

 Another, and perhaps the true reason is, that the sound is developed by th© 

 lightning in passing through the air, and consequently separate sounds are 

 produced at every point through which the lightning passes. 



, . . Thunder-storms prevail most in the torrid zone, and decrease 



Where do thun- . . ^ , .,, , ^ , . . , , 



der storms in u'equency toward either pole. In the arctic regions thunder- 

 most prevail ? gtorms seldom or never occur. As respects time, they are 

 most frequent in the summer months. 



What is called a thunder-storm may be considered to 

 be merely an effort of nature to effect an equilibrium of 

 forces which have become disturbed. 



"767. A knowledge of tho laws of electricity hag enabled 



When vpre 



lightning con- man to protect himself from its destructive influences. Light- 



diictors first ning-rods, or conductors, were first introduced by Dr. Frank- 

 mtroduced ? ° ' ' •' 



lin. He was induced to recommend their adoption as a means 



of protection to buildings, etc., from observing that electricity could be quietly 



and gradually withdrawn from an excited surface by means of a good x;on- 



ductor, which was pointed at its extremity. 



^^^t is a As ordinarily constructed, a lightning-con- 

 lightning-rod? (j^ctor consists of a metal rod fixed in the 

 earth, running up the whole height of a building and ris- 

 ing to a point above it. 



The best metal that can be used for a lijirht- 



How should a. . •/•• • 111 



li-htning-rod niDg-rod IS coppcr ; it iron is used, the rod 

 should not be less than three quarters of an 

 inch in diameter. When only one rod is used, it should 

 be continuous from the top to the bottom, and an entire 

 metalhc communication should exist throughout its whole 

 length. This law is violated when the joiuts of the several 

 parts that form the conductor are imperfect, and when the 

 whole is loosely put together. 



The rod should also be of the same dimensions through- 

 out. The rod is best fastened to the building by wooden 

 supports. If there are masses of metal about the build- 



