140 THE REASON WHY. 



" All ye inhabitants of the world, and dwellers on the earth, see ye, when he 



lifteth up an ensign on the mountains ; and when he bloweth a 



trumpet, hear ye." ISAIAH xvni. 



surfaces of clouds, and in its efforts iofind an equilibrium it causes 

 the clouds to roll in heavy masses, having dark outlines. 



The fact that electricity resides in, and is conducted by, the surj 'aces of bodies, 

 is well established, and should receive due attention in the protective measures 

 adopted to secure life and property against the effects of lightning. A practical 

 suggestion that arises out of this fact is, that tubes of copper would form far 

 more efficient conductors than bars of the same metal. A copper tube, of half 

 an inch diameter, would conduct nearly double the amount of electricity which 

 could be conveyed away by a bar of copper of the same diameter. The upper 

 extremity of the tube should be open obliquely, that the electric current might 

 be induced to pass over both the inner and outer surfaces. 



CHAPTER XXIX. 



614. What is thunder? 



Thunder is the noise which succeeds the rush of the electrical 

 fluid through the air. 



615. Why does noise follow the commotion caused by elec- 

 tricity ? 



Because, by the violence of the electric force, vast fields of air 

 are divided ; great volumes of air are rarefied ; and vapours are 

 condensed, and thrown down as rain. Thunder ig therefore caused 

 by the vibrations of Ike air, as it collapses, and seeks to restore its 

 own equilibrium. 



616. Why is the thunder-peal sometimes loud and con- 

 tinuous ? 



Because the electrical discharge take place near the hearer, and 

 therefore the vibrations of the air are heard in their full power. 



617. Why is the thunder-peal sometimes broken and 

 unequal ? 



Because the electrical discharge takes place at a considerable 

 distance, and the vibrations are affected in their course by 

 mountains and valleys. Because, also, the forked arms of the 



