METEOROLOGY. 141 



The phenomena of lightning are best observed from the tops 

 of mountains which extend above the clouds; from such a 

 position the flashes have been obsevered to extend for several 

 miles in length. The frequency of succession, and length of 

 the luminous streaks, are supposed to depend upon the imper- 

 fect conducting power of the clouds and vapor between them. 



The question is now settled, that lightning rods, by con- 

 ducting off the fluid, serve as a protection to buildings. The 

 rod protects a circle, the diameter of which is four times the 

 length it extends above the highest point of the building: 

 hence the failures of lightning rods have been owing to their 

 not extending sufficiently high. 



THUNDER. 



The noise produced by the passage of lightning or electricity 

 through the air, from one cloud to another, or from a cloud to 

 the ground, is termed in common language, thunder. The 

 .loudness of thunder depends upon the magnitude and prox- 

 imity of the explosion, the .relative position of the clouds, the 

 character of the surrounding country, and the position of the 

 observer. 



The sharp crashing noise sometimes heard, is caused by 

 lightning striking near us : the low rumbling noise is the effect 

 of distant thunder : the rattling sound is occasioned by a quick 

 succession of explosions from a highly charged cloud. The 

 same species of snapping noise attends the discharge of sparks 

 from the prime conductor of a charged electrical machine. 

 "And when we consider how trifling a portion of electric 

 matter is put in action by the most powerful means of artifi- 

 cial excitement, compared with* the quantity stored up in a 

 full charged thunder cloud, the discrepancy between the 

 appalling crash of the one and the insignificant snap of the 

 other, it will appear surprising that both should originate in 

 the same cause." [Brande.] 



Lightning is the light attendant upon electrical action, and 



