640 



THE NATURE BOOK 



an outlet must be found for it in some 

 way or other. Now air, unlike copper or 

 silver, or metals of any kind, is a \-ery bad 

 conductor of electricity, and since the 

 cloud is surrounded on all sides by this 

 meduun. this electric energy finds no 

 readv means of escape. Sometimes a 

 hea\-y shower of rain is sufficient to carry 

 off the electricity and reduce the potential 

 of the cloud to that of the earth. 



With very rapid accumulation of elec- 

 tricity, however, this safety xalve may 

 not be sufficientl}^ quick acting. When 

 this occurs the potential of the cloud 

 may become greater than that of the 

 earth below it or greater than that of 

 a neighbouring cloud, with the result 

 that a disrupti\-e discharge through the 

 intervening air takes place, and we have 

 the phenomenon of a flash of lightning. 



The discharge is called " disruptive " 

 because the resistance of the air being 

 so great, much of the electric energy of 

 the flash is transformed into heat, and 

 the air particles are put into such violent 

 vibration that they become luminous. 



Although the above reason has been 

 given to explain the origin of the electrical 

 condition in thunderstorms, the reader 

 must still remember that no final decision 

 has been arrived at. ^^'hatever may be 

 the source from which the electric dis- 

 charge obtains its energy, there is no 

 doubt about the enormous power pent up 

 in such violent flashes. Thunder, the 

 direct result of such discharges, makes 

 one familiar with the forces at work. 



So highly heated becomes the column 

 of air through which the discharge makes 

 its way that the air, in trying to accom- 

 modate itself to the rapid change of 

 temperature, expands with all speed. 

 This expansion and subsequent contrac- 

 tion — the air particles rushing in to fill the 

 partial \-acuum caused b\' the former — 

 give rise to a series of air waves, pro- 

 ducing sounds, throughout the entire 

 length of the column. These waves are 

 as much emitted by the main stream of 

 the discharge as they are by the branches 

 or ramifications of the flash, so that the 

 sources of the different noises are at a 

 great number of different distances from 

 the observer. If the flash be a short one, 

 and the observer near it, the sound will 

 be almost simultaneous, and this is known 



as a thunderclap. It is hke a sharp 

 rattling explosion, accompanied some- 

 times by a hissing noise. This hissing 

 noise seems to be directly connected with 

 the lightning flash, and is familiar to one 

 who is acquainted with sparks made in 

 the laboratory- It is therefore correctly 

 referred to in the following quotation 

 from the " Ingoldsby Legends " : 



■"It was in such an hour as this 



On such a wild and wintry night 

 The forked lightning seemed to hiss." 



If. on the other hand, the flash is of con- 

 siderable length and has many branches, 

 the sound waves, emanating simultane- 

 ously from the various parts of the dis- 

 charge, have distances of various lengths 

 to travel, and so do not reach the ear at the 

 same moment. The reason for this is 

 that while light tra\'els almost instantane- 

 ously, namely, 186,000 miles per second, 

 sound only covers about 1,100 feet in the 

 same time. The greater the distance 

 the flash is away the longer is the time 

 interval between the discharge and the 

 hearing of the thunder. 



The noise impressed on the ear by 

 distant flashes is of a rolling or rumbhng 

 nature, and is usually referred to as a 

 thunder-peal. Although the lengthening 

 of the duration of a thunderclap may be 

 to a great extent caused by echoes from 

 the earth's surface, there seems reason to 

 believe that a considerable portion of it 

 may be due to the successive arrivals of 

 the air wa\'es from the ^'arious parts of 

 the numerous branches of the main dis- 

 charge. 



It is sometimes stated that the rolling 

 is due to the noises being reflected from 

 the clouds, but in the face of the Rev. 

 J. M. Bacon's experiments this cannot be 

 claimed. Mr. Bacon made his experiments 

 by exploding cartridges from underneath 

 the car of his balloon, and although he 

 always heard the echo from the earth, he 

 recorded no instance when they were 

 returned by clouds. His observations are 

 so interesting that I quote one of them, 

 wliich is of special importance here : 



" First, when about 3,000 ft. above the 

 cloud layer, we lowered a cartridge, and, 

 stop watch in hand, listened carefully for 

 the result. It exactly repeated our former 

 experience. Absolute silence ensued, until 



