40 Introduction to the Study of Science 



first equalizes, for a time at least, the difference in electrical 

 energy between two clouds or a cloud and the earth. In its 

 passage through the air, much of its energy is changed into 

 heat, the air particles being thrown into swift vibration and 

 becoming white hot, incandescent, or luminous (page 166). The 

 transfer of electrical energy is so rapid that only a flash is visible. 

 The white-hot air expands suddenly and violently, and instantly 

 contracts, or cold air rushes into the partial vacuum, giving 

 forth a succession of vibrations that produce the noise of 

 thunder. 



The distance of a thunderstorm from an observer may be 

 readily estimated. The flash of lightning is seen almost as 

 soon as it occurs. The sound or report of the thunder travels 

 more slowly, about eleven hundred feet per second at ordinary 

 temperature. The distance is approximately the product of 

 the rate at which sound travels and the number of seconds 

 elapsing between the flash and the thunder. 



From the account of the accumulation of electricity upon 

 raindrops, it should be evident that a heavy downpour of rain 

 will greatly diminish the electrical charge of the cloud. When 

 occasion offers, observe whether the sharpest lightning precedes 

 rainfall or not ; and whether or not there is a diminution of the 

 lightning after the beginning of rainfall. 



When directed toward the earth, an electrical charge follows 

 the path of least resistance, " striking " usually the highest 

 points, as towers, steeples, or trees which are especially good 

 conductors because of their water or sap. In passing down a 

 tree to the earth, the electricity follows the moisture, which, 

 instantly vaporized by the heat, expands and explodes, tearing 

 off the bark or splitting and shattering the tree. Lightning 

 striking dry sand fuses it and makes what are called " lightning 

 tubes." 



Kinds of discharge. In certain circumstances when the air 

 is heavily charged with electricity, there may occur brush-like 

 flames on high, sharp-pointed objects, as trees, flagpoles, or 



