December 17, 1909] 



SCIENCE 



863 



ing inductively that accounts for such 

 pressures. A combined stress is set up 

 towards the earth or towards another cloud 

 mass of opposite charge. The lightning 

 stroke results from a breakdown of the in- 

 sulating air layer between them, and also 

 all through the cloud itself, and for a time 

 a partial neutralization or electric equi- 

 librium is effected. This continues until a 

 further redistribution of charges is re- 

 quired and until again the breakdown 

 potential is reached. The continued coal- 

 escence of charged water particles which 

 were not discharged at the fii-st breakdown, 

 repeats the original condition, and so on. 

 Unlike the case of a .suspended charged 

 metal body, a single discharge does not 

 usually equalize the electric potential of 

 cloud and earth. Instead, many succes- 

 sive discharges occur. It is probably 

 fortunate for us that the process is as 

 gradual as it is, for the ordinary partial 

 discharges of the cloud are each terrific 

 enough and tax our resources sufficiently 

 when we seek to protect ourselves and our 

 effects from them. 



Various hypotheses have been proposed 

 to account for the presence of electric 

 charges in cloud masses, but there is no 

 time to discuss them here, and there is in 

 fact little that is really known as to the 

 origin of the electricity of clouds. We 

 shall briefly refer to the phenomena which 

 characterize or accompany the electric 

 discharges. The usual form which the 

 discharge takes is that known as disrup- 

 tive spark or fork lightning, a long flash or 

 electric spark, joining earth and cloud, or 

 cloud and cloud, and branching within 

 the cloud mass like a tree. Oftentimes 

 between cloud and earth there is seen the 

 single streak zigzag in its course, but 

 within the cloud it ramifies or branches 

 extensively in several directions. In this 

 way only can any considerable part of the 



cloud contribute its portion to the main 

 discharge path, for, as stated before, the 

 cloud can not act as a conducting body. 



Some authorities treat lightning as a dis- 

 charge of very high frequency like the 

 ordinary discharge of a condenser or 

 Leyden jar. In fact it has not been un- 

 usual to assume that such apparatus can 

 be substituted and inferences drawn as to 

 the nature and character of the lightning 

 discharge from experimentation and tests 

 with these laboratory appliances. There 

 is, however, abundant reason to doubt that 

 lightning discharges are really oscillatory. 

 If they oscillate the conditions are such as 

 to forbid such oscillation being of a high 

 frequency order. The cloud discharge 

 represents what Ls known as a discharge 

 of a large capacity, and the length of the 

 path or spark may reach thcmsands of feet 

 or even many miles; a long inductive 

 path, while the heat and light given out in 

 every part of the path indicate a high 

 resistance to the passage of the discharge. 

 All of these conditions are together 

 known to be inconsistent with the idea 

 of high frequency oscillation. But the 

 breakdown or discharge is extremely sud- 

 den and involves an almost in.stant rise 

 of the current to a large value, so that 

 the inductive effects upon surrounding- 

 structures, such as electric lines or cir- 

 cuits, are very energetic and sharp like- 

 a quick blow struck; and these lines or 

 structures become the seat of rapid vibra- 

 tion or high frequency oscillation.s. The- 

 sudden blow of the hammer on a bell ia 

 like manner brings out all the rates of the 

 vibration, fundamental and overtones, of 

 which the bell is capable and in which the 

 hammer itself takes no part. 



The very .sudden startling character of 

 a lightning discharge leads to an exag- 

 geration in the popular estimate of its 

 more evident effects. The amount of light 



