268 APPENDIX 



In order to illustrate my meaning we may usefully 

 ponder earth conditions during a thunderstorm, in relation 

 to contour and nature and conductivity of soil. 



Let us disregard for the moment the terms positive and 

 negative and substitute for them the words " plus " and 

 " minus." 



The air, the upper stratum and, hypothetically, stretch- 

 ing upwards to infinity, is always " plus " ; the earth, 

 normally, " minus." 



Between the charged cloud and the comparatively 

 uncharged earth there is an air-space the spark-gap and 

 unless the tension of the cloud is sufficiently high to bridge 

 it no discharge can take place. Suppose the surface o^ 

 the earth to be flat 



arth 



Fig. 144. 



or, alternatively, the surface to be very dry or composed of 

 some more or less dielectric material. The cloud would 

 unless the tension were extraordinarily high travel over 



Fig. 145. 



such ground without discharging. When, however, by 

 reason of contour, the distance between earth and cloud 

 was lessened to one that the tension of the cloud could 



