APPENDIX 



overcome, or, alternatively, tension being sufficient, a 

 point was reached where the soil favoured conduction, a 

 transfer of potential from the plus cloud to the minus earth 

 would at once take place, in exactly the same manner that 

 a spark is obtained from a Ley den jar or induction coil when 

 the conducting knobs or points are approached near 

 enough to each other. Scientifically this is termed a 

 disruptive discharge. It occurs when the air becomes 

 strongly strained by the potential difference, and, suddenly 

 yielding, allows the discharge to pass, not freely as through 

 a conductor, but by a violent disturbance of the molecules 

 of air along the path, which become strongly heated, and 

 make the visible spark. This takes a zigzag and forked 

 path which in all probability is the line of least resistance, 

 and is due to irregular distribution of conducting motes in 

 the air, or to its hygrometrical condition. 



However this may be, we will imagine that at the point 

 A (Fig. 1 46) the sub-soil is of such a nature that the charge 

 which it has just received from the cloud cannot be readily 

 dissipated, and that another cloud which has discharged 

 itself in the immediate vicinity passes over it within a 

 distance over which the spark-gap can be bridged. The 



Jfartk 



result must be that discharge will take place from earth to 

 cloud, because the cloud is the minus and the earth the 

 plus quantity ; but it does not necessarily follow that such 

 discharge must be from the exact area which first received 



