94 
ME. C. T. R. WILSON : INVESTIGATIONS ON LIGHTNING DISCHARGES 
the picture as a whole) that a charge of electricity had been concentrated in a 
comparatively small volume in the head of the cloud, and that the discharges took 
place approximately along lines of force. 
The mean height of the upper ends of the two discharges—the height of the 
centre of the charged cloud-head according to this view—must have been just under 
2 km., if its horizontal distance from the camera is taken as 11'7 km. The distance 
and height may in fact have been somewhat greater, since the track of the long flash 
may at some point of its course have been nearer the camera than the vertical flash, 
and the distance deduced from the interval between the lightning and thunder is 
that of the nearest point of the discharge. 
VIII. Electric Field of a Thunder-cloud. 
It is much more difficult to obtain direct information about the electric field of a 
thunder-cloud than about the sudden changes produced in the field by lightning 
discharges. The observed field may be the resultant of the fields of several thunder¬ 
clouds superimposed upon the normal electric field ; while a single instantaneous 
change in the field will in general be due to the passage of one lightning flash, of 
which the approximate distance may frequently be determined. Nothing approach¬ 
ing a direct survey of the electric field of a thunder-cloud has yet been attempted : 
some general conclusions may be reached by a study of the photographic records of 
the potential gradient in thunderstorms. 
It might perhaps naturally have been thought that the actual field due to a 
distant thunder-cloud would greatly exceed in magnitude the sudden changes due to 
the lightning discharges from it, each flash removing from the cloud only a small 
part of its whole charge. This is disproved by the observations ; only when there 
has been, in addition to the more distant thunder-cloud, a heavy shower-cloud over¬ 
head or in the immediate neighbourhood of the place of observation, has the actual 
potential gradient greatly exceeded the instantaneous changes; the main part of the 
observed field has in all such cases obviously been due to the nearer cloud and not to 
the comparatively distant thunder-cloud which was in action at the time. The 
potential gradient due to a distant thunder-cloud has apparently never greatly exceeded 
in magnitude the sudden changes produced in the field by the lightning discharges 
from the cloud. Very frequently each discharge has approximately destroyed or 
even reversed the previously existing potential gradient, the field has then been 
rapidly regenerated, to be again nearly neutralised or reversed by the next discharge. 
The magnitude of the vertical electric force at the ground due to a thunder-cloud at 
a given distance is thus probably of the same order as has been found for the 
change produced by the average lightning discharge at the same distance. 
Potential gradients exceeding 30,000 volts per metre (i.e., °f the sparking 
value) have not been recorded: it is doubtful, however, if any of the records were 
obtained when the centre of a storm was nearly overhead. 
