AND ON THE ELECTRIC FIELD OF THUNDERSTORMS. 
93 
lightning flashes. A few are quoted by Hann, # the length of vertical flashes to 
earth generally ranging from 1 to 3 km. It is only rarely, in the photography of 
lightning, that the distance of the flash has been recorded, so that its length may be 
deduced. Fig. 4 is a reproduction of a photograph taken with this object in view 
Fig. 4. 
and for which the necessary data are available ; it is, moreover, of interest in other 
ways. It was taken on May 22, 1918, at about 22h. 45m., the camera pointing 
north. The interval between the lightning flash and the moment when the thunder 
began to be heard was 35 seconds, corresponding to a distance of 117 km. Two 
flashes are shown in the photograph, both passing between the cloud and the earth ; 
they must have been nearly simultaneous, since the camera lens was covered as soon 
as a flash was observed. One discharge has initially passed upwards from the cloud 
and reached the ground by a curved path at a horizontal distance of nearly 4 km. 
from its starting point. The other has taken a nearly vertical course to the ground, 
its image is somewhat faint and ill-defined in the photograph : the discharge was 
probably within a heavy rain shower, a considerable thickness of which had to be 
traversed by the light on its way to the camera. The starting points of the two 
discharges in the cloud are comparatively close together, suggesting (as indeed does 
* Hann, ‘ Lehrbuch der Meteorologie,’ p. 632, 1901. 
