Progressive lightning

A method of obtaining direct information as to the mode of development of the lightning discharge has been described by Boys.¹ it consists in the use of a camera with a fixed plate and two lenses revolving in a circle at opposite ends of a diameter. The two pictures of the lightning flash thus obtained show distortions, due to the motions of the lenses, which are in opposite directions. From a comparison of the two pictures and a knowledge of the velocities of the lenses, it is possible to deduce the direction and speed of the developing discharge.