June, I934.1 CURRENT TOPlCS. 77I
followed exactly the same path that the leader had taken, to the cloud.
The leaders were invariably thin, of uniform width, and unbranched. Their speeds ranged from 8IO to 19,9oo miles per second and averaged 515o miles. The length of the strokes observed varied from 1.6 to 4.7 miles. The longest time occuped by a leader stroke was 167 Β° millionths of a second for a 4-7 mile stroke.
The main discharge is entirely different in character. It is thicker and brighter, and the thickness diminishes upward. Also it travels faster. The speeds range from 14,9oo to 68,400 miles per second, the average being 28,500 or about 15 per cent. of the speed of light. Often the main upward discharges were branched--but they branched downward. After each branch the main stem thinned but did not pause in its upward motion.
The time required for the main discharge to reach the cloud varied from 44 to 65 micro-seconds. The bright luminosity at the base lasted from I2 to 164 micro-seconds. A faint luminosity, however, lasted much longer, sometimes as much as I/5oth of a second, as though the path had been heated and continued to glow. In conformity with the theory proposed by Dr. N. E. Dorsey in 1926, the authors believe that the leader consists of an "electron avalanche" which goes ahead and ionizes the air, thus preparing a conducting path for the main discharge that follows.
C.