Two distinct explanations have been offered of multiple flashes as seen in photographs. Ac cording to one, the first flash makes, as it were, a hole in the air, and succeeding discharges quickly pass up and down through this tube, while the general wind of the atmosphere carries it along for a few feet. According to the other explanation the hole or tube of rarefied air lasts so short a time that the motion of the wind is inappreciable, but it is of such a great diameter that the successive discharges passing up and down within it side by side, or one within the other, appear as separate streaks of light. Neither of these explanations can he said to be satisfactory. Somewhat similar flashes can be produced artifieially by blowing across a dis charge between two electrodes. Photographs often show a dark flash among the bright ones, but this is known to be a photo-chemical effect on the sensitive plate and not atmospheric.
The researches of Trowbridge seem to show that the resistance of atmospheric air to elec trical discharges becomes less as the voltage increases beyond some high number such as a mil lion volts; when the first or pilot discharge has taken place the air oilers less resistance to the succeeding discharges. Under powerful electric
stress a resisting medium seems completely to lose its power of resistance and is immediately ruptured, Consult: Lodge. Lightning ('onduct ors and Lightning Gunrds 1892), and time bulletin of the United States Weather Bureau on Protection from Lightning, by Alexander Mc Ad ie.
The frequency of occurrence of lightning and thunder storms is published regularly in the Prised States Monthly Weather Berke% annual summary shows that the number of (lays on which thunder-storms were reported varies from 100 at any station in Louisiana and Florida to 5 or even 1 per annum at special stations in Arizona. California. Maine. Nevada. North Da kota. Oregon, and Washington. it is suspected that there is some relation hetween the aurora and the lightning, since on many occasions numer ous thunder-storms have been recorded around a central region where clear skies and auroras were observed.