Page:Popular Science Monthly Volume 64.djvu/225

Rh When the cloud spread over Fort de France these necessarily rolled out into long billows and irregular kidney-shaped forms, the lower surface of which might sometimes be seen when illumined by the lightning flashes. Discharges from node to node, with the 'end on' view, would be small points of light, like the sparks between the electrodes of an induction machine when the two are pushed together. They would also be very numerous and incessant as the individual nodes were numerous, small and possessed of intensest vertical velocities over the direct discharge from the volcano's throat. As the mushroom spread to a distance the individual dust eddies became larger and more diffuse, and the discharge between them would become less frequent and more irregular, giving rise to the worm stage. With further increase and more massive movements within the cloud, the same continually growing in volume and rising to greater height as well as spreading across the line of vision of the observer below, the lightning flashes decreased in frequency, and increased in length, intensity and irregularity, producing finally the long orange-colored serpents described below—the color being due to the dust layer which partially obscured the dazzling flashes.

Whatever the cause, the lightning clearly showed a gradual change in character; from incessant, shifting, scintillating points it changed to less frequent worm-like lines and short forking discharges, the divergence being at first downward toward the crater. One flash was noticed in a direction N. 30° W. having a distinct short S shape with three inflections, the lying horizontal. At no time were any lightning flashes seen to pass from earth to cloud or vice versa; all of these discharges were across the sky. At 9:30 the continuous grumbling from the volcano died away.

At 9:43 the middle of the dust arc southward had the bearing S. 20° E. This point corresponded to what at the start was the zenith point of the dust balloon.

At 9:45 very brilliant sheet lightning occurred showing no definite points or lines, but illumining the whole vault in dazzling flashes. The lightning seemed to be more concentrated on the periphery of the cloud. Meanwhile white cumulus clouds lifted towering thunder heads to the southeast and these marked the beginning of local thunder-storms over all the high points of the island. The high tops of these clouds lighted by the sheet lightning showed their billowy profiles white against the dark vault of dust above; some of them appeared to rise to a great height, but their summits were far below the black dome of volcanic dust, affording an excellent measure of the stupendous height of the volcanic cloud and its electrical phenomena—both entirely distinct from the rain-bearing thunder-storms.

At 9:52, looking southwest, the thin edge of the dust cloud against