-18591 WRITINGS OF JOSEPH HENRY. 361 



It is asserted by Mr. Wise that the thunder-cloud, when 

 viewed on one side from a sufficient elevation, presents the 

 appearance of an hour-glass, the upper and the lower ends 

 spreading out almost into two distinct clouds, as seen in 

 Figure 19. 



We find that the same form of the thunder cloud has been 

 described by other aerial voyagers, also by Volta ; and we 

 are inclined to consider it the usual one presented by this 

 meteor, since it is precisely that which would be produced 

 by the self-repulsion of the upper and lower parts of the 

 cloud, each charged as it is throughout its mass with the 

 same kind of electricity. The middle of the perpendicular 

 dimensions of the cloud as illustrated by the perpendicular 

 conductor, Figure 15, will be neutral, and hence no tendency 

 to bulge out at this point will exist. Mr. Wise also states 

 that flashes of sheet lightning are constantly seen at c, in the 

 middle space ; and sometimes intense discharges from the 

 upper to the lower part of the cloud; — appearances in exact 

 conformity with the views here presented. 



The immense number of discharges of lightning from a 

 single thunder-cloud in its passage over the earth, through 

 a distance in some cases of more than 500 miles, indicates a 

 constant supply of electricity; and this is found in the con- 

 tinued rushing up of new portions of moist air, and in the 

 successive renewals of the perpendicular column with fresh 

 materials, the electrical equilibrium of which is disturbed 

 by induction. 



In the case of a tornado or water-spout, the ascending cur- 

 rent of air is confined to a very slender column, in which 

 the action is exceedingly intense; and since it is scarcely 

 possible that the rushing in from all directions of the air 

 below to supply the upward spout can be directed to pre- 

 cisely the same central point, a whirling motion must be 

 produced. This will tend to limit the diameter of the spout, 

 and to create a partial vacuum at the axis of the column, in 

 which the moist air will have its vapor condensed by the 

 cold of the sudden expansion, and a conductor will thus be 

 formed extending from the cloud to the earth. Through 



