58 Whirlwinds excited by Fires. 



nificent scale. This effect, it appears, is most fully produced in 

 the volcanic exhibitions above mentioned. 



7. These cases may serve to illustrate, in some degree, the fa- 

 voring influences which are presented in the calm latitudes, near 

 the equator, and on the exterior limits of the trade winds in mid- 

 ocean, for the production of vorticular action, either in the form of 

 squalls, thunder gusts, or water spouts. The main horizontal 

 movements of circulation which are common to our atmosphere, 

 being here comparatively sluggish or inert, opportunity is thus 

 afibrded for the minor influences of rarefaction to come into play ; 

 by which means, frequent squalls, water spouts, and other local 

 movements in the lower atmosphere, are excited in these regions. 



8. The velocity and strength of the wind in the rotative action 

 of a vortex, the axis of which is in a vertical position, as in the 

 above cases, greatly exceeds that which is exhibited in other cir- 

 cumstances. 



9. We may perceive, also, in these cases, how effectually the 

 heated stratum of air nearest the earth's surface, may be caused 

 to penetrate a homogeneous and colder overlying stratum of cur- 

 rent, by means of the vorticular action ; a movement which is oth- 

 erwise physically impossible, except in a partial degree, by means 

 of other concurrent movements or favoring circumstances. 



10. These considerations, and others which suggest them- 

 selves, will enable us to account for the sudden and rapid forma- 

 tion of hail, in summer hail storms and tornadoes ; and in the struc- 

 ture of the hail stones, in successive layers, and their occasional- 

 ly fractured condition, we may perceive both the evidence and the 

 effects of the violent vortical action, and the successive changes 

 of temperature and hygrometric condition in which they have 

 been successively placed, before leaving the vortex, and while 

 falling to the earth. 



11. These cases, viewed in connection with the natural phe- 

 nomena which have been exhibited in a multitude of instances, 

 appear fully to confirm the opinion of Franklin, in favor of the 

 general identity of the columnar whirlwinds with the so called 

 water spouts. 



12. By the clue which is here afforded us, in the peculiar noise 

 and rumbling detonations of these violent whirlwinds, we may 

 become apprised of the existence of a violent vortical action in the 

 atmosphere, or within the envelopement of a visible thunder 



