102 



By this forward motion the direction of the air particles, at first per- 

 pendicularly upwards towards the point of rupture, attains obliquity 

 before gaining entrance dc facto, while those that started at the rear 

 become still more slanting in their course, having to hurry forward at an 

 increased rate, but impinging upon those coming from the sides and the 

 opposite direction, are forced out of it, their motion thus becoming side- 

 ways, forward, and upward — i.e., spiral. These in front are forced aside 

 by their adhesion and the forward pressure of the up-rushing current, and 

 the inertia of the comparatively unaffected volume of the adjacent air. 

 In this manner the spiral mo ion is obtained. 



The upward force of the ascending current is further augmented 

 (and, derivatively, the lateral motion) by the downward pressure of the 

 superincumbent strata of the air, by forcing the lower into the (funnel- 

 like) vortex, which latter assumes the functions of a partial vacuum. 

 The commotion produced in the vortex gradually affecting the adjacent 

 mass of air, the spirally-ascending currant assumes naturally in progres- 

 sion the form of a descending inverted cone — observed at times in the 

 case of waterspouts, etc.— which finally reaches the surface of the earth, 

 there exerting the maximum of force because here the weight (gravitation) 

 of the atmosphere, etc., is greatest, and reaches the earth at a point from 

 which the action rapidly extends on all sides. The spiral motion is 

 strongest near the circumference, while at the centre of the current — the 

 whirlwind — it is directly upwards, but in a more or less oblique direction, 

 on account of the forward motion of the whole and the friction with the 

 surrounding air. 



Generally the gyrations of the whirlwind cease as suddenly as they 

 begin — first in the centre, then at the circumference. A dust column, 

 seen at a distance, appears invariably to break off at some distance above 

 the ground, the objects comprising the lower part rapidly descending, 

 while those in the upper portion seem slowly to float upwards, con- 

 spicuous articles of small weight being often carried to considerable dis- 

 tances ; the light dust preserves its columnar form sometimes for a 

 quarter of an hour or more without apparently changing its place. 



In conclusion, the following is a short resume of what is intended 

 to be shown in the foregoing, viz : — 



1. Whirlwinds are upward currents of heated air, endowed with 

 a spiral motion at the circumference, progressive motion and small 

 diameter in proportion to their height. 



