Ch. XVI.] 



WHIRLWINDS AXD CYCLOXES. 



305 



surface, which was in a state of unstable equilibrium. 

 This continued until the heated stratum was able, at 

 some point where the ground favoured a comparatively 

 greater accumulation of heat, to break through the over- 

 lying strata of air, and force its way upwards. An 

 opening once made, the whole of the heated air moved 

 towards it and was drained off, the heavier layers sinking 

 down and pressing it out. Sir George Airy has sug- 

 gested to me that the reason of the particles of air not 

 rising as they are heated, when there is no wind blowing, 

 may be due to their viscosity ; and this suggestion is pro- 

 bably correct. That air does not always rise when 

 heated, may be proved by the hot winds of Australia, 

 which blow from the heated interior towards the cooler 

 south, instead of rising directly upwards. Sultry, close 

 weather, that sometimes lasts for several days, would 

 also be impossible on the assumption that air rises as 

 soon as it is heated. 



This explanation supplies the force that is necessary to 

 drive the air with the great velocity with which it moves 

 in whirlstorms. The upper, colder, and heavier air is 

 pressing upon the heated stratum, and the greater area 

 over which the latter extends, the greater will be the 

 weight pressing upon it, and the greater the violence of 

 the whirlwind when an opening is formed for the ascent 

 of the heated air. There is a gradual passage, from the 

 small dust eddies, through larger whirlstorms such as 

 that at Lough Neagh, to tornados and the largest 

 cyclone ; every step of the gradation might be verified 

 by numerous examples ; and if this book were a treatise 

 on meteorology, it might be admissible to give them ; 

 but to do this would take up too much of my space, and 





