Mr. T. Belt on the Origin of Whirlwinds. 49 



process has been repeated^ until the conclusion became inevitable, 

 that those whirlwinds were the channels that carried off the 

 heated air from the surface to the higher regions. 



It is generally supposed that, as the strata of air next the 

 surface become heated, the rarefied particles rise and are mixed 

 with the higher and cooler layers ; but 1 expect to be able to 

 show that this equalizing action does not always take place in 

 such a regular and placid manner, but is accompanied by, and 

 the cause of, those commotions of the atmosphere known as 

 hurricanes, typhoons, and whirlwinds. 



In calm or nearly calm weather during the summer months 

 in this colony, the strata of air next the ground become heated ; 

 and unless there is sufficient wind to carry them off and mix 

 them with cooler portions of the atmosphere, they remain next 

 the surface in a state of unstable equilibrium, and heat goes on 

 accumulating until the elastic force of the heated strata becomes 

 so great, that at special points, where some peculiarity of the 

 ground has favoured a comparatively greater accumulation of 

 heat, they are enabled to pierce through the overlying masses 

 of air and force their way upwards. An opening once made, 

 the whole of the heated strata will move towards it and be carried 

 off, the heavier layers sinking down and pressing them out. 



The eddies of air I have described, are the points where the 

 heated air from next the surface is escaping through the denser 

 superincumbent atmosphere ; and as it has not only to force 

 upwards but to contend against the pressure on the sides of the 

 ascending column, it will readily be perceived how this double 

 action gives to it its rotatory or spiral motion. The behaviour 

 of an eddy of air is similar to what occurs when an opening is 

 formed through the bottom of a shallow cistern of water. 



As heat is an active agent in eliciting electrical action, it may 

 be that the lower strata are prevented from mixing with the 

 upper by their peculiar electrical conditions. 



The hot winds of Australia prove that air does not always 

 rise as it is heated, for in them we find a warm current of air 

 actually displacing the cooler atmosphere. 



Analogy with hurricane, ^-c. — If the violence of the whirlwind 

 is caused by the great pressure of the upper masses of air 

 forcing out the lower strata, its dimensions and force will be in 

 proportion to the extent of the rarefied layers, — so that whenever 

 large tracts of air next the surface arc liable to be gradually 

 heated, wc may expect whirlwinds to occur at intervals similar 

 to the Australian eddies (which may be called the initial phase of 

 a whirlstorin), but of greater extent and violence in proportion 

 to the larger tracts of air drained off. 



Perhaps the next in violence to the eddies of air is the whirl- 

 Phil Mag. S. 4. Vol. 17. No. 111. Jan. 1859. E 



