100 



clouds form. They appear rather to act as producers of the concentra- 

 tion of electricity in the clouds than otherwise, but when talcing place 

 under an overcharged cloud no doubt act as channels of exchange and 

 discharge, and in this way — two forces acting conjointly— their effective- 

 ness would be increased immensely. 



To get a fair understanding of the phenomena, the following points 

 must be considered, viz. : — 



1. The time of day they take place. 



2. The accompanying circumstances. 



3. The actual facts observable — a, at the start ; b, during progress; 

 c, at the end. 



4. The mechanical agencies, by means of which the observed facts 

 can be explained. 



In no instance can I remember ever to have witnessed a real whirl- 

 wind either much before 10 a.m. or after 4 p.m. ; the period of maximum 

 frequency being between noon and 3 p.m., thus limiting their occur- 

 rence to the hours of maximum daily temperature. By this observa- 

 tion it is not implied that they cannot occur at some other time 

 (for I have noticed often small momentary whirling gusts at other 

 times, when obstructions of peculiar form offered themselves to the 

 winds), but that their normal occurrence falls within the periods 

 named. 



The weather at the time is distinguished by calm, intercepted by 

 short, fitful blasts of wind from every point of the compass irregularly, 

 and considerable warmth. If a hot wind is prevalent at the time, 

 whirlwinds invariably occur in a temporary lull. In respect of size 

 they range from a few inches in height and diameter, over an ash- 

 heap or in large fireplaces, to lateral dimensions of several hundreds 

 of feet, and vertically to several thousands of feet in elevation. 



The producing causes may be found in the sudden temporary escape 

 of overheated air, when in unstable equilibrium, through a resisting 

 medium, viz., a stratum of denser, colder, rapidly-moving current above. 

 By this escape a partial vacuum being formed, the gravitation of the sur- 

 rounding still air will assist in forcing the lower strata directly under the 

 vent, powerfully up and towards the same, by which means the increased 

 force of the whirling current can be easily accounted for. 



The effect of the upper polar current upon the ascending heated air par- 

 ticles is very great, because there is a mechanical law, according to which 

 a fluid or gas in rapid motion acts to some extent like a solid in pre- 



