9 



The observations hitherto made, seem, however, to have established some 

 important principles, which I may brieily recapitulate from the ^vritings of 

 Franklin, Eedfield, Colonel Capper, Colonel Reid, and Professor Espy. 



A tornado (from the Spanish toniar, to turn round), is defined as a sudden 

 and violent storm of wind, acce)mpanied by thunder, lightning, and heavy 

 rain, occurring about the time of the equinoxes or (in the tropics) at the 

 changes of the monsoons. The storm continues in one place a short time, days 

 or hours, during which time the wind blows at once or successively from all 

 points of the compass. The storm moves onward in a straight line for some 

 distance, but at the same time revolves around a centre, and the rush of air 

 can-ies ujjward objects which are included within the vorte.x. Sometimes the 

 force is so great, that even trees, houses, and other large oVijects are whirled ' 

 upward or torn to pieces, which fall in opposite directions. The centre may 

 be absolutely calm, while objects at the outside of tlie whirl or vortex are 

 exposed to wnds of immense f iny and violence. Xhe vortex may be many 

 miles or merely a few yards in diameter, while the jiath of the .storm may 

 extend for hundreds or even thousands of miles. The cause is supposed to 

 be the rarefaction of the air in a given spot, usually within the tropics, which 

 liberates the electric fluid ; the rarified air ascends, and the cool air all around 

 it rashes in to the partial vacuum thus produced. The particles of air, forming 

 the different currents caused Ijy ol)stnictions on the earth's surface, strike each 

 tither at different angles, and thus acijuire spiral or circular motion. These 

 whirlwinds seem to be always connected with electrical discharges. Some- 

 times, the storm commences with contrary currents of air forcing Ijlack clouds 

 in opposite directions ; the rarefaction causes the lower stratum to ascend, 

 the clouds come into collision, and there is a thunderstorm, followed by a 

 whirlwind. At other times the whirlwind is preceded by an unusual stillness 

 of the atmosphere, in winch the v>'hirling motion commences suddenly, and is 

 followed by thunder, lightning, and rain. At other times, as on Sunday, 

 July 7th, the whirlwind seems to occupy a central jiosition between two 

 storms ; but in most cases the rotary motion r.-ipidly increases as the vortex 

 passes on, until its fury is spent. The duration of its extreme violence does 

 not continue in any one place or district more than half an hour, and in most 

 cases the period is very much shorter. A few minutes is about the average. 

 Our little whirlwind of Sunday week, July .3, lasted for a period of time 

 variously estimated by observers from two to five minutes. Tlie rate at 

 which the vortex travels varies from oO to 100 miles per liour. 



In a whirlwind, it has been found that the direction of the rotary motion 

 in the Northern hemisphere is invariably from N. roimd by W. to S. and 

 I'L ; in the Southern hemisphere the reverse. 



The precise mode in which the result is produced is a matter upon which 

 there is a difference of opinion. For example. Dr. Hare, of the University 

 of Philadelphia, considers that the immediate causes of those storms ai-e 

 electrical discharges between the earth and the clouds, which liberate heat and 



