i ' WINDS. 2S7 



whatareTor- 632. Tomadoes may be regarded as hurri- 

 nadoes? canes, differing chiefly in respect to their con- 

 tinuance and extent. 



Tornadoes usually last from fifteen to seventy seconds ; 

 their breadth varies from a few rods to several hundred 

 yards, and the length of their course rarely exceeds twenty 

 miles. 



The tornado is generally preceded by a calm and sultry state of the atmos- 

 phere, when suddenly the whirlwind appears, prostrating every thing before 

 it Tornadoes are usually accompanied with thunder and hghtning, and 

 sometimes showers of hail. 



Tornadoes are supposed to be generallv pro- 

 How are tor- , , , i i i • n 



nadoes pro- ciuced by the lateral action ot an opposmg 

 wind, or the influence of a brisk gale upon a 

 portion of the atmosphere in repose. 



Similar phenomena are seen in the eddies, or little whirlpools formed in 

 water, when two streams flowing in different directions meet. They occur 

 most frequently at the junction of two brooks or rivers. 



Whirlwinds on a small scale are often produced at the corners of streets in 

 cities, and are occasioned by a gust of wind sweeping round a building, aud 

 striking the calm air beyond. 



The whirl of a tornado, or whirlwind, appears to originate in the higlier 

 regions of the atmosphere ; it increases in velocity as it descends, its base 

 gradually approaching the eartli, until it rests upon the surface. 



Great conflagrations sometimes produce whirlwinds, in consequence of a 

 strong upward current, which is produced by the expansion of the heated 

 air. A remarkable example of this is recorded to have happened at the 

 burning of Moscow, in 1812, where the an- became so rarefied by beat, that 

 the wind rose to a frightful hurricane. 



It has been noticed as one of the curious effects of a tornado, that fowls and 

 birds overtaken by it and caught in its center, are oflen entirely stripped of 

 their feathers. In a theory propounded some years since to the American 

 Association for the Promotion of Science, by an eminent scientific authority, 

 it was supposed that in the vortex, or center of the tornado, there was a 

 vacuum, and the fowls being suddenly caught in it, the air contained in the 

 barrel of their quills expanded with such force as to strip them fix)m th» 

 body. 



Avhat is a ^^^' ^ watcr-spout is a whirlwind over the 

 Water-spout T surfacc of Water, and differs from a whirlwind 

 on land in the fact that water is subjected to the action 

 of the wind, instead of objects on the surface of the earth. 

 In diameter the spout at the base ranges from a few feet 



