82 SCIENCE OF COMMON THINGS. 



Tornadoes-~cause of. Eddies in water. 



520 How great is the surface over whicJi they prevail ? 



Mr. Redfield, of New York, lias estimated the great 

 Cuba hurricane of 1844 to have been not less than 

 eight hundred miles in breadth, and the area over 

 which it prevailed during its whole length was com- 

 puted to be two million four hundred thousand square 

 'miles an extent of surface equal to two-thirds of that 

 of all Europe. 



521 What curious fact have mariners noticed when in the centre or 

 vortex of the hurricane ? 



An awful calm prevails, described as the lull of the 

 tempest, in which it seems to have rested only to 

 gather strength for greater efforts. 



522 In what respect does a tornado differ from a hurricane f 



Tornadoes may be regarded as hurricanes, differing 

 chiefly in respect to their continuance and extent. 



523 How long do they usually last f 



From fifteen to seventy seconds. 



524 What is their extent f 



Their breadth varies from &few rods to several hun- 

 dred yards, and the length of their course rarely ex- 

 ceeds twenty miles. 



525 What phenomena generally attend them f 



The tornado is generally preceded by a calm and 

 sultry state of the atmosphere, when suddenly the 

 whirlwind appears, prostrating everything before it. 

 Tornadoes are usually accompanied with thunder and 

 lightning, and sometimes showers of hail. 



526 What is supposed to be the origin of tornadoes f 



They are supposed to be generally produced by the 

 lateral action of an opposing wind, or the influence of 

 a brisk gale upon a portion of the atmosphere in repose. 



527 How are the eddies or whirlpools produced which occur in water, 

 and which in their formation resemble some tornadoes f 



Eddies or whirlpools are most frequently formed in 

 water when two streams flowing unequally meet, They 

 may be seen at the junction of two brooks or rivers. 



