XXX11 INTRODUCTION. 



the meteorologist will obtain from it ; the two former for their own 

 benefit, the latter for science, which is useful to all. 



The facts which result from the numerous documents which 

 Mr. Espy has placed in the hands of the committee, are the fol- 

 lowing : the motion of the air in the meteor under consideration, 

 called tornado or water-spout, if it is violent, and of small extent; 

 a storm, if it covers many degrees of the earth's surface ; the 

 motion of the air, we say, is always convergent, either towards a 

 single centre, when the tornado has a circular form, and limited 

 extent, or towards a diametrical line, when the tornado or storm is 

 of a lengthened form and extends over many hundred leagues. 



If the tornado is very small, in which case the violence of the 

 motion of the air is greater, a cloud is frequently seen in the cen- 

 tre, whose point descends more and more until it touches the earth 

 or sea. Water-spouts are small tornadoes, and the force of these 

 meteors in the south and east of the United States is such, that 

 trees are carried up in the air, and the heaviest objects are over- 

 turned, displaced, and transported. Finally, we have only to call 

 to mind the well known storms of the Antilles, which change even 

 the form of the ground over which they pass. We will adopt the 

 technical word tornado to designate the meteor in question, what- 

 ever may be its extent or violence. China and the neighboring 

 seas, Central Africa and the south west part of the Indian Ocean, 

 are, like the West Indies, the theatre of meteors of the same na- 

 ture, and not less disastrous. 



In observing at the same moment the force and direction of the 

 wind, which is shown by the overturned trees, the displaced mov- 

 able objects, in a word, by the traces impressed upon the soil, 

 Mr. Espy proves that in the same instant the motion of all parts 

 of the air which is reached by the tornado is tending towards a 

 central space, point or line, so that if the wind on one side of the 

 meteor blows towards the east, it blows with the same violence 

 towards the west on the other side of the tornado, and frequently 

 at a very short distance from the first place, whilst in the centre, 

 an ascending current is formed of astonishing rapidity, which, 

 after having risen to a prodigious height, spreads out on every 

 side to a certain limit, which we shall soon determine by the ob- 

 servations of the barometer. This ascending current loses its 

 transparency at a certain height, and becomes a true cloud of the 

 kind called cumulus, the base of which is horizontal, and whose 

 height is determined by the temperature and humidity of the at- 

 mosphere. The central cloud of the tornado is constantly repro- 

 duced, in proportion as it is carried off by the rapid current of the 

 centre ; and, according to Mr. Espy, when rain or hail proceeds 

 from this meteor, which is generally the case, it is the cold, caus- 

 ed by the expansion of the air carried into the higher regions of 



