INTRODUCTION. XXX111 



the atmosphere, which condenses the water. Electricity, when 

 it appears in the tornado, is not, according to Mr. Espy, essential 

 to the phenomenon. 



The existence of an ascending current of extreme violence 

 once placed beyond doubt by the phenomena of the rising of the 

 air, and its motion towards a centre or towards the great diameter 

 of the oblong space occupied by the tornado, being well estab- 

 lished by facts, Mr. Espy examines the progressive movement of 

 the whole meteor, which is very slow, compared with the velocity 

 of the wind in the mass of air which becomes at each instant a 

 part of the tornado. Mr. Espy shows that near the latitude of 

 Philadelphia, where cirrus clouds, very elevated as is known, 

 move towards the east, the centre of the tornado moves almost 

 always towards the east, as well as in Europe, where the west 

 wind is predominant ; whilst, in the inter-tropical regions, (Bar- 

 badoes, Jamaica, the north of the Indian Ocean,) the meteor moves 

 towards the west or north west, following the course of the trade 

 winds. These assertions are also verified with regard to China 

 and the Indian Ocean, according to the maps of Berghous. The 

 barometer, in the centre of the meteor, is sometimes nearly 2.25 

 of an inch (sixty millimetres,) lower than towards its border, and its 

 limit is marked on all its outline by a closed curve, along which 

 the barometer is found to be at its " normale " height, whilst, on 

 the other side of this line, further from the centre, the barometer 

 is observed to rise, which rise in small tornadoes is .08 of an 

 inch, (two millimetres,) but which may be forty or forty-eight 

 hundredths of an inch, (ten or twelve millimetres) in very ex- 

 tended storms. If the centre of the tornado moves, (which may 

 take place in any direction, when compared with the diametrical 

 line,) and the effects produced by the motion are examined, it is 

 always found that if the meteor has followed in its motion the line 

 of its greatest diameter, the tree which fell the first, indicates a 

 point anterior in the path of the meteor, and the tree which fell 

 last, a posterior point. Thus it is constantly found that the trees 

 which were overthrown with their tops turned towards positions 

 anterior to the centre of the tornado, are covered by trees falling 

 in the direction of the centre at a posterior period. In short, in 

 this same case, the branches of the trees not overthrown, growing 

 on the side farthest from the opposite side of the line which the 

 centre of the meteor takes, have followed the wind and are twisted 

 around the trunk of the trees. 



The circumstances favorable to the sudden production of a tor- 

 nado, large or small, are, according to Mr. Espy, a warm and hu- 

 mid atmosphere, covering a country sufficiently level and ex- 

 tended, still enough to allow that part of the air which is acci- 

 dentally the least dense, to rise to a great perpendicular height 

 above the middle of the heated space which is charged with trans- 



