INTRODUCTION. 



ature of the ascending current becomes that of the dew point, 

 which itself depends evidently upon the degree of dampness of 

 the air. This theory further explains how, in the small tornadoes, 

 whose violence is remarkable, an expansion takes place in the 

 centre of the meteor, at a very small height, sufficient to condense 

 vapor by the cold and consequently to produce this kind of ap- 

 pendage which particularly distinguishes small tornadoes, or com- 

 mon water spouts. Let us add that the calculations of Mr. Espy, 

 upon the density of the warm column, its comparative levity, the 

 ascending force of the current, the central depression which is 

 the consequence of it, the rapidity with which the surrounding air 

 rushes towards the place where the pressure is diminished, finally 

 all the conclusions drawn from the physical data of the phenome- 

 na have been proved and ascertained with sufficient exactness to 

 leave no doubt as to this portion of Mr. Espy's theory. 



One word remains to be said relative to the progressive move- 

 ment of the meteor. This movement may depend upon an ordi- 

 nary wind, which, imparting a common motion to the whole atmo- 

 sphere, would not disturb the ascension of the column of moist 

 air. But as these phenomena are produced suddenly in the midst 

 of a great calm, Mr. Espy thinks that, in accordance with ob- 

 served facts, the motion of the meteor should be attributed to the 

 winds, which predominate in the upper part of the atmosphere, 

 and that in modern latitudes, this motion should thus take place 

 towards the east, whilst in the equatorial regions this motion should 

 be directed towards the west, as the current of the trade winds. In 

 a word, the slight surcharge which is owing to the spreading out of 

 the air, around the top of the meteor, accounts for the trifling 

 elevation of the barometer, which the invasion of the tornado, in 

 every place presents, and can even, according to Mr. Espy, serve 

 as a prognostic of it. 1 Another result is, that beyond the limits of 

 the meteor, a feeble wind ought to be observed, as is the case, 

 whose direction is opposite to that of the air which is violently 

 rushing towards the centre of the tornado. 



The consequences which Mr. Espy deduces from this theory, 

 are, that in many localities, in Jamaica, for example, the sea 

 breezes cause a motion of the air perfectly analogous to that which 

 constitutes a tornado, and that the results of it are the same, name- 

 ly, rain and tempest at stated hours, on each day of summer. The 

 same circumstances produce the same effects in other well known 

 localities, volcanic eruptions, great conflagrations of forests, with 

 the favorable circumstances of tranquillity, heat, and moisture, 

 ought also to produce ascending currents and rain. Tn the midst 

 of all the theoretical deductions of Mr. Espy, it should be remark- 



1 The reader will recollect (hat in the " Report," tornado is used to signify 

 both large and small storms. 



