XXXIV INTRODUCTION. 



parent vapor; moreover, in the higher regions, a cold and dry air, 

 whose situation and especially whose density contrasts with that 

 of the ascending current which dilates, cools, loses its transpar- 

 ency by the precipitation of its dampness, keeping notwithstanding 

 a specific gravity less than that of the air which surrounds it, and 

 by its expansion, presenting the form of a mushroom or the head 

 of a pine with or without the prolongation or appendage towards 

 the base, which appendage, cloudy and opaque, shows a space 

 where the expansion and the cold are at their maximum, and 

 where, consequently, the precipitation of vapor commences almost 

 immediately above the ground or the surface of the sea. 



Such are then the principal points which Mr. Espy has obtained 

 from numerous observations. The motion of the air towards the 

 centre of the meteor, the depression of the barometer in the cen- 

 tre, the centra! ascending current, the formation of cloud at a cer- 

 tain height, and its circular expansion after this cloud has attained 

 a prodigious height, an expansion accompanied with rain and hail, 

 and finally, the motion of the whole meteor, en masse ; these, I 

 say, are the points which the extensive labors of Mr. Espy, his 

 own observations, and the documents which he has collected, and 

 which he intends publishing immediately in a special work, have 

 placed beyond doubt, and which seem even to have triumphed 

 over every objection, and to have rallied all opinions to his own. 



Let us now see the theory upon which he bases his observa- 

 tions, or rather which is based upon these facts well observed, 

 well proven, and always reproduced in nature with similar circum- 

 stances. 



Mr. Espy thinks that if a very extended stratum of warm and 

 humid air at rest, covers the surface of a region of land or sea, 

 and that by any cause whatever, for example a less local density, 

 an ascending current is formed in this mass of humid air, the as- 

 cending force, instead of diminishing in consequence of the ele- 

 vation of the rising column, will increase with the height of the 

 column, exactly as though a current of hydrogen was rising 

 through the common air, which current would be pushed towards 

 the top of the atmosphere, with a force and velocity in proportion 

 to its height. This column of heated air may also be compared 

 to that in chimneys and stove-pipes, of which the draught is in 

 proportion to the height of the pipe containing the warm air. 

 What then is the cause which renders the warm and humid as- 

 cending current, lighter in each of its parts, than the air which is 

 found at the same height with these different portions of the as- 

 cending column ? 



This cause, according to the sufficiently exact calculations [tres 

 suffisament exact] of Mr. Espy, is the constantly higher tempera- 

 ture which the ascending column retains, and which proceeds from 

 the heat furnished by the partial condensation of the vapor mixed 



