THEORY CONFIRMED BY PHENOMENA. 47 



vortex will go on increasing in size and perpendicular height, 

 until the air immediately below it, being pressed downwards 

 with less and less weight as the cloud above increases in 

 height and levity, will, by expanding more than the air 

 which preceded it in the vortex, be cooled down to the point 

 of deposition, before it reaches the elevation of one thousand 

 yards. And if, in this case, the column should rise to a 

 height sufficient to produce a diminution of pressure under 

 it of one pound to the square inch, the cone of visible vapor, 

 or cloud, will reach down to a point four hundred yards 

 from the earth's surface. And, in general, the nearer the 

 dew point is to the temperature of the air, the lower will 

 the visible spout descend ; so that, if they had been assumed 

 only six degrees apart, in the above case, the apex of the 

 spout would have descended to the earth. And, if they had 

 been assumed still nearer together, the spout would not only 

 have descended to the earth, but it would have been of some 

 considerable size there. Thus we find that this mode of 

 calculation not only enables us to account for the more fre- 

 quent appearance of these spouts in the day than in the 

 night, but also to assign a reasonable, hypothetical cause, 

 why these spouts, or storms, are sometimes broad, and some- 

 times narrow, and sometimes even do not reach down to the 

 surface of the earth. 



78. It is known, also, that spouts, and violent storms, are 

 always preceded by calms. The fact, also, is easily ex- 

 plained by the theory. For, in the first place, it is known 

 that a calm favors the production of a high dew point, which 

 is an essential ingredient in these storms ; and, second, a 

 vortex of great strength cannot be formed, unless it can rise 

 nearly perpendicular to a great elevation, which never can 

 happen where there is a strong wind. This will readily be 

 admitted, when it is considered that the wind is always 

 stronger at some distance above the surface of the earth, 

 than at the surface itself; and, therefore, no vortex of any 



