THEORY CONFIRMED BY PHENOMENA. 45 



surface of the earth, or sea, and the air would have to ascend 

 a little more than three hundred yards, before condensation 

 would commence. I say more than three hundred, because, 

 though the temperature sinks one degree for every hundred 

 yards of elevation, the dew point also sinks a little from the 

 expansion of the air, and the same quantity of vapor occu- 

 pying a larger space. But, if the dew point in the above 

 case had been only six degrees below the temperature of the 

 air, then the spout, or visible cone of vapor, would have 

 reached the earth. 



Now, it is highly probable that a spout, in passing over 

 the surface of the earth, would meet with slight variations 

 in the dew point, and, if so, it would rise as the dew point 

 fell, and fall as the dew point rose ; and thus the theoretical 

 deductions correspond exactly with the facts. 



75. Again, the direction of the two spouts, as also of the 

 great storm with two veins of hail, mentioned before, was 

 from the south west to the north east; and Pouillet says, 

 that a large majority of these storms are known to move in 

 this direction. I presume he means those which occur in 

 France. Now, it is manifest that these storms, according to 

 the theory, must move in the direction of the upper current 

 into which they may ascend, for the top of the vortex will 

 lean in that direction ; and as theory demonstrates, and 

 observation agrees with that demonstration, that the upper- 

 most current of air in the temperate zone moves constantly 

 from the south of west, towards the north of east, this will 

 satisfactorily account for the general tendency of these 

 storms in that direction, all over the northern temperate 

 zone, or, at least above lat. 30. For, from that latitude, 

 down to the tropic of Cancer, the uppermost current of 

 air moves nearly towards the north, and, within the tropic, 

 it moves towards the north west; and so the theory 

 would lead us to presume that, in these regions, the 

 storms will be found to move in these directions. Such 



