462 PHILOSOPHY OF STORMS. 



direction, and, as nearly as can be estimated, with the same 

 velocity as the air near the surface of the ground. Indeed, 

 I have never, in a single instance, observed clouds remain- 

 ing stationary during a very heavy rain, when the wind 

 underneath was blowing strongly. Besides, the edges of 

 clouds are frequently of such a ragged and marked charac- 

 ter, and retain the same distinctive configuration for such a 

 length of time, that if there was any rapid upward vortex 

 of air underneath, such as is stated to be the case in your 

 reports, the upward movement of the edges of the cloud 

 would have been long since observed, and universally ad- 

 mitted ; whereas, though horizontal movements of clouds 

 during rain are constantly observed when there is any 

 wind, an upward movement from underneath never has 

 been noticed, and is not visible, so far as I am aware. 1 



1 In our great north east storms, when they approach Philadelphia from the 

 south west, within three or four hundred miles, the wind begins to blow 

 from the north east, and at the same time the top of the cloud from the storm 

 generally makes its appearance, coming from the south west ; and those two 

 currents in opposite directions, continue for several hours before many clouds 

 form below and even when they do begin to form below, by the gradual 

 sloping of the air upwards, the upper clouds are still seen through the 

 openings, coming and thickening from the south west. 



It is true, indeed, when the storm of rain or snow comes on, the upper 

 clouds are concealed entirely from view, and the lower clouds, being in the 

 under current, are seen moving in the same direction with the wind. 



Moreover, in violent summer thunder showers, which are sometimes only 

 ten or twelve miles wide, I have frequently seen the clouds in the lower part 

 move with great rapidity towards the centre of the cloud from all sides, and 

 before I knew of the upward motion of the air in the middle, I have stood 

 looking on with amazement, at not seeing them overlap, but seem to lose 

 themselves in the centre, and others succeed in their turn. But when I cal- 

 culated the effect produced by the evolution of the caloric of elasticity which 

 is given out during the formation of cloud, and found that the volume of 

 air in which the cloud was formed would be increased about six times as much 

 by receiving this caloric, as it would be diminished by the condensation of 

 the vapor into water, the mystery was immediately explained. And I think 

 the reader will find, that this single principle, in connexion with other laws, 

 previously well known, will leave but few mysteries in meteorology, except 

 the luminous meteors, unexplained. 



