156 On the Causes of the Tornado, or Water Spout, 



gravity ; it is in the next place only necessary to advert to facts per- 

 fectly virell known, in order to point out a cause of acceleration sufFi= 

 cient to account for the well known violence of the tornado. 



At the height of fifteen miles, the air has been ascertained to have 

 less than one thirtieth of the density of the stratum next the earth. 

 Of course this substratum would exercise a force nearly equal to the 

 atmospheric pressure, or about fourteen and a half pounds to the 

 square inch, in order to attain the space occupied by the rare me- 

 dium, to which allusion has been made. It follows that if the weight 

 of the superincumbent air were removed or counteracted, that the 

 inferior stratum would rise with explosive violence. 



While the air is thus carried upwards by the concurrent influence 

 of electrical attraction, and the reaction of its own previously con- 

 strained elasticity, other bodies are lifted, both by electrical attrac- 

 tion, and the blast of air to which it gives rise. Hence houses within 

 the sphere of the excitement are burst by the expansion of the air 

 which they contain, their walls being thrown outwards, and their 

 roofs carried away ; while, by the afflux of the atmosphere requisite 

 to the restoration of its equilibrium, trees, houses, and other bodies 

 are thrown inwards towards the vertical current, from before, as well 

 as from either side. 



When once a vertical current is established, and a vortex produ- 

 ced, I conceive that it may continue after the exciting cause may 

 fiave ceased to act. The eiFect of a vortex in protecting the space 

 about which it is formed, from the pressure of the fluid in which it 

 has been induced, must be familiar to every observer. In fact, 

 Franklin ascribed the water spout to a whirlwind produced by the 

 concourse of the atmosphere to a given point. His hypothesis was, 

 as I conceive, unsatisfactory, because it did not assign any adequate 

 cause for the concentration of the wind, or for the hiatus which was 

 presumed to be the cause. This deficiency is supplied, if my sug- 

 gestions be correct. 



One fact, of which I am myself a witness, cannot be explained 

 without supposing a gyratory force. About six feet of a brick chim- 

 ney, without being thrown down, were so twisted on the remaining 

 inferior portion, as to be left with its corners projecting. 



I have hardly deemed it necessary to advert to the cause of the 

 progressive motion of a. tornado, since that would appear evidently 

 due to the current of the atmosphere within which it may be created. 



