THEORY DEDUCED FROM PHYSICAL LAWS. 7 



umn being relieved from a pressure equal to an elevation 

 of more than ten hundred yards, will fall in temperature 

 more than 10, and of course the cloud will reach the earth, 

 unless the temperature of the air should be about 10 above 

 the dew point, in which case it will reach very near to the 

 earth. 



19. In this case there will be a spout, and the air below 

 the cloud reaching to the surface of the earth, the trees will 

 be thrown inwards, and also forwards, if the spout has a 

 motion along the surface of the earth. 



20. The spout must have a motion on the surface of the 

 earth, if there is a current of air at the upper end of the 

 column, for this current will move the upper end of the 

 column in its own direction, and the lower end will imme- 

 diately advance with it. And as it is known that the upper- 

 most stratum of air in which clouds appear, moves con- 

 stantly at Philadelphia, and probably throughout the north- 

 ern temperate zone, from a point a little south of west ; and 

 as it is certain that the upper end of the spout reaches far 

 into this stratum, the motion of the spouts in this climate 

 should be generally in this direction, or to a point a little 

 north of east. Indeed, they will always move in this direc- 

 tion unless they meet with a middle stratum of air moving 

 in a different direction. 



21. The narrower the spout is, the more readily will the 

 air at the upper end be able to spread out and leave the 

 column below free from the pressure of accumulation, and 

 the more violent will be its effects at the surface of the 

 earth. 



22. If the dew point should be much below the tempera- 

 ture of the air, the visible spout or cloud will not reach the 

 surface of the earth, and then the rain falling through the 

 lower atmosphere may, partly by its impulse and partly by 

 its cooling influence, (thus increasing its specific gravity) 

 cause the air at the surface of the earth to move outwards 



