346 Prof. J. P. Espy on the Law of Storms. 



In great storms, the wind, on both sides of the line of mini- 

 mum pressure, blows towards that line directly or obliquely. 



I have examined also Redtield's, Reid's, Piddington's, and 

 Thorn's storms in the following manner : — In reading the account 

 of a storm, I spread the chart on which was laid down the posi- 

 tion of the ships in the storm, and drew arrows showing the di- 

 rection of the wind at a particular moment, and I found that all 

 the arrows of the ships within the violence of the hurricane 

 pointed inwards to a central space of no great magnitude. 



So of tornadoes : they have been examined as they pass through 

 forests and cities, by Bache, Henry, Walter Johnson, Loomis, 

 Olmsted, Prof. Eustice and the Rev. ^Ir. Brooks ; and they all de- 

 clare, without a dissenting voice, that the air blew inwards, and 

 threw down the trees at the sides perpendicularly towards a cen- 

 tral line in which the tornado moved, occasionally throwing 

 bodies backwards by the front of the tornado, and then forwards 

 by the rear. 



The inward motion below, indeed, might be inferred from the 

 upward motion in the middle, which was so violent as to caiTy 

 up all kinds of materials from the surface of the earth, and let 

 them fall many miles from where they were taken up, along with 

 a severe shower of hail, imbedding in it sand and leaves, and 

 limbs of trees covered with a thick coating of ice. 



In storms then the air comes inwards on all sides towards the 

 centre. 

 Therefore, It goes upwards. 



It comes under less pressure. 



It expands. 



It growscolder about one degree for 100 yardsof ascent. 



It begins to condense its vapour at a certain height*. 



It liberates the caloric of the condensing vapour. 



It then cools only half as fast as in going up, as dry 

 air would. 



When vapour enough is condensed in the cloud to make one 

 inch of rain, sufficient latent caloric will be evolved to hinder the 

 air from cooling, so much that the whole column of air contain- 

 ing the cloud, from the surface of the earth to the top of the 

 atmosphere, if the heat was equally distributed, would be 10° 

 warmer than the surrounding air. 



The column then containing the cloud would be j-^jj hghter 

 than the surrounding air (the mean temperature of the air being 

 zero). The air then would run in at the base of the light co- 

 lumn, and push it up with great velocity, and carry in its vapour 

 with it, which, on going up, would condense as before, give out 



* At 1000 yards if the dew-point is 10° below the temperature of the air, 

 and at 1500 yards if the dew-point is 15° below the temperatui-e of the air. 



