OF THE TORNADO OR WATER SPOUT. 307 



distance from the npmoving current, notwithstanding it is 

 in the midst of an npmoving current, which, it is generally 

 supposed, has a tendency to cause a fall in the barometer. 

 At the surface of the earth, ihe fall of the barometer, and an 

 upmoving current of air, are simultaneous effects of the 

 same cause, a diminution of weight of the whole column of 

 atmosphere to the top ; but the quantity of atmosphere over 

 a will be greater because it is swelled up over a to the 

 very top of the atmosphere, and of course runs outwards 

 in all directions from a. 



Again, if, instead of making holes in the receiver, we 

 remove the entire sides, it is manifest that the heated 

 column of air, which we suppose to be the same as before, 

 would spread out laterally in ascending, in the form of an 

 inverted cone, or mushroom, as exhibited by the dotted 

 lines p : 5, ra, e. And this spreading out would take place 

 sooner, and be more flaring, if heavy bodies were carried 

 up in the middle of the ascending column. But still, as 

 long as the barometer indicated a depression of one inch in 

 the middle of the base of the upmoving column, the velocity 

 with which the air would move upwards there would be 

 240 feet per second. 



When the air near the surface of the earth becomes very 

 much heated or very highly charged with aqueous vapor, 

 such an ascending column as is here imagined may actually 

 take place, and be kept up for a long time. The difference 

 of temperature of the ascending column and that of the at- 

 mosphere through which it passes may be much greater 

 than that here supposed, partly caused by its greater tem- 

 perature below ; but chiefly from the great quantity of 

 latent caloric evolved by the condensation of vapor into 

 cloud. 



It becomes then a matter of high interest to calculate the 

 shape of such an ascending current, and its specific gravity 

 when compared to that of the surrounding air through which 



