OF THE TORNADO OR WATER SPOUT. 305 



the whole, or one-twentieth of the whole bulk. Now as the 

 air on the outside of the receiver, from the surface of the 

 earth to the top of the receiver, weighs 20 inches of mer- 

 cury, the air on the inside will weigh only 19 inches of 

 mercury, and of course it will be pressed upwards against 

 the upper end of the receiver at a, with a force of about 

 half a pound on the square inch, the bottom of the receiver 

 being open at c. 



Also, if a barometer should be placed in the inside of the 

 receiver at the top a, it would stand an inch higher than 

 one on the outside at the same height ; therefore if a small 

 hole should be made in the top of the receiver the air 

 would spout out with a velocity due to a head of pressure 

 equal to one inch of mercury. This is equal in weight to 

 about 900 feet of air of mean density at the earth's surface. 

 The velocity with which it would spout on supposition of 

 its having this density will be found by the common for- 

 mula for spouting fluids Sy'yOO ' = 240 feet per second. 

 But the air at the top of the receiver is only about one third 

 the density of that at the surface of the earth, provided no 

 allowance is made for temperature ; and as the velocity of 

 spouting fluids under equal pressure is inversely as the 

 square root of their densities, the real velocity with which 

 the air will spout out at the top of the receiver will be 

 240X^=415 feet per second. 



If now we suppose the whole top of the receiver to be 

 taken off, the velocity will be the same, if there is no fric- 

 tion up the sides of the receiver, and the air gets freely in 

 at the bottom, an allowance of course being made for the 

 reaction of the air in the upper part of the receiver on the 

 air below, in consequence of the velocity increasing all the 

 way up. 



This is a case, however, which could not occur, for there 

 would be friction, and the present state of science does not 

 enable us to estimate its amount. 



39 



