388 The Iowa ae Illinois Tornado. (July, 
From the calculations of the velocity of the wind it is 
not probable that, when we assume—at the distance of 
roo yards from the centre—an average velocity of 120 miles 
per hour, we overrate it. The angle which the wind made 
with the tangent was probably about 25°. The wind, there- 
fore, on these bases enters a cylinder of 100 yards radius 
at the rate of 50 miles per hour, or 4400 feet per minute. 
From the calculations of the height of the centripetal 
current we have seen that it probably extended over 
2 miles. Let us assume that its height was 2 miles. 
Let us assume, also, that the velocity with which the wind 
entered the cylinder was equally great throughout its length. 
We have then the following data for calculating the quantity 
of air which enters this cylinder of too yards radius, or 
628,318 yards circumference per minute. It amounts to 
(628,318 x 3) X 4400 x (2 X 1760 X 3) c. ft. = 87,582,502,656,000 
cubic feet. Since this volume is calculated from the me- 
chanical effects or force of the wind, and since the relation 
between the force and the velocity of the wind is calculated 
for the average pressure, it matters not, in calculating the 
mass of the above volume of air, what the barometric 
pressure was at the distance of 100 yards from the centre. 
The difference of pressure due to elevation must, however, 
be calculated. Since the average pressure at sea-level is 
about 30 inches, at an elevation of 1 mile about 24°60 inches, 
and at an elevation of 2 miles about 20°20 inches, and taking 
into account the correction due to the elevation of the 
country over which the tornado passed, 24 inches would be 
about the mean pressure up to 2 miles of elevation. The 
density of the air varies with the temperature. The tem- 
perature around the tornado must have been, at a fair 
exposure, about 76°, but was in deep wooded ravines no 
doubt much greater: since the temperature diminishes 1 for 
every 300 or 400 feet of elevation, we may assume the 
average temperature to have been, within the limits of the 
centripetal current, about 60°. The relative humidity was 
about 65 per cent. 
The weight of a cubic foot of air, at a pressure of 
24 inches, a temperature of 60°, and a relative humidity of 
65 per cent, is 426 grs., or thereabout. The total weight of 
air, therefore, entering the column per minute amounts to— 
87,582,502,056,000 X 426 _ 
7000 
This weight of air moves, by hypothesis, at the rate of 
120 miles per hour, or of 176 feet per second. This velocity 
is acquired from the acceleration due to gravity in falling 
5330,020,875,922 lbs. 
