392 The Iowa and Illinois Tornado. (July, 
initial pressure was 28°5, ie = the pressure when the 
dew point is reached and the funnel commences = 15°69. 
This gives a diminution of 12°81 inches, corresponding to a 
pressure of about 6°40 lbs. per square inch, or 922 lbs. per 
square foot,—an amount of potential energy amply sufficient 
to account for all the phenomena. 
The form and dimensions of the visible funnel did not 
therefore depend upon the diminution of pressure alone, but 
also upon the temperature and relative humidity of the in- 
rushing winds. Its presence accordingly does not indicate 
a certain invariable velocity of the wind. When the air is 
moist, as it always Is over the sea, a very small diminution 
of pressure, and consequently a comparatively light wind, 
would suffice to develop the phenomenon of a visible funnel. 
When, on the contrary, the air is very dry, winds of the ut- 
most possible violence could be produced without a visible 
funnel. The funnel of dark cloud, therefore, by no means 
indicates the limits of the ascending current. This renders 
the explanation of the hovering up and down of the lower 
end of the funnel easy of comprehension. Varying humidity 
along the path of the tornado would be sufficient to produce 
this phenomenon. 
As the air ascends the pressure diminishes, and the funnel 
consequently increases in width. The diminution of pressure 
perpendicularly is symmetrical, and the result is the sym- 
metry of the form of the funnel. 
The Sound.—Its loudness was extreme, yet of such a 
nature as to drown all other sounds without stunning. A 
man could stand by his house, as it was shivered to pieces, 
and not hear the noise of its breaking. 
B. Whitaker heard it, together with others, at the dis- 
tance of 60 miles. He says that even at that distance it 
indicated great force. The barometric gradient inclined 
from Mr. Whitaker to the tornado ; consequently the propa- 
gation of the sound received no help from the wind. Thunder 
has very seldom been heard at a greater distance than 
to miles. The sound of the tornado was therefore thirty-six 
times louder than the loudest thunder. Heavy cannonading 
has been heard, it is said, at the distance of go miles, and 
“the report of a volcano at St. Vincent was heard at Deme- 
rara, 300 miles off.” The voice of the tornado is, therefore, 
one of the strongest known to art or nature. 
The Tornado in Illinois—The general appearance of the 
tornado, as a whole, in Illinois, was somewhat different from 
that which it presented in Iowa. ‘The ascending column of 
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