382 The Iowa and Illinois Tornado. (July, 
new paths than that on the surface when the friction is so 
important an element. Inthe forward march of the tornado 
the base would thus be somewhat left behind. The warm, 
moist air of the ravine would increase this tendency on ac- 
count of its specific levity. 
But the inrushing winds follow the path of least re- 
sistance toward the area of greatest rarefaction. The 
bending, therefore, cannot proceed far before the south-west 
winds, whose direction of motion is carrying them away 
_ from the base of the inclined funnel, will find less resistance 
and a shorter path towards the greatest rarefaction by striking 
up into the nearest point in the side of the funnel. 
A streak of rarefaction great enough to produce conden- 
sation is thus generated. This arm of the funnel affording 
the shortest way for the winds will increase with rapidity. 
The other will decrease, becoming mainly confined to the 
north-west winds. Meanwhile the intervening air will 
become rarefied and set in motion; the two arms will 
suddenly unite, and the funnel present again its original 
proportions. ‘These transformations must take place within 
a brief space of time, for the tornado is travelling at the 
rate of half a mile per minute. When we consider, in 
addition, the darkness, we shall be at no loss to conceive why 
these two branches should present the appearance of one 
funnel with its larger end down, and why to a less favourably 
situated or more superficial observer the whole should appear 
as one. 
One difficulty yet remains to be obviated. Since the arm 
moves forward ina spiral to the centre, and the velocity of 
the inrushing winds at the funnel far exceeds 100 miles per 
hour, how is it possible for the observer in the darkness to 
notice the appearances distinétly ? Mr. Marbourg gives the 
distance between the two arms at 200 feet. This space 
would be traversed in one second by winds of so great 
velocity, and the condensed vapour would fill the whole 
space before it had time to disappear. ‘The very velocity 
itself would render distinct vision difficult. ‘We must not, 
in short, confound the path of the winds with the winds 
themselves. While the winds were rushing up ina slanting 
and curving direction along these arms, the arms themselves 
might be moving forward with a much less rapid motion. 
If it were possible to determine the inclination of these 
arms to the perpendicular it would aid in calculating the 
force of the wind. Mr. Marbourg stated that the distance 
between the bases of the two arms was 200 feet, and the 
height, when they joined together, 50 feet. He gave 80 to 
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