336 The Iowa and Illinois Tornado. (July, 
z. A house, exposing 280 square feet to the wind, having 
a floor of 480 square feet, and weighing 20 tons, was 
pushed 6 feet from its foundations. It was then 
lifted over the tops of trees 20 feet high. 
. A house, presenting to the wind a surface of 176 square 
feet without the roof, and weighing Io tons, was car- 
ried 24 yards while falling 3 feet. The wind struck 
it nearly on the end. 
4. A granary, exposing 128 square feet of surface, and 
weighing 60,000 lbs., was pushed 14 feet. 
5. A house, exposing a surface of 392 square feet, + the 
slanting roof, and weighing 20 tons, was carried for- 
ward 18 feet while falling 2 feet. 
6. A house, exposing 533 square feet, and weighing about 
25 tons, was pushed from its foundations. 
. A granary, exposing a surface of 256 square feet to the 
wind, and weighing 55,000 lbs., was carried 21 yards 
while falling 6 feet. 
SS) 
“NJ 
If we estimate the static friction of a frame house upon 
a stone foundation, or upon posts firmly fixed into the 
ground, and to which it is nailed, at one-half the weight of 
the house, it does not seem likely that we shall err by excess. 
Upon this basis it is easy to calculate the minimum velocity 
of a horizontai wind necessary to push it from its founda- 
tions. The difficulty is that we are dealing with a wind 
having a perpendicular component to its velocity. If the 
wall of the house was perfectly smooth, and since the air is 
nearly perfectly elastic, the only force exerted upon it by awind 
blowing at any angle to its surface would be perpendicular 
to that surface. Moreover, whatever force the perpendicular 
component may exercise upon the wall, owing to its rough- 
ness, must have upon the house the effect of an overturning 
rather than a lifting power. But, since friction is indepen- 
dent of extent of surface, this overturning power would not 
assist in overcoming friction. The fact, also, that the 
houses, while being carried bodily through the air, were not 
made to spin round on an axis by this perpendicular com- 
ponent,—there being then no resistance, beyond inertion, to 
motion in a circle,—would seem to prove that its effect was 
not great. It would therefore appear that an approximation 
may be made to the velocity of the horizontal wind, by 
simply calculating—upon the given assumption of the rela- 
tion of weight to friction—the minimum force required to 
push these houses from their foundations. The following 
are the results for the given cases :— 
