1874.] The Iowa and Illinois Tornado. 391 
pressure immediately expands and blows the house asunder, 
as with gunpowder. There was not, so far as I was able to 
learn, a single case in which a house was so destroyed. 
The houses were invariably demolished or blown from their 
foundations by a wind blowing in a certain direction. They 
were generally pushed or carried a considerable distance 
bodily, floor and all. After striking the ground with the 
foremost edge they turned partially over, thus exposing the 
floor to the wind. The wind, being obstru¢ted by the wall 
of the house resting on the ground, became immediately 
compressed by the pressure from behind. The floor, unable 
to stand this pressure, went upwards, and the whole interior 
of the building became subjected to a strong pressure due to 
the compression of the air. This pressure was more than 
balanced on the side struck by the wind. On the other sides 
there was nothing to resist it, and the house was accordingly 
burst asunder. 
We have therefore ample proof of explosion by com- 
pression, but none whatever of explosion by rarefaction. 
The temperature at the centre is easily calculated. We 
have assumed, from the data, an average temperature of 76° 
on the surface of the ground, and a relative humidity of 
65 percent. This gives a dew point of 63°, or thereabout. 
This sudden decrease of 13°, together with the wet, is suffi- 
cient to account for the coolness experienced. 
The lowering of the temperature at the centre of the tor- 
nado is due to the work performed by the air in expanding 
under diminishing pressure. The amount of this expansion, 
and consequently of the diminution of pressure, is easily 
obtained from the equation poe = a (“* Zeuner’s 
I 
Grundzuge,” p. 131), when a = coefficient of expansion 
= 491, t = initial temperature — 32, ¢, = final temper- 
ature — 32, v = initial volume, v, = final volume, and k 
= the relation between the thermal capacities of the air 
with constant volume, and with constant pressure = 1°41. 
The initial temperature in the case of the tornado = 76°, and 
the final temperature 63°. Consequently, t=76—32=44, 
and ¢,=63—32=31. Letustakev=1. Then— 
SH (2), or 535 — v4, or 1,= (339) 41 = 
491+31 I 522 
= Oa ke 
A cubic foot of air therefore expands, in passing from the 
circumference to the centre, to 1°8219 cubic feet. But the 
pressure is inversely as the volume. Therefore, since the 
