42 W. Ferrel on motions of Fluids and Solids 
of pressure between one part and the surrounding parts, the 
equilibrium would be at once restored if the motions toward the 
centre produced by this difference of pressure, did not give rise 
to a cyclonic motion, and thus to a centrifugal force combined 
with the deflecting force arising from the earth’s rotation which 
in a great measure counteracts the force arising from a difference 
of pressure between the external and internal parts. If therefore 
the earth had no rotation, the oscillations of the barometer woul 
be no greater in any part of the earth than they are at the — 
equator. 
33. When the disturbance of equilibrium is great, but extends 
over a small area only, the centripetal force is much greater than 
in the case of large cyclones, and the gyrations are then ve 
rapid and very near the centre, as in the case of tornadoes. 
Tornadoes generally occur when the surface of the earth is very 
warm, and the atmosphere calm. For then the strata near the 
surface becomes very much rarefied, and are consequently in a | 
kind of unstable equilibrium for a while, when from some slight — 
cause, the rarefied atmosphere rushes up at some point through ~ 
the strata above, and consequently flows in rapidly from all sides © 
below, and then, unless the sum of all the initial moments of 
generally st 6 small in comparison with that of the initial state — 
centre of 
e 
° 
5B 
sistances. ce fe evclon 
while small tornadoes, depending principally upon the 
initial gyratory state for their violence, are soon overcome by 
the resistances. a ee 
. On account of the centrifugal force arising from the rapi 
gyrations near the centre of a tornado, it must frequently be 
nearly a vacuum. Hence, when a tornado passes over a build- 
