378 The Iowa and Illinois Tornado. (July, 
motion of the upper currents of the heavy masses of clouds 
they bear. There appears to be nothing in the nature of the 
tornado itself which can determine the motion of either the 
upper or under current more towards any one point of the 
compass than toward the others. This direction of motion 
relative to the ascending column depends upon the dire¢tion 
and velocity of motion of the latter and of the atmospheric 
strata in which the influx and efflux take place, modified to 
some extent by the differing velocities of revolution of the 
surface of the earth at different parallels of latitude, by the 
form of the earth’s surface, and by the variation in the con- 
stitution of the atmosphere. If the tornado column, and the 
atmospheric strata which it penetrates, move in the same 
direction and with the same velocity, the influx and efflux 
will take place in nearly equal quantity on all sides of the 
column. Ifthey move with different velocities the dire¢tions 
of exaggeration and diminution of the influx and efflux can be 
calculated in the same way as the direction of a wind-vane 
on a ship’s mast, given the directions and velocities of the 
motion of the wind and of the ship. 
General Idea of the Tornado of May 22, 1873, in Iowa.—A 
huge, dark cloud covered an area at least 30 miles in 
diameter. Under the south-west edge of this cloud there 
moved a perfectly opaque funnel-shaped appearance, reach- 
ing from the ground to the clouds. Towards its base the 
wind, in spirals, rushed violently from all sides, overthrow- 
ing, when in immediate proximity to or within the opaque 
vortex, whatever opposed its progress. Towardsits summit, 
where it disappeared in overhanging horizontal cloud, long 
streaks of clouds rushed in spirals from all directions. 
Viewed from a distance they appeared to come from opposite 
directions, and move swiftly towards each other at right 
angle to the observer’s line of vision. Between the surface 
current and these centripetal clouds the air doubtless obeyed 
the same forces, and rushed in spirals with ever-increasing 
velocity towards the opaque funnel. 
Under the remainder of the lofty cloud which defined the 
limits within which the outspreading of the ascending air 
was taking place, and which lay chiefly to the N.E. of the 
rising column, there raged a tremendous storm of hail and 
rain, accompanied by incessant and brilliant electrical 
phenomena. 
More Particular Description, Dimensions of the Meteor, and 
of the Centrifugal or Upper Current.—The data for deter- 
mining the horizontal extent of the cloud are not very pre- 
cise. There is a general concurrence of the witnesses that 
