140 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1918. 



center, by spiraling and ascending winds of terrific violence; strong 

 enough to crush and wreck the strongest buildings; ascending with 

 sufficient velocity to carry aloft objects so heavy that for wind to 

 lift them seems almost impossible. The surface winds wdiich take 

 part in the vorticular inflow and ascent seem to be chiefly responsible 

 for the damage and loss of life. There is, however, an additional 

 factor. The central " core," surrounded by its whirling winds, has its 

 pressure greatly reduced by the centrifugal force of the whirl. It 

 therefore exerts a powerful explosive effect upon near-by air at ordi- 

 nary pressures, within buildings or in other more or less well-inclosed 

 spaces. This curious but very widely-attested explosive effect ac- 

 counts for many tornado " freaks " which can not be explained by 

 any controls, either of radially or spirally inflowing winds, whatever 

 their velocity. 



The damage done by tornadoes may be roughly classified as fol- 

 lows : (1) That resulting from the violence of the surface w T inds, blow- 

 ing over buildings and other exposed objects, crushing them, dashing 

 them against each other, etc.; (2) that caused by the explosive 

 action; and (3) that resulting from the uprushing air movement 

 close around the central vortex. Carts, barn doors, cattle, iron chains, 

 human beings, are carried through the air, whirled aloft, and dashed 

 to the ground, or they may be dropped gently at considerable dis- 

 tances from the places where they are picked up. Iron bridges have 

 been removed from their foundations; beams are driven into the 

 ground ; nails are forced head first into boards ; cornstalks are driven 

 partly through doors; harness is stripped from horses; clothing is 

 torn from human beings and stripped into rags. The damage is 

 greater and extends farther-from the center on the right of the track 

 than on the left, for the wind velocities are greater on the right, as 

 in the " dangerous semicircle " on the right of the track of tropical 

 cyclones. 



The explosive effects are many and curious. The walls of build- 

 ings fall out, sometimes letting the roof collapse onto the founda- 

 tions ; or the roof may be blown off, leaving the walls standing. The 

 accompanying photograph (pi. 1) illustrates some of the damage 

 which was done by the St. Louis, Mo., tornado of May 27, 1896. 

 The surface of the ground may be swept clean, as if with a broom. 

 Articles may be blown out of houses and carried to great distances. 

 Empty bottles are uncorked; feathers plucked from barnyard 

 poultry; doors and windows blown out; soot rises from chimneys; 

 mud penetrates clothing. 



Property damage in the United States due to tornadoes varies 

 greatly from year to year, depending, as it does, upon the "acci- 

 dental" passage of tornadoes through well-populated or through 

 sparsely settled districts. In half an hour the St. Louis tornado 



