302 The National Geographic Magazine 



still retains the cold of winter. This 

 abnormality is accentuated in the south- 

 east quadrant of the cyclone, wherein 

 southerly winds still further add to the 

 heat of the lower stratum and increase 

 the humidity. An unstable condition 

 then ensues, in which heavier air is 

 superposed on lighter and much warmer 

 air. This unstable equilibrium is more 

 often relieved by the breaking through, 

 here and there, of masses of the heavier 

 air and its horizontal rolling along the 

 surface of the earth, with the warm and 

 cold masses driven together by electric 

 explosions ; these are thunder-storms. 

 But at times dry and extremely cold air 

 are brought together with humid and 

 very warm air, with the result that a 

 narrow vertical whirl is set up which 

 develops great vortical energy ; this is 

 the tornado. The tornado also may be 

 caused, and many times is, by the cy- 

 clone whirling together on the same 

 level the cold currents from the north- 

 west and the warm ones from the south- 

 east, especially at an elevation of a few 

 thousand feet, in the interior layers of 

 the cyclone. The vortex then burrows 

 downward to the surface of the earth, 

 or dances along with the tail of the 

 funnel whipping from side to side, and 

 touching only the high places or noth- 

 ing at all. 



Tornadoes mostly occur between 2 and 

 5 in the afternoon, and generally move 

 from the southwest to the northeast ; 

 their tracks may vary in width from a few 

 hundred feet to one mile ; their velocity 

 of translation is usually about that of 

 an express train ; their speed of gyration 

 can be measured only approximately, 

 but as it is sufficient often to drive straws 

 a half inch into the bodies of trees it 

 must equal or exceed the velocity of a 

 rifle bullet. Professor Biglow says : 

 "The vortex of a tornado obeys the 

 laws of the movements of fluids in gyra- 

 tory circulation. If a mass of air 6,000 

 feet in diameter is rotating at a half- 

 mile level and it runs into a vortex so 



that the tube is 100 feet in diameter at 

 the top, and supposing the outer edge 

 of the upper vortex makes 7 miles 

 an hour, then at the rim of the bottom 

 of the vortex we should have a velocity 

 of 200 miles an hour. This causes an 

 enormous centrifugal force in the lower 

 tube, a high vacuum, and low temper- 

 ature. The vacuum tube causes the 

 explosive and disastrous effects upon 

 objects in its path." The cold of ex- 

 pansion condenses the vapor that makes 

 the tube visible, and the sudden con- 

 densation causes electric discharges of 

 great violence. A roaring like the 

 sound of a thousand express trains ac- 

 companies the tornado, whose track is 

 usually 5 or 10 miles in length, and 

 whose rate of movement is about 30 

 miles per hour. 



The writer visited St Louis the day 

 after the tornado of May 27, 1896. He 

 was especially impressed with the fact 

 that many of the buildings seemed to be 

 burst outward at their upper stories, 

 indicating that at the time of their de- 

 struction they were near the center of 

 the rotating mass of air, where centrifu- 

 gal force had reduced the air pressure 

 on the outside to such an extent that 

 the expansion of the air in the upper 

 stories of the houses whose windows 

 and doors were closed had produced an 

 explosion of the building. In one case 

 all the four walls of the upper story of 

 a house were thrown outward, leaving 

 the lower story intact and the roof rest- 

 ing in proper position one story lower 

 than in the original building. Again, 

 great structures seemed to have been 

 crushed over or taken up bodily and 

 scattered in all directions. 



The fact that this tornado traveled 

 with destructive force through several 

 miles of brick buildings and yet left the 

 city with greater force than it possessed 

 on entering it illustrates the futility of 

 planting forests to the southwest of 

 cities for purposes of protection, as 

 some have advocated. The strongest 



