SHORT MEMOIES ON METEOROLOGICAL SUBJECTS. 443 



barometric variations nowhere be greater than they at present are at 

 the equator. 



When the disturbance of equilibrium is large, but only extends over 

 a small area, then is the centrifugal force greater than in the case of 

 extended cyclones. The rotations are then very rapid and close to the 

 center, as in tornadoes. These occur at times of great heat and calms, 

 Trhereby the equilibrium in vertical direction is disturbed. The lower 

 strata rise upward, a rapid inflow takes place from all sides; and when 

 the sum of all the initial moments of rotation is not equal to zero, which 

 will seldom be the case, then must the mass of air near the center exe- 

 cute rapid rotations, and a tornado is the result. We can well represent 

 this by the flow of water out of a basin through a hole in the bottom. 

 When, at the beginning of the motion, the fluid is in perfect rest, the 

 efiiux takes place without rotation; but when the slightest initial rota- 

 tion is present, then the outflow is accompanied by rapid rotations. 



In tornadoes, which are always of small dimensions, the influence of 

 the earth's rotation upon the production of rotation is very slight in 

 comparison with the influence of the initial condition of the atmosphere. 

 Therefore the direction of the rotatory movement in a tornado is some- 

 times from right to left, sometimes the reverse. Therefore at the equa- 

 tor tornadoes can occur, but no great cyclones. In these latter, the initial 

 condition of the atmosphere is of little consequence in comparison to 

 the influence of the earth's rotation, and this latter is constant, while 

 the former is easily nullified by resistances. Therefore the great cyclones 

 have a long existence, while the tornadoes, depending principally on the 

 initial intensity of their rotation, are soon brought to rest by the resist- 

 ances. 



In consequence of the great centrifugal force near the center of a tor- 

 nado, caused by the rapid rotation, almost a vacuum must exist there. 

 When, therefore, a tornado passes over a building, the exterior pressure 

 is suddenly removed, and the air within presses with nearly 10,333 kilo- 

 grams per square meter; hence is produced a destruction as if by an 

 explosion. 



The progressive motion of cyclones can be explained by the inequality 

 of the centrifugal forces on the polar and equatorial sides of a cyclone. 

 The term of the gradient depending on 2 n sin ^ is greater on the 

 polar than on the equatorial side, while the other moments remain the 

 same. The cyclone, therefore, moves toward the direction of the greater 

 diminution of pressure or toward higher latitudes. It is therefore not 

 necessary to assume that a real transfer takes place from the equator to 

 the pole of the mass of air that forms the cyclone. The deviating force 

 and the motions are greater on the polar side of the cyclone, and on this 

 side new portions of the atmosphere are continually drawn into the 

 movement, since on this side n sin <p is increasing steadily, while on the 

 equatorial side the motion ceases by reason of the frictional resistance 

 and the inertia of the air. Thus the center of the cyclone is continually 

 being formed anew during the progress toward higher latitudes. 



