118 MISSOURI STATE HORTICULTURAL SOCIETY. 



of the air around this center, produced by the rotation of the earth 

 around its axis, especially in high latitudes toward the poles. In con- 

 sequence of this rotation, every limited area of the earth's surface has 

 a gyratory motion around its center, which is greatest at the poles, and 

 vanishes at the equator. If a large shallow basin with water in it has 

 a gyrating motion around its center, and the water is allowed to run 

 through a hole at the center, so that there is a gradual motion from all 

 sides toward this center, it is well understood that the water will run 

 into a gyration around this center. So it is in the case of air, where 

 it is drawn in from all sides toward a central part. Such a gyration 

 over a large area of the earth's surface, say 500 or 1,000 miles in diame- 

 ter, is called a cyclone. In a cyclone the centrifugal force of the gyra- 

 tions drives the air away a little from the central part, so that the air 

 pressure there, as indicated by the barometer, is less. In consequence 

 of the ascending current in the inner part of the cyclone, this is always 

 more or less a region of cloud and rain. Most of our large rain-storms 

 over a considerable area of country are connected with cyclones of 

 greater or less gyratory violence ; and so where there is rain the barome- 

 ter generally stand lower. The falling of the barometer, therefore, is 

 generally a good indication of coming rain. 



TORNADOES. 



The local smaller disturbances, called tornadoes, are similar to 

 cyclones, though small in comparison, and arise from a peculiar state 

 of the atmosphere called the unstable state. In this state the lower 

 strata of the atmosphere become so warm in comparison with the upper 

 ones that whenever there is the least initial upward start given to the 

 air, there is a tendency in the air of the lower strata to burst up through 

 those above. The air that flows in from all sides toward the center to 

 supply the ascending current, generally runs into gyration of great vio- 

 lence around this center. This may also be illustrated by means of a 

 basin of water in which the water runs through a hole in the bottom at 

 the center. In these disturbances of limited area the earth's rotation 

 has no sensible influence. We must, therefore, in this case, suppose 

 the basin to be at rest. But it is well known that even when this is 

 the case the water cannot be so quiet, before it is allowed to run 

 through the hole in the bottom, that it will not run into a violent gyra- 

 tion at and very near the center. So the air in a tornado, in running 

 toward the center to supply the ascending current, always runs into a 

 gyration of destructive violence near the center. The part of very vio- 

 lent gyration being small, the tornado in its progressive motion leaves 

 only a very narrow path of great destruction. The principal difference 



