Year: 1873

The secondary cause of a storm is a

 reduced alr-pressure over any particular

 part of the earth's surface which is

 indicated by depression of the mer-

 cury 1n the barometer within that region.

 Hence the central district of storm is | technically called " an area of low bar

 ometer." Towards this region the sur

 rounding air rushes in to restore m equilibrium, and that rush of air is

 known as wind. The resulting circular

 motion, around the central point, is due

 to the rotation of the earth upon her

 axis, from west to east.

 It is easy to calculate that the velocity

 of rotation at the equator, 24,9OO miles

 ill "A hours, is 17.3 miles per minute.

 while at the pole it is nothing. In the

 latitude of 40 degrees it is 13i miles :; in

 the latitude of 45 degrees it is 123 miles

 per minute. The earth carries the at mosphere along with it, and the velocity

 of the atmosphere at any point is the ve- locity of thatpoint, which decreases with

 m increase in the latitude. It is evident

 that, if a mass of air at any point be

 moved northward of southward, from

 any cause, it will preserve for time the

 eastward velocity of the point from which

 1t started; just as a stone thrown from the window of moving railroad car is

 carried forward by the impetus of the train. Hence, mass of air moving southward, ill this hemisphere, will lag

 behind in the rotation, while the relative motion will be faster than that of the surface if it move northward, because of the change ill velocity of the surface due to the latitude.

 Now suppose the top line of ouf dia- gram to represent the parallel of 45 de- grees of latitude, and the length of the horizontal arrow indicate the motion per minute on the parallel. Let the lower line represent the parallel of 40 degrees; then the length of the arrow near it will indicate the velocity of the air there per minute. In each case the surface, and It's superlncumbent air, moves in the direction of the arrow. If an area of low barometer be at c, nearly midway between these two parallels, the air will move towards that area from every point of the compass; and the result is indicated in the diagram, where the dotted lines show the tendency, and the full lines show the direction, of | the actual movement. A mass of air coming from the north at N, moves eastward with 8 velocity 12 miles per minute, while the storm-centre moves 124 miles per minute. Hence, the nov ing air drags behind at the rate of ] mile per minute, or 30 miles per hour. and takes the direction of the arrow towards the circumference of the small | circle, instead of its centre at c. Simi- larly mass of air moving northward. | from s, has an initial velocity of 13l | miles per hour greater than that of the | md H.. direction of the arrow from s, instead | of the dotted line. Similarly, we may show that the same deviation occurs in the case of the air coming from every direction towards c; and the conse quence is that the air moves round the centre of the area of low barometer, in a direction contrary to that of the hands of & watch that is laid with it's face upward.

 The vertical motion resulting from all directions towards storm-center is even more decided than indicated by the diagram, which only represents the secondary tendency. The central rush Is opposed, or counteracted, by the elasticity of the several volumes of air as they approach each other and their impetus Carries them around in the direction of the movement already established, so that the whirl extends nearly as far from the center as the distance from which the winds blow. All the air that tends towards the storm-center moves round it in an ap- proximata circle in just the same way that water courses round in funnel, for similar reason. and in the same direction, the only difference being that the central column of air moves up- ward, while the central column of water ill the funnel moves downward.

 A knowledge of these facts enables us not only to tell the direction of the storm-center, but often to know whether it is approaching towards us, or re- cedlng, or is passing us at harmless distance. If we face the wind the storm-center will always be on the right hand :; or if we turn so that the wind blows from left to right we shall face the storm-center. If, at the same time, the mercury in the barometer con- tinues to fall, we may be sure that the vortex is approaching us while, if the barometer rises, we may count on it's having passed by. The general direc- tion of the movement of the center is from west to east but it may depart several points from the cardinal line.

 The preceding generalization of posi- tion apply to the Northern Temperate Zone. A little consideration will help