The common toy known as a gyroscope illustrates the last peculiarity also. It consists of a wheel within a metal frame, which has a peg like a top. If the wheel be made to revolve rapidly, the whole may be balanced on the peg: when the frame will begin to slowly revolve in the opposite direction: and if placed upon a smooth level surface, like the top it will tend to describe the same course.
Still other illustrations of this principle are even more familiar than the spinning of a top. Any one who has seen the game of soldiers in a bowling alley knows that in order to make the ball turn to the left as it moves forward, it must spin the other way; that is, with the hands of a watch. To travel or curve to the right, it must spin in the contrary direction. So in our “great national game,” base-ball, the pitcher curves the ball any way he pleases merely by following this law. It is not necessary to take into account, as many do, the return trades, as occasioning the travel of a whirling storm; and the fact is, that the cyclone frequently travels more rapidly than the ordinary wind moving in the same direction.
Now, the motion of the planets is similar: rotating in one direction, they travel in the other. So we find the general law is,
All revolving bodies, left free as to direction, travel in a curve in a direction opposite to that of their rotation. This curve is usually some form of conic section: an ellipse, parabola or hyperbola. The planets, and some comets, move in ellipses. Some comets travel parabolas or hyperbolas. And the parabola is the customary path of the cyclonic storm. As the cyclone in the northern hemisphere rotates from right to left, and in the southern from left to right, their paths must necessarily be in opposite directions, as may be seen by the accompanying diagram. So in either case, the direction of the path is always away from the equator.
As far as the United States are concerned, most non-cyclonic storms originate in the Saskatchewan country, or along the southeastern slope of the Rocky Mountains. By far the greater number pass over the St. Lawrence valley. A small number are developed in the Gulf, or in the Pacific: but these are much affected, often broken up, in crossing the Rocky or Appalachian systems. The usual course is somewhat north of east; but there are a few notable exceptions. The immense amount of vapor wafted up the Mississippi valley induces some low area storms to move southward from Manitoba into the upper Mississippi valley. In like manner, the excessive moisture along our north Pacific coast causes occasional storms to move southward from Alaska to Oregon.
But the course of a cyclonic storm, we have seen, must be different.