Page:Physical Geography of the Sea and its Meteorology.djvu/448

422 of meteorology will enable us to warn the ships in our Gulf ports, as well as those of Cuba, of the approach of every hurricane or tornado that visits those regions.

791. The changing of the wind in a cyclone.—But, returning to the cyclone theory: though the wind be blowing around in spirals against the hands of the watch, yet, from the fact that the centre about which it is blowing is also travelling along, the changes of the wind, as observed by a vessel over which the storm is passing, will not, under all circumstances, be against the sun in the northern, or with the sun in the southern hemisphere. The reason is obvious. This point is worth studying, and any one who will resort to "moving diagrams" for illustration will be repaid with edification. Piddington's horn cards are the best; but let those who have them not cut a disc of paper of any convenient diameter, say 2½ inches, and then cut out a circle of 2 inches from the middle; this will leave a ring half an inch broad upon which to draw arrows representing the course of the wind. Suppose them to be drawn for the northern hemisphere, as in the annexed diagram; lay the paper ring on the chart: suppose the ship to be in the N.E. quadrant of the storm, which is travelling north, the centre of the storm will pass to the west, but the wind will change from S.E. to S., and so on to the west, with the hands of a watch, though it be revolving about the centre against the hands of a watch; still the rule for finding the direction of the centre holds good: Face the wind, and the centre in the northern hemisphere will be to the right; in the southern, to the left.

792. The wind stronger on one side than the other.—Suppose that in the case before us the storm is travelling to the north at the rate of 20 miles the hour, and that the wind is revolving around the centre also at the rate of 20 miles the hour: when the vortex bears west of the ship, the wind will be south. It is going 20 miles to the north with the body of the storm, and 20 miles around the centre; total force of the wind, 40 miles an hour on the east side. Now imagine yourself on the other side, that is, that you are in the north-west quadrant, and that the storm is travelling due north as before; the vortex will pass east of you, when the wind should have changed from N.E. to north, turning