symptoms, such as a southeasterly wind and an increase in temperature, it is safe to count on a period of stormy weather.

20. The Formation of Storms. Storms are due primarily to the formation and persistence of areas of low barometer. Under ordinary circumstances, the reading of the barometer differs but little from the mean. The atmospheric pressure over a certain region having fallen below the average, air rushes in from all sides to restore the equilibrium, just as water rushes in from all sides to fill a vacant space in its midst. Under certain complicated conditions the influx of this new air, instead of filling up the area of low barometer, serves only to increase it; the depression becomes deeper, the influx of air becomes stronger, and a storm is established. As the storm gains in strength, the area of low barometer assumes a roughly circular shape, and attains a diameter varying from 200 to 500 miles, and even greater.

21. The Storm Center. - The area of lowest barometer is known as the storm center, inasmuch as it generally coincides very nearly with the center of the area over which the storm prevails. As we go outwards from the center in any direction, the atmospheric pressure increases and the barometer rises; and if we draw the isobars, or lines along which the barometric pressure is the same, say for 29.00, 29.10, 29.20, etc. inches, we will find that they form closed curves, nearly circular or elliptical in shape, one outside of the other, having for a common center the area of lowest barometer.

Neither during the course of their formation, nor afterwards, are these storms, or areas of low pressure, stationary. They move constantly from west to east, or from southwest to northeast, with a velocity ranging from 15 to 40 miles per hour. The storms that cross the lakes during the season of navigation have their origin, not on the lakes themselves, but at a distance, some in the southwestern states, Kansas, Indian Territory, and northern Texas, whence they move

northeastward, the center reaching the lake region during the second or third day of its existence.

As such a storm approaches the lakes from the westward— across the states of Missouri and Illinois, if it belongs to the first class, across Minnesota and Wisconsin, if to the second class - the barometers in advance of it begin to fall and continue to fall as long as the storm center continues to approach. The center having passed and begun to recede to the eastward, the barometer begins to rise.

22. Average Storm Track. - The path followed by the storm center in moving eastward is known as the storm track. By comparing the observations of many years, plotting the successive positions of the center of each storm as it approaches the lakes, and drawing the tracks, it

becomes apparent that for each class of storms there is a certain strip, or belt, along which the storm center seems to travel in preference to any other. This belt is known as the average storm track, and is shown for the Great Lakes and the country to the west in Fig. 6. Occasionally a storm travels outside the general belt and takes an erratic course; but it seldom takes a direction differing materially from the general progressive movement of the majority. It is very necessary, however, that the average path should not be blindly accepted as the real path of any particular storm.

23. Wind and Weather in the Various Parts of the Storm Area. - Observation has shown that there are certain conditions of wind, cloud, temperature, etc., that are peculiar to certain portions of the storm area, the same conditions being always encountered in the same portion. Of no feature is this so true as of the direction of the winds around the center of the storm, and it is by carefully observing this direction, and the manner in which it changes-the "shifts" of the wind- added to the behavior of the barometer, that the mariner is enabled to decide whether he is in advance of the storm, with the latter bearing down full upon him, or in the rear, having already withstood its worst severity; whether he is to the north of the track, or to the south; and if compelled to heave-to, whether he must do so on the port tack or the starboard, in order to escape the danger of being overborne by the heavy seas, or struck aback by some sudden squall of wind.

As soon as a barometric depression is formed at any point, there is a tendency of the air to rush in from all sides toward that point until the depression is filled up and the barometric level is restored. If, then, barometric difference were the only thing to be considered, we would find surrounding a storm a series of winds all blowing straight toward the center, just as the spokes of a wheel are all directed toward the hub. This simple condition of things is never realized, owing to the fact that, in the first place, the storm center itself is in rapid motion, and a wind constantly directed toward this center would therefore of necessity be continually shifting; and in the second place, to the fact that all winds in the northern hemisphere, owing to the rotation of the earth on its axis, undergo a constant deflection toward the right. Under these two influences, the wind, instead of blowing directly toward the storm center along straight lines, follows a constantly incurving path.

24. Rotation of Wind. - In the northern hemisphere, we have therefore the following law governing the rotation of the winds: Around a center of low barometer, the winds circulate

in a direction contrary to the motion of the hands of a watch. To the east, then, in advance of a barometric depression, southerly and southeasterly winds will prevail, marking the storm's approach. To the west, in the rear, northerly and northwesterly winds mark its departure. North of the center, we have easterly and northeasterly winds; south of the center, westerly and southwesterly winds.

25. Bearing of the Storm Center. - The circulation of the winds around the storm center, or area of low pressure, is represented in Fig. 7. For the sake of simplicity,

the storm is made circular, the dotted lines representing the isobars and the full lines the direction of the winds. The latter is, in general, inclined 2 compass points to the left of the former. An observer on the deck of a vessel with his back to the wind will therefore find the storm center, or point of lowest barometer, 6 points to his left; or, if he face the wind, the center will be 10 points to his right; the two statements are identical. Each of these rules will thus suffice to fix at any moment the direction of the storm

center from the observer. It should, however, be borne in mind that the incurvature of the winds toward the center is greater over the land than over the water, and, consequently, in the case of the Great Lakes, the proximity of the land will not permit the accuracy in fixing the center that is sometimes attained on the open sea.

The movement of the winds around this storm center in the northern hemisphere should here be compared with the typical circulation shown in Fig. 8. Their revolving character in the direction indicated, viz., in the direction contrary

to the motion of the hands of a clock, is at once evident. In front, or to the east, of such an advancing storm, during the fall of the barometer that it occasions, southerly and southeasterly winds are seen to prevail, while during the rise of the barometer succeeding the passage of the storm center, the mariner will evidently find northwesterly gales of greater or less severity. As the storm center advances, its circulating system of winds travels with it, the result being that a vessel hove-to to the north, or left hand, of the track, as at A, will experience in turn easterly, northeasterly, northerly, and northwesterly winds, the winds shifting to the west through the north, or against the sun; while if her position be to the south, or right hand, as at B, the winds will "veer," or shift, to the west through the south, or with the sun, the order being southeasterly, southerly, and southwesterly.