STORMS are violent commotions of the atmosphere, occurring in all climates, partic ularly in the tropics, and differing from other atmospheric disturbances in the extent over which they spread themselves, their destructive power, and the sudden changes which take place in the direction of the wind. There is, perhaps, no question in science in which there has been so large an admixture of speculation with fact, as in the attempts made to reduce the phenomena attendant on storms under general laws; the reason being, that meteorological observatories were too few in number, and too wide apart, to enable any one to give the barometer pressure, the general course of the winds, and the rainfall, without drawing largely on conjecture. Now, however, owing to the growing popularity of meteorology, and the countenance happily given to it by most civilized nations, sufficient data may be obtained for a fuller and more satisfactory state . ment of the facts.
We subjoin two charts of Europe, showing, from actual observations made at upward of 100 localities scattered over that continent, the barometric pressure, and direction and force of the %rind, at 8 A.M. of the 1st and 2d of Nov., 1863, during part of the course of two storms which passed over Europe at that time. The isoharometrie lines, or lines showing where, at the above hours, the height of the barometer was the same, are given for every two-tenths in the difference of the pressure. Bence, where these lines approach near each other, or crowd together, the difference of pressure, or the atmospheric disturb ance, was the greatest; and thd least where they are most apart--a distinction of the utmost importance in determining where the storm may be expected to rage in greatest fury. The arrows show the direction of fhti wind, being represented flying with it. The force of the wind is shown (1) by plain arrows, which repre sent light and moderate winds; (2) by 'arrows feathered on one•side only whiell represent high winds; (3) by arrows feathered on Lath sides; which represent strong gales, storms, or hurricanes.
The mean atmospheric pressure, at the level of the sea, may be stated to be 29.9 inches. When, therefore, the barometer falls below 29.9, the equilibrium of the atmos
phere is more or less destroyed, according to the amount of the fall, and it is within this area of Cow barometer that a storm may be expected to occur. Bence, while we trace these low pressures, as they advance over the earth's surface from day to day, we trace at the same time the progress of the storms.
Form and Extent of Storm Areas.—The circular isobarometiic lines on the charts repre present very accurately the general shape storms assume. The area of almost every storm is either circular or slightly elliptical, and when elliptical, the major axis of the ellipse sel dom exceeds twice the lei'lgth of the minor axis. Rarely in Europe. hut in America less rarely. the form of storms is much more elongated. The outline is occasionally very irreg ular. but in all such cases the storm will be found to have parted into two or more distinct storms, which rem.on separate for some time and then reunite. This circular form of storms, which an examination of some hundreds. especially in Europe. has shown to be their general characteristic, is a most important feature, whether as determining the practical rules for the guidance of sailors in storms, or for the forecasting of storms at particular sea-ports, in respect of time direction from which they may be expected to come, and the veerings of the winds during their continuance. Time extent over which storms spread themselves is very variable, being seldom less than 600 miles in diameter, but Often two or three times that amount, or even more. Almost the whole of Europe is sometimes overspread by a single storm at one time. The area of storms is by no means constant from day i to day, but varies in size, sometimes expanding and sometimes con tracting. And it is worthy of remark that when a storm contracts its area, the central depression gives signs of filling up, and the storm of dying out. On the other hand, when it increases iu extent, the central depression becomes deeper, the storm increases in violence, and occasionally is broken up into two, or even three, depressions, which become separate storms. with the wind circling round each.