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in Iceland, 29.510 inches ; at Aberdeen, 29.737 inches; in London, 29.956; at Algiers, 30.119 inches, &c.; in other words, suppose the pressure to be exactly the mean of January, as represented in Plate II., then, by the method of equal barometric differences, these pressures would all be reckoned as 0—that is, no barometric disturbance would be indicated. But under such circumstances there would be considerable barometric disturbance, which would occasion a general flow of the atmosphere from the very south of Europe northward towards Iceland. Hence no method but that of reduction of barometric observations to sea-level represents the whole of the disturbing force at any time prevailing over wide areas. Except at very great heights, reductions to sea-level by the usual methods may be accepted as close approximations. The pressures which prevail at great heights must not be mixed up with pressures which prevail at lower levels in examining storms. The only use to which pressures at great heights can be legitimately put is in their relation to the upper currents, or to the winds which prevail at these heights, and not to the winds which blow on the earth's surface. On this point R. Russell, in his Climate of America,' has made some ingenious and valuable remarks.
501. Third Method.-By this method account is taken only of the line of minimum and the line of maximum barometer; in other words, a line is drawn through all places at which at the moment the barometer has fallen to the lowest point, but has not yet begun to rise; and another line through all places where the barometer has risen to the highest point. By this method only one feature of the pressure is given, which is included in the first method. It needs scarcely be said that this method is very defective, since by it no information can be got as to where the greatest differences of pressure are,—the most essential element of the storm, as pointing out where the wind is strongest,—the shape and position of the storm as defined by differences of pressure, and the exact direction towards which the body of the storm is moving. This is the method which was adopted by Espy in examining the storms of America, and it is at present advocated by Signor Matteucci and R. Russell.
STORMS OF EUROPE.
502. I have charted a very large number of European storms, laying down in all cases the isobarometric lines, or lines of equal atmospheric pressure, and the direction and force of the winds; and, in a great many instances, the lines of equal thermometric disturbance, and the rainfall, cloud, and clear sky. I have also completed many of the daily charts of the weather published in the · Bulletin International,' by filling in the observations from Iceland, Faro, Norway, Scotland, England, Austria, and other places. Hence the results which will be stated as applicable to the storms of Europe are based on a sufficiently large number of instances. Plate VII. is given as illustrative of the general features of these storms. It is a synchronous or synoptic Chart of Europe, giving, from observations made at about one hundred and forty places scattered over this continent, the atmospheric pressure
and direction and force of the wind at 8 A.m. of the 2d of November 1863, by which are shown the position and character of two storms passing over Europe at the time. The isobarometric lines, or lines showing where at that hour the height of the barometer reduced to 32o and the level of the sea, was the same, are given for every two tenths of an English inch in the difference of the pressure. They are the red lines of the chart. Hence, where these lines approach near each other, or crowd together, the difference of pressure, or the atmospheric disturbance, was greatest; and least where they are most apart-a distinction of the utmost importance in its relation to the force of the wind. The force of the wind is shown by arrows represented as flying with the wind. A plain arrow - shows that the pressure does not amount to 1 on the scale 0 to 6; that the pressure is 1 on the same scale; that it equals 2; and so on up to mar,
which represents 6, a violent storm or hurricane. This arrow E → indicates that the force of the wind is not known; and represents a calm.*
503. The average atmospheric pressure over Europe at the level of the sea may be stated roughly at 29.9 inches. When, therefore, the barometer falls below 29.9, the equilibrium of the atmosphere is more or less disturbed in proportion to the extent of the fall, and it is within this area of low barometer that storms occur. Hence, by tracing 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.
504. Form and Extent of Storms.—The curved isobarometric lines on the chart represent the shape storms generally
The area of European storms is generally either circular or elliptical, and when elliptical, the major axis of the ellipse seldom exceeds twice the length of the minor axis. Rarely in Europe, though less rarely in America and in the southern part of the Indian Ocean, the form of storms is more elongated. The outline is occasionally very irregular, but in such cases the storm will be found to have parted into two, or more rarely three, distinct storms, which remain separate for some time, and afterwards reunite ; sometimes, however, they continue separate, and diverge, one taking one direction, the other a different. The irregular shape of the iso barometric lines may arise from two storms distinct from each other, though appearing in the chart at the same time, as on Plate VII. The circular or elliptical form of storms, which an examination of some hundreds proves to be their general characteristic, is a most important feature, whether as determining practical rules for the guidance of sailors in storms, or for the foretelling of storms at particular seaports.
I have been enabled to make this Chart more complete than in the first edition,-to make it indeed one of the most complete Synoptic WeatherCharts of Europe hitherto published, -through the courtesy of Professor Mohn, Norway, Dr C. Jelinek, Austria, and A. 0. Thorlacius, Stykkisholm, Iceland, in sending me observations made in these respective countries. The Russian observations since published are also added.