which represents 6, a violent storm or hurricane. This indicates that the force of the wind is not arrow H 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 assume. 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 isobarometric 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. O. Thorlacius, Stykkisholm, Iceland, in sending me observations made in these respective countries. The Russian observations since published are also added. The extent over which storms spread is very variable, being seldom less than 600 miles in diameter, but oftener two or three times that amount, or even more. Thus the storm in the chart which had at that time its centre at Liverpool, extended in one direction at least as far south as Bordeaux, or 600 miles. Its diameter was therefore about 1200 miles, or about the average extent. Sometimes a space greater than the whole of Europe appears to be involved in a single storm at one time. The area of storms is by no means constant from day to day, but varies in size, sometimes expanding and sometimes contracting. It is worthy of remark that, when a storm contracts in area, the central depression gives signs of filling up, and the storm of dying out. Thus the storm which was at Liverpool on the 2d November had, on arriving at Jutland on the following morning, contracted to one-fourth part of its former diameter, and the central depression, instead of 28.9 inches, was only 29.3 inches. Similarly the storm seen in the north of Sweden covered a much wider area while passing over Great Britain. On the other hand, when a storm increases in extent, the central barometric 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 spirally round each. 505. Direction in which European Storms advance.-It may be premised that by the direction of the progressive motion of a storm is meant, not the direction of the wind, but the path followed by the central area of disturbance, as shown by the lowest barometric pressures. About half the storms of middle and northern Europe travel from S.W. or W.S.W. towards N.E. or E.N.E., and nineteen out of every twenty, at least,, travel towards some point in the quadrant of the compass from the north-east to the south-east. Thus the general direction from which storms come is westerly, understanding by the term any point of the compass from S.W. to N.W. Several storms could be adduced as having advanced on Great Britain from N.W.; still more which have come from W.N.W. and W.; but the larger proportion appear to come from W.S.W. and S. W., and advance towards E.N.E. and N.E. These proportions are given only as conjectural, or as the impression left on my mind from the examination of a very large number of storms, and from the examination of the monthly schedules of the observers of the Scottish Meteorological Society for upwards of seven years, before sending them to be reduced to the Royal Observatory, Edinburgh. Before the proportions could be stated numerically, or before positive statements should be ventured, at least a thousand weather-charts would require to be constructed, showing the progressive movements, from day to day, of all storms that have passed over Europe during, say, the last six years; and such examination should include, if possible, observations from Iceland, the Azores, and the east coast of the United States of America. In Plate VII. the two blue lines represent the tract of the centre of each of the storms. Thus the storm which on the morning of the 2d November 1863 was in the north of Sweden, had arrived thither from the west of Ireland and Shetland; its course being thus from south-west to north-east. The other storm, the centre of which was at Liverpool on the 2d November, had advanced from the south-west of Ireland; on the two following days it continued its eastward course to Denmark and the Baltic Sea, where it died out. The first of these directions, as just stated, is the most general, and the second the next most common. Some storms, however, travel towards the southeast, over Great Britain, Austria, and the Black Sea, into Asia; of this class was the disastrous storm of November 1854, which inflicted so much damage on the British and French fleets during the Crimean war. Others descend over the North Sea, Germany, Austria, and Italy, thus following a course from north to south. From the 27th to the 30th March 1864 a storm passed successively over the Bay of Biscay, Gulf of Lyons, Corsica, Croatia, the valley of the Theiss, Warsaw, Dantzic, and the south of Sweden; its track being thus nearly semicircular. These two directions are of comparatively rare occurrence. 506. The least common, or rather rarest, direction in which storms travel, is toward some westerly point. The great snowstorm which occurred in Europe in January 1836, and which has been admirably traced and described by Professor Loomis, America, began, as the barometric curves conclusively prove, in the north-east of Russia, and thence proceeded southwest to the Swiss Alps, where its westward course would appear to have terminated. A few other storms might be mentioned, particularly during winter and spring, which have advanced from eastern or north-eastern Russia westward as far as Norway or Denmark, where generally their western course is arrested; they then retreat on their course and proceed eastward. Hence, then, though the weather-changes of Europe generally advance from west to east, it is a mistake to suppose that this is always the case; and weather-prediction proceeding on this assumption must occasionally lead to Further, all the weather-changes which conspire to produce the dry east winds in the British Islands are instances of weather-changes advancing in a westerly direction. error. 507. But again, storms have been repeatedly observed to remain stationary for several days in succession over the Bay of Biscay; and occasionally, though less frequently, over the Gulfs of Bothnia and Finland. It should be here noted that both these regions lie to the north-east of extensive mountainranges bordering on the Atlantic. Hence any system of weather-warnings is incomplete without daily telegrams from these regions, especially the Bay of Biscay, Norway, Sweden, and the north of Russia, including, if possible, Archangel. For a stationary low pressure around Bayonne-which, for example, prevailed for nearly a week during May 1866-influences the direction of the winds in the south of Great Britain by drawing them towards it; and a low pressure in the Baltic or Gulf of Bothnia, either stationary or having advanced thither from the east, influences the winds at the seaports of Great Britain by giving to them a westerly direction. It is during the occurrence of these storms in Russia and Sweden that strong westerly gales occur in Great Britain, |