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being poured all around into the area of the storm, and since, notwithstanding these accessions tending to increase the pressure, observation shows that the pressure is not thereby increased, but on the contrary sometimes diminished, we are forced to the conclusion that from a large area within and about the centre of the storm a vast ascending current must arise into the upper regions of the atmosphere; and arriving there must flow away over into neighbouring regions. The physical cause of the ascending currents is to be found in the moist and warm, and therefore light, air which all observation shows to prevail in the front and in the central part of storms. And since most of the rain which accompanies storms falls in those parts of the storm, the barometer will be still further reduced by the removal of the elastic aqueous vapour which is condensed into rain-drops, and by the latent heat set free in the condensation of the
vapour. 568. These considerations, taken in connection with what has been advanced in Chap. XI., with reference to the lower and upper currents of the atmosphere in their relation to the lines of equal barometric pressure, suggest that the general movements of the atmosphere over the globe and in storms, are due to the same physical causes acting in the same way. Indeed, what has been expressed in par. 481 equally expresses what takes place in storms. But a theory of storms which would account for all the phenomena is a very different, and a very difficult, subject. To be satisfactory, it must account for all forms of storm areas, from the circular to the form of the ellipse so elongated as to appear trough-like; for the direction in which they move, and the changes in that direction ; for the track they usually take in different parts of the world ; and, above all, for the saturation of the atmosphere with vapour often over a most extensive region, which must be considered as the necessary precursor of storms. Till synchronous charts be constructed, embracing, at least, the greater part of North America, the West Indies, the North Atlantic, Europe, and eastern and northern Asia, plausible or ingenious theories may be propounded, but since the chief
facts are wanting, nothing can be said which can be regarded as satisfactory or convincing.
569. The part of the track of the West Indian hurricanes after turning to the N.E., is in accordance with this theory ; but how account for the first part of their course, which is at right angles to the prevailing trade-winds of that region ? The usual place where the vapour brought by the tradewinds is condensed, is the region of calms, where heavy rains and thunderstorms daily occur. But since this condensation takes place simultaneously over a somewhat broad belt of the earth's surface, which for the time is stationary, it follows that the storm is neither rotatory nor progressive, the only effect of the condensation being the flow of the regular tradewinds towards the belt where it takes place. When the condensation is more copious than usual, the effect will be the acceleration of the speed of the trade-winds. This is the most probable explanation of the harmattan which occurs in December, January, and February, on the coast of Africa ; it always blows in one direction from the land, and does not increase quite to the violence of a gale, and then dies away.
Copious rainfall in the belt of calms opposite that part of Africa, and at some distance, is the probable cause of these winds. Similar in some respects to these are the tornados of Western Africa, which blow invariably off the land, first with little force, but ultimately they rise to the strength of a heavy gale, and then after an hour, or sometimes two hours, die away. The direction of the wind remains all the time unchanged; and the barometer varies little, if any, during their continuance. They are generally accompanied with rain and thunder, though sometimes they are quite dry winds. They are probably caused either by very heavy rains in the region of calms, falling over a limited region and lasting only for a short time, or by a sudden increase of pressure in the interior through the upper currents. On the other hand, it has been seen that when the region of calms lies in a slanting position, as in the Indian Ocean from November to May, storms are originated there. Is this slanting position of the region of calms
the chief cause of the stormy character of the weather of the Indian Ocean during these months ? If so, then it is due to the diminution of pressure arising from the heating of South Africa at this season, and the increase of pressure to the north, arising from the cooling of the land north of the equator, which there slants from W.S.W to E.N.E.
570. Let us suppose that the atmosphere of the West Indian Islands has from some cause become exceptionally warm and moist, and that, at the same time, a high barometer is interposed between that region and the belt of ealms. In such circumstances, as the trade-winds cannot flow towards the belt of calms, the usual provision for draining them of their moisture is taken away, and a rapid accumulation of aqueous vapour takes place to the north of the high barometer, ready to burst in any instant in rain and tempest. This would appear to have been the case before and during the Bahama hurricance of October 1866, Plate VIII. The following table shows the atmospheric pressure and winds at St Croix, and on board H.M.S. Buzzard, which sailed from Barbadoes, on the 26th September, for England. St Croix is in lat. 17° 44' 29" N. and long. 64° 41' W., and the mean height of the barometer at sea-level is 30.080 inches.
BAROMETRIC PRESSURE AND WINDS OBSERVED AT St Croix, WEST
INDIES, AND ON BOARD H.M.S. BUZZARD, FROM 26TH SEP-
From this table it is seen that during the hurricane, which in all probability began a little to the north of Barbadoes on the evening of the 26th or morning of the 27th Septeinber, atmospheric pressure was above the average to the S.E. and E. of the region traversed by the storm. This being the case, the air would naturally flow from the high towards the low barometer—that is, a general movement of the atmosphere would set in to the N.W., which was the course pursued by the storm for the first four days, a course at right angles to the usual trade-winds. During this part of its course, the rate at which it travelled was slow, not exceeding fifteen miles an hour. A progressive motion, as slow, nay often much slower, is common to these storms until they enter the region of the return trades, when the course is changed to N.E., the onward movement becomes greatly accelerated, and the storm itself spreads over a wider area.
571. Newspaper accounts of the weather before the hurricane describe it as close and sultry, the whole atmosphere being abnormally heated and excessively loaded with moisture. At Bermuda, at 9 A.M. of the 4th October, the temperature of the air was 79o.4, and the dew-point 73o.6, giving a humidity of 83 : and this kind of weather had been prevailing for fully a week before. On the following day, the 5th, at 9 A.M., when the storm had just passed, the temperature was only 71°, the dew-point 56°, and the humidity 59o. Next night the temperature fell to 56°, whereas, before the storm, it did not fall on any night lower than 72°. It would be interesting to know if West Indian hurricanes occur simultaneously with a high pressure to the south, being thus interposed between that region and the usual position of the belt of calms at that season. If so, a considerable step will be gained toward understanding the causes of their origin, and how it is that they are, happily, phenomena of very rare occurrence.
572. Perhaps no writer on meteorology denies the influence of low pressures in drawing the wind towards them ; but the effect of this influence in accounting for the rotation of storms round their centres is often tacitly subordinated to a theory by which the storm is conceived to rotate from some force, not explained, but quite distinct from that arising from differences of pressure; and in virtue of which the air is driven to the outside of the whirl, thus diminishing the pressure at the centre, -much in the same way as, in whirling round a pail of water, part of the water leaves the centre and rises up against the sides of the vessel. In applying this theory to cyclones, the wind at every point within the storm is stated to be under the influence of two forces, the one arising from the rotatory motion of the cyclone, and the other from its progressive motion. To illustrate this by an example : Suppose a storm advancing over England to the eastward at the rate of twenty-five miles an hour, and taking the simplest case of the wind whirling round within the storm at the rate also of twentyfive miles an hour, then both the rotatory motion of the winds and the progressive motion of the storm are from the west, at the extreme south point of the storm ; hence there the wind ought to be due west. At the extreme east point of the storm, since the direction of the wind due to the rotatory motion would be south, and by the progressive motion west, the wind ought to be seen blowing there from the south-west. At the extreme north, since the rotatory motion would be east, and the progressive west, and both being equal, there should be no wind at all. Lastly, at the extreme west point of the storm, the rotatory motion being north, and the progressive west, the wind should be northwest. Further, since in the south half of the storm the direction of the two motions is generally the same, and in the north half generally opposite, the gale ought to be most severely felt in the southern half of the cyclone, attaining its height at places immediately south, and least at those immediately north of the centre. When the rotatory motion much exceeds the progressive, the deviations from the true circular course of the winds when laid down on a synchronous chart would be less than those stated above ; but in all cases, unless when the cyclone was stationary, the winds in the