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comparing it with the observations at each place as it is passed over, it is proved beyond a doubt that all these storms turn round upon a centre. But the kind of the rotation is not proved-that is, it is not proved whether the winds blow round the centre in perfect circles, modified only by the progressive motion of the storm, the ideal of the cyclone; or whether they blow round and in upon the centre. I have examined many, especially tropical storms, by synchronous charts, in the hope of lighting on some one that might illustrate the purely cyclonic behaviour of the winds round the centre, but have not yet found one. On the contrary, every storm examined in Europe, in America, or within the tropics, shows beyond a doubt that storms are vorticose. This, be it noted, is not a mere opinion, but the simple statement of the position of the winds in storms as shown by synchronous charts on which the winds are entered exactly as observed at each place. It is therefore a statement unassailable by argument, and must be accepted as the law in accordance with which the course of the wind in storms is regulated. Professor Taylor, followed by Sir John Herschel, has shown that these characters of storms are the result of the "law of rotation" of the wind as devoloped by Dové, which takes effect when, owing to a local excess of heat or of aqueous vapour, the air over some extensive region rises in a vertical column. This rotatory feature of storms becomes intelligible when the winds are regarded as drawn in upon a centre of low atmospheric pressure along the globular surface of the earth rotating eastwards.*

565. Since storms are formed of spirally in-moving currents of air, it follows that the fall of the barometer at the centre is not the effect of centrifugal force. For if this was the cause of the low barometer, the wind would blow round the

* This view of the spirally in-moving course of the winds in storms, which is essentially Redfield's theory, has been adopted by the following meteorologists: Dr Buys Ballot, Utrecht, whose system of forecasting the weather, proposed by him in 1857, is based on it; Dr Joseph Henry, Secretary of the Smithsonian Institution, America; Maury; Dr H. Lloyd, Dublin; Sir John Herschel; William Stevenson; &c. &c.

centre in circles, subject only to a slight modification from the onward motion of the storm,—a state of the winds which in none of the storms examined has been found to exist, not even in those cases when the storm was increasing in area and deepening the depression at the centre. Professor Loomis has shown that any observed velocity of the winds round the centre is altogether inadequate, on the principle of centrifugal force, to bring about the low pressure at the centre. Thus, if the wind blows round a circle 300 miles in diameter at the rate of 70 miles an hour, the centrifugal force would only depress the barometer at the centre a little more than 0.02 inch; whereas in the Bahama hurricance the difference of the pressure at that distance from the centre was 2 inches, or 1000 times greater than that due to centrifugal force, on the supposition that the mean velocity of revolution of the portion of the atmosphere, 300 miles in diameter, was 70 miles an hour.

566. An interesting question may be here suggested-viz., Does any portion of the atmosphere as it turns round and in upon the centre ever make one or more complete revolutions round the central area of least pressure, whilst it continues to blow as a wind in contact with the earth's surface, or before it rises into the upper regions of the atmosphere? It is probable that this seldom occurs, when it is considered that the qualities of the air, both as regards temperature and moisture, are generally essentially different in the front, as compared with the rear of a storm. Mr Meldrum, however, gives an example which occurred in May 1863, when a vessel belonging to Dundee, called the Earl of Dalhousie (Captain Campbell), scudded, at the rate of 10 to 13 miles an hour, three times round the centre of a revolving storm, which happened to be nearly stationary, till at length it reached the central calm.

567. But the spiral rotation, instead of the purely circular rotation, of the winds in storms, completely alters the whole complexion of the question of the theory of storms. For since it follows from it that enormous quantities of air are constantly

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 calms. 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 SEPTEMBER TILL 4TH OCTOBER 1866.

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