The tornados of North America and the coasts of Africa, as well as the typhoons in the sea of China, have long been known as violent tempests in which the wind has a revolving motion of this kind, but these terms are commonly applied to such storms as are of short dura tion and comparatively of small extent, the diameters of the vortices varying from a few hundred yards to one or two miles. It is now ascertained, by such evidence as leaves scarcely any doubt of the fact, that in all or most of the great storms which agitate the atmosphere the wind has a rotatory movement, and that the diameter of the circle within which the gyration is performed is sometimes equal in extent to several hundred miles : in great whirlwinds the axis appears to be either vertical or nearly so, but in those of small extent its inclination is often inconsiderable, and it is sometimes parallel to the horizon.
As early as the middle of the 17th century the revolving motion of the wind, during the great hurricanes which take place iu the West Indies, appears to have been noticed ; and in a description of them, which was given at that time in the ' Philosophical Transactions,' it is stated that, after a cessation of the trade-winds, the storm begins from the north; that the wind afterwards goes round to the north-west and then to the south, the storm subsiding when the wind comes to the south-east : and in Colonel Capper's Work on the ' Winds and Mon soons; which was published in 1801, the gyratory nature of the storms in the Emt Indian seas is inferred from the recorded in the directions of the wind during the stones of 1760 and 1770. Whirl wind atones appear however to have been thou considered as local and temporary ; and we owe to the late 31r. Redfield, of New York, the discovery that they have a progrmaive as well as a revolving motion. Dr. Franklin ascertained that the storm which he witnessed at Phila delphia in 1743, took a certain time to arrive at Boston, but ho did not pursue the subject, and, from a mistaken estimate of the distance between those cities, his opinion of the rate of movement is now known to be erroneous.
Currents of air are frequently, as at the changes of the monsoons in the East Indian seas, impelled obliquely against each other, and thus rotatory motions in the atmosphere may b e produced, exactly as eddies or whirlpools are formed in currents of water. [Wuinttoot..] Mr. Redfield, in his' Observations on Storms,' in the ' Transactions of the American Philosophical Society; 1341, offers an opinion that generally during a gale there is, in the lower part of the atmosphere, a spiral motion inclining downwards and towards the centre; and in the higher regions a like spiral motion inclining upwards and towards the exterior. Ile adds that, in storms of great extent, there is sometimes
found a considerable area within which the winds are moderato and blow in various directions. These characters of a revolving storm appear to be verified by the manner in which trees were prostrated during the hurricane which occurred in New Brunswick in Juno, I835; when, about the centre, bodies of great weight were carried spirally upwards, and, on opposite sides of the storm's path, the trees were thrown in contrary directions. It is observed that when a storm rages violently, the doors and windows of houses are often forced outwards, either from the centrifugal force caused by the revolving motion, or from the expansion of the air within, when a temporary rarefaction takes place on the exterior; and from the move ments of the clouds it appears 'often that a storm, in passing over a place, is in activity at a considerable altitude before it descends to the earth's surface.
That a whirlwind may have a progressive as well as a revolving motion may be easily understood if it be observed that, as the atmos phere in the tropical regions moves from east to west with respect to the surface of the land or sea, it may, after crossing the Atlantic and Pacific oceans, be arrested in its progress westward by the continents of America and Asia, and deflected from thence towards the poles : the whirlwinds formed by electricity or otherwise in the general current of air will consequently be carried with the deflected branches into high northern and southern latitudes; and it may occasionally happen that, from the nature of the deflecting forces, the path of the axis of a revolving storm in either brauch is a curve line like a segment of a circle or a parabola. Sir John F. W. Herschel, at the mooting of the British Association in 1838, suggested that the Gulf-Stream may be the cause of the nearly varabolic curves assumed by the paths of the storms on the coast of North America : the paths nearly coincide with the course of this stream ; and the warmth of the water, by increasing the temperature of the air above it, must disturb the equilibrium of the atmosphere, and maintain the storms which had their origin in a lower latitude.