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WHIRLWINDS AND CYCLONES.
soon increased greatly in size and violence. Franklin followed it on horseback, and saw it enter a wood, where it twisted and turned round large trees: leaves and boughs were carried up so high that they appeared to the eye like flies. Again there was no condensation of vapour.
We thus see that whirlwinds of great violence occur when the air is dry, and there can be no condensation. When, however, they are formed at sea, and occasionally on land, the air next the surface is saturated with moisture; and this moisture is condensed when it is carried to a great height, forming clouds, or falling in showers of rain and hail. This condensation of vapour is an effect, and not a cause, and takes place, not in the centre, but at the top or at the sides of the ascending column. This is well shown in an account, by an eye-witness, of a whirlwind that did great damage near the shore of Lough Neagh, in Ireland, in August 1872.* It was about thirty yards in diameter. It destroyed several haystacks, and carried the hay up into the air out of sight. It partially unroofed houses, and tore off the branches of trees. The railway station at Randalstown was much injured; great numbers of slates, and two and a half hundredweight of lead were torn from the roof. When passing over a portion of the lake, it presented the appearance of a water-spout. On land everything that it lapped up was whirled round and round, and carried upwards in the centre, whilst dense clouds surrounded the outside and came down near to the earth.
As above mentioned, I had in Australia many opportunities of studying the dust whirlwinds; and as I looked upon them as the initial form of a cyclone, I
* "Nature,” vol. vi. p. 541.
paid much attention to them. On a small plain, near to Maryborough, in the province of Victoria, they were of frequent occurrence in the hot season. This plain was about two miles across, and was nearly surrounded by trees. In calm, sultry weather, during the heat of the day, there were often two at once in action in different parts of it. They were only a few yards in diameter, but reached to a height of over one hundred feet, and were often, in their higher part, bent out of their perpendicular by upper aërial currents. The dust and leaves they carried up rendered their upward spiral movement very conspicuous. No one who studied these whirlwinds could for a moment believe that they were caused by conflicting currents of air. They occurred most frequently when there was least wind; and this particular plain seemed to be peculiarly suitable for their formation, because it was nearly surrounded by trees, and currents of air were prevented. They lasted several minutes, slowly moving across the plain, like great pillars of smoke.*
When attentively watched from a short distance, it was seen that as soon as one was formed, the air immediately next the heated soil, which was before motionless, or quivering as over a furnace, was moving in all directions towards the apex of the dust-column. As these currents approached the whirlwind, they quickened and carried with them loose dust and leaves Ch. XVI.]
* A friend of mine tells me that he saw a similar whirlwind rise at noon one still summer day, and traverse the dusty road on the Chesil Bank between Portland and Weymouth. It travelled fully half a mile, about as fast as he could walk; and the point where it met the ground was not thicker than his walking stick. By-and-by it swept out to sea, where the dust gradually fell.
WHIRLWINDS AND CYCLONES.
into the spiral whirl. The movement was similar to that which occurs when a small opening is made at the bottom of a wide shallow vessel of water: all the liquid moves towards it, and assumes a spiral movement as it is drawn off.
The conclusion I arrived at, and which has since been confirmed by further study of the question, was, that the particles of air next the surface did not always rise immediately they were heated, but that they often remained and formed a stratum of rarefied air next the surface, which was in a state of unstable equilibrium. This continued until the heated stratum was able, at some point where the ground favoured a comparatively greater accumulation of heat, to break through the overlying strata of air, and force its way upwards. An opening once made, the whole of the heated air moved towards it and was drained off, the heavier layers sinking down and pressing it out. Sir George Airey has suggested to me that the reason of the particles of air not rising as they are heated, when there is no wind blowing, may be due to their viscosity: and this suggestion is correct. That air does not always rise when heated, appears from the hot winds of Australia, which blow from the heated interior towards the cooler south, instead of rising directly upwards. Sultry, close weather, that sometimes lasts for several days, would also be impossible on the assumption that air rises as soon as it is heated.
This explanation supplies us with the force that is necessary to drive the air with the great velocity with which it moves in whirlstorms. The upper, colder, and heavier air is pressing upon the heated stratum, and the
greater the area over which the latter extends, the greater will be the weight pressing upon it, and the greater the violence of the whirlwind when an opening is formed for the ascent of the heated air. There is a gradual passage, from the small dust eddies, through larger whirlstorms such as that at Lough Neagh, to tornadoes and the largest cyclone; every step of the gradation might be verified by numerous examples; and if this book were a treatise on meteorology, it might be admissible to give them ; but to do this would take up too much of my space, and I shall only now make some observations on the largest form of whirlstorm-the dreaded cyclone.
Just as over the little plain at Maryborough, protected by the surrounding forest from the action of the wind, the heated air accumulates over the surface until carried off in eddies, so, though on a vastly larger scale, in that great bight formed by the coasts of North and South America, having for its apex the Gulf of Mexico, there is an immense area in the northern tropics, nearly surrounded by land, forming a vast oceanic plain, shut off from the regular action of the trade-winds by the great islands of Cuba and Hayti, where the elements of the hurricane accumulate, and at last break forth. In this and such like areas, the lower atmosphere is gradually heated from week to week, and, as in Australia the quivering of the air over the hot ground foreshadows the whirlwind, and in Africa the mirage threatens the simoom, so in the West Indies a continuance of close, sultry weather, an oppressive calm, precedes the hurricane. When at last the huge vortex is formed, the heated atmosphere rushes towards it from all sides, and Ch. XVI.]
EXPLANATION OF CYCLONES.
is drained upwards in a spiral column, just as in the dust-eddy, on a gigantic scale. Unlike the air of the dust-eddy, that of the hurricane coming from the warm surface of the ocean is nearly saturated with vapour, and this, as it is carried up and brought into contact with the colder air on the outside of the ascending column, is condensed and falls in torrents of rain, accompanied by thunder and lightning.
I advanced this theory to account for the origin of whirlwinds in a paper read before the Philosophical Institute of Victoria, in 1857. It was afterwards communicated by the Astronomer-Royal to the London Philosophical Magazine, where it appeared in January 1859. A suggestion that I at the same time offered, that the opposite rotation of cyclones in the two hemispheres was due to the same causes as the westerly deflection of the trade-winds from a direct meridional course, has been generally adopted by physicists, and I am not without hopes that the main theory may also yet be accepted; but whether or not, I am confident that a study of the smaller eddies of air is the proper way to approach the difficult question of the origin of cyclones.