Ch. XVI.]

WHIRLWINDS AND CYCLONES.

305

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 Airy 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 probably correct. That air does not always rise when heated, may be proved by 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 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 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 tornados 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 dust 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 by the direct rays of the sun during the day, by radiation from the sea during the night; 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 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

Ch. XVI.]

EXPLANATION OF CYCLONES.

307

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 received 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.

CHAPTER XVII.

Cattle-raising-Don Filiberto Trano's new house-Horse-flies and wasps-Teustepe-Spider imitating ants - Mimetic speciesAnimals with special means of defence are conspicuously marked, or in other ways attract attention-Accident to horse -The Mygale-Illness-Conclusion of journey.

AFTER crossing the trachytic plain, we reached a large cattle hacienda, and beyond, the river Chocoyo, on the banks of which was some good, though stony, pasture land. We saw here some fine cattle, and learnt that a little more care was taken in breeding them than is usual in Nicaragua. The country, with its rolling savannahs, covered with grass, is admirably suited for cattle-raising, and great numbers are exported to the neighbouring country of Costa Rica. Scarcely any attention is, however, paid to the improvement of the breeds. Few stations have reserve potreros of grass; in consequence, whenever an unusually dry season occurs, the cattle die by hundreds, and their bones may be seen lying all over the plains. Both Pará and Guinea grass grow, when planted and protected, with the greatest luxuriance; and the latter especially forms an excellent reserve, as it grows in dense tufts that cannot be destroyed by the cattle. When not protected by fencing, however, the cattle and mules prefer these grasses so much to the native ones, that they are always close-cropped, and

Ch. XVII.]

A PEASANT FARMER.

309

when the natural pasturage fails there is no reserve of the other to fall back on. I planted both the Pará and Guinea grasses largely at the mines and at Pital, and we were able to keep our mules always in good condition with them.

About four o'clock in the afternoon our animals were getting tired, and we ourselves were rather fatigued, having been in the saddle since day-light, with the exception of a few minutes' rest at Tierrabona. We halted at a thatched cottage on some high stony savannah land, and were hospitably received by the peasant proprietor, Don Filiberto Trano. He informed us that we had entered the township of Teustepe, and that the town itself was eight leagues distant. The family consisted of Don Filiberto, his wife, and four or five children. They had just prepared for their own dinner a young fowl, stewed with green beans and other vegetables, and this they placed before us, saying that they would soon cook something else for themselves. We were too hungry to make any scruples, and after the poor, coarse fare we had been used to, the savoury repast seemed the most delicious I ever tasted. I think we only got two meals on the whole journey that we really enjoyed. This was one, the other the supper that the padre's housekeeper at Palacaguina cooked for us, and I have recorded at length the names of the parties to whom we were indebted for them.

Don Filiberto had about twenty cows, all of which that could be found were driven in at dusk, and the calves tied up. As they came in, the fowls were on the look-out for the garrapatoes, or ticks; and the cows, accustomed to the process, stood quietly while they flew up and picked them off their necks and flanks. The calves are