Ch. XVI.] TIGER-BEETLES AND BUTTERFLIES. 299 ditches that were seen here and there amongst the brushwood. As we got further south the alluvial flats in the valleys increased in size and fertility, and the cultivated fields were enclosed with permanent fences. On some of the ranges we crossed the rocks were amygdaloidal, containing nests of a white zeolite, the fractured planes of which glittered like gems on the pathway. Eight leagues from Matagalpa we reached the small town of Tierrabona, where, as the name implies, the land was very good. Every house had an enclosure around it, planted with maize and beans; and though it was evident that the land was cropped year after year, it still seemed to bear well. We stopped at a small brook just outside the town, and eat some provisions we had brought from Matagalpa. Some speckled tiger-beetles ran about the dusty road; and on wet, muddy places near the stream groups of butterflies collected to suck the moisture. Amongst them were some fine swallow-tails (Papilio), quivering their wings as they drank, and lovely blue hairstreaks (Thecla). The latter, when they alight, rub their wings together, moving their curious tail-like appendages up and down. Great dragon-flies hawked after flies; and on the surface of still pools "whirligigs " (Gyrinida) wheeled about in mazy gyrations, just as they are seen to do at home. Savannahs, sparingly timbered, were next crossed; then we reached one of those level plains, with black soil and blocks of porus trachyte lying on the surface, which are swamps in the rainy season, and have for vegetation sedgy grasses and scattered jicara trees, cactuses and thorny acacias. Up to the time we passed, there had been no rain in these parts, and the plain was The dry and bare, with great cracks in the black soil. grass had not sprung up; not a breath of air was stirring; and the heated air quivered over the parched ground, forming in the distance an imperfect mirage. Directly overhead the noonday sun hung in the hot, hazy sky. As we moodily toiled over the plain, my attention was arrested by a dust whirlwind that suddenly sprung up about fifty yards to our left. The few dry leaves on the ground began to whirl round and round, and to ascend; and in a minute a spiral column was formed, reaching, perhaps, to the height of fifty feet, consisting of dust and dry dead leaves, all whirling round with the greatest rapidity. The column was only a few yards in diameter; and it moved slowly along, nearly parallel with our course, but only lasting a few minutes; so that before I could point it out to Velasquez, who had ridden on ahead, it had dissolved away. I had been very familiar with these air eddies in Australia, and had hoped to carry on some investigations concerning them, begun there, in Central America; but, though common on the plains of Mexico and of South America, this was the only one I witnessed in Central America. The interest with which I regarded these miniature storms was due to the assistance that their study was likely to give in the discussion of the cause of all circular movements of the atmosphere, including the dreaded typhoon and cyclone. All the chief meteorologists who have discussed this difficult question have approached it from the side of the larger hurricanes. There is a complete gradation from the little dust eddies up through larger whirlwinds and tornados to the awful typhoons and cyclones of China and the West Indies; Ch. XVI.] WHIRLWINDS AND CYCLONES. 301 and it has long been my opinion that if meteorologists devoted their attention to the smaller eddies that can be looked at from the outside, and their commencement, continuance, and completion watched and chronicled, they could not fail to obtain a large amount of information to guide them in the study of all cyclonic movements of the atmosphere. Unless the smaller whirlwinds are quite distinct from the larger ones in their origin, the theories advanced by meteorologists to account for the latter are certainly untenable. According to the celebrated M. Dove, cyclones owe their origin to the intrusion of the upper counter trade-wind into the lower trade-wind current.* More lately, Prof. T. B. Maury has stated that "the origin of cyclones is found in the tendency of the south-east tradewinds to invade the territory of the north-east trades by sweeping over the equator into our hemisphere, the lateral conflict of the currents giving an initial impulse to bodies of air by which they begin to rotate." Cyclones having thus originated, Prof. Maury considers that they. are continued and intensified by the vapour condensed in their vortex, forming a vacuum.† Humboldt had long ago ascribed whirlwinds to the meeting of opposing currents of air. There is this dynamical ‡ objection to the theory. The movements of the air in whirlwinds is much more rapid than in any known straight current, such as the trade winds; and it is impossible that two opposing currents should generate between them one of much greater force and rapidity. * "Law of Storms," p. 246. "Quarterly Journal of Science," 1872, p. 418. But even if this fundamental objection to the theory could be set aside, the small whirlwinds could not thus arise, as they are most frequent when the air is nearly or quite motionless. Then, again, when we turn to Prof. Maury's theory that the cyclones, having been initiated by the conflict of contrary currents, are continued and intensified by the condensation of vapour in their vortex forming a vacuum, we find it negatived by the fact that in the smaller whirlwinds the air is dry, and there is consequently no condensation of vapour; and yet, in comparison with their size, they are of as great violence as the fiercest typhoon. Tylor describes the numerous dust whirlwinds. he saw on the plains of Mexico,* Clarke those on the steppes of Russia, and Bruce those on the deserts of Africa; and nowhere is there mention made of any condensation of vapour. I myself have seen scores of whirlwinds in Australia, some of them rising to a height of over one hundred feet; yet there was never any perceptible condensation of vapour, though some of them were of sufficient force to tear off limbs of trees, and carry up the tents of gold-diggers into the air. Franklin describes a whirlwind of greater violence than any of these. It commenced in Maryland by taking up the dust over a road in the form of an inverted sugar-loaf, and 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. Ch. XVI.] WHIRLWINDS AND CYCLONES. 303 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 eyewitness, 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. In Australia I had many opportunities of studying the dust whirlwinds; and as I looked upon them as the initial form of a cyclone, I 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, * "Nature," vol. vi. p. 541. |