When these occur on a larger scale, they are called whirlwinds, and are often very destructive in their effects, unroofing houses, and doing various other mischief. They are sometimes occasioned by draughts of air being disturbed in their course by mountains or hills, and meeting each other. But the largest are caused by two or more currents of wind, produced by what ordinarily influences the direction of the wind, meeting from different quarters, and then twisting round each other just as two strings, with weights at their ends, would do if you swung them forcibly together so as to meet about the middle. A whirlwind occurred some years ago, near where I was living; it lasted about ten minutes, and produced some very curious effects. It first met with a milk-maid, who was carrying a pail of milk upon her head, and tore off her bonnet along with the pail, and carried both to a great distance, where they were not found till some days afterwards. It next twisted a wagon in pieces, and blew most of the fragments over a wall; it unroofed a house, and carried some of the tiles to a great distance; next it dashed through the window of the room where I was sitting, swept all the ornaments off the mantelpiece, and made strange havock with some of the furniture. It then passed on to a neighbouring park, where it tore up several trees. The wind had not been extremely violent before, neither was it immediately afterwards. Suppose a cloud happens to be exactly in the point of union of two currents of wind, meeting as they did in this whirlwind, it then becomes twisted in along with them, and partially condensed; and if it is over the land, this is all that seems necessary to form the water-spout. And if it happens to be over the sea, the wind, as it eddies round, works up the waves into a ferment, and much spray and foam is produced, which is twisted in with the whirlwind in the same manner as the cloud, and carried upwards to meet it. Whether this is just the way in which the thing takes place, or not, it is pretty certain that the water-spout is caused by the meeting of winds from various quarters, from what Dr. Franklin tells us in one of his letters which I read the other day. He says that a sailor informed him that he was in one of three vessels which chanced to be placed as at the three corners of a triangle; a water-spout was formed between them which seemed to be to the leeward of each of them. [Illustration] The cut will enable you to understand what I mean. The star in the middle represents the water-spout, and the three arrows the directions in which each of the three vessels found the wind come, which shows that three diverse currents of air all set towards the water-spout. CHAPTER VII. CORAL REEFS AND ISLANDS.