Great Barrington and Stockbridge, Mass., in the month of April, 1783, and is thus related by Mr. T. Dwight, who beheld it. "In an open field, a large quantity of prush-wood was lying in rows and heaps for burning, perfectly dry and combustible. On a certain day, when the atmosphere was entirely calm, the brush was ignited on all sides of the field at once. I was residing at this time, at the distance of about half a mile from the fire, when suddenly my attention was aroused by a loud, roaring noise, like heavy thunder. Upon going to the door, I beheld the fire covering the field, and the flames collected from every side into a fiery column, broad at the base, tapering upward, and extending to the height of 150 or 200 feet. This pillar of flame revolved with an amazing velocity, while from its top proceeded a spire of black smoke, to a height beyond the reach of the eye, and whirling with the same velocity as the fiery. column. During the whole period of its continuance, the column of flame moved slowly and majestically around the field. The noise of the whirlwind was louder than thunder, and its force so great, that trees six or eight inches in diameter, which had been cut, and were lying on the ground, were whirled aloft to the height of forty or fifty feet." 144. During the terrible conflagration of Moscow, in 1812, the air became so rarefied by the intense heat, that the wind rose to a frightful hurricane; the roar of the tempest being heard even above the rushing sound of the conflagration. 145. RESULTS OF CENTRIFUGAL ACTION. By the centrifugal action of the whirl, the air is driven outward, as in the case of hurricanes, and at the same time spirally upwards, on account of the pressure of the surrounding atmosphere: a great rarefaction, therefore, occurs at the centre. As long as the base of the whirlwind is above the ground, the warm air of the earth will stream under and upwards, into this partial void Give the cases. State the result of centrifugal action, from every quarter; while, at the same time, the cold air will descend into it from the higher region of the atmosphere. By this union, a powerful condensation of vapor occurs; causing the precipitation of rain and hail, and the development of electricity. 146. These, however, constitute no essential part of a whirlwind; for, if the currents of air that give rise to this phenomenon are very dry, the violence of the wind is the only remarkable circumstance. This was shown in the case of a small whirl, which involved two persons, who were going one cloudy day from Halle to Giebichenstein. Suddenly they were separated by a gust of wind; one being driven against a wall, and the other thrown into a field; while the people who were near had not discerned the slightest disturbance in the atmosphere. 147. When the base of the whirlwind descends to the earth, it touches the surface, either partially or wholly, according as the axis is inclined or vertical. In the first case, the inward flowing currents will be partially, and in the second entirely, arrested by the centrifugal action of the storm. The same results often occur when it covers a building. Hence, the atmosphere becomes exceedingly rarefied, both above, and around the edifice; and if it hap pens to be closed, and the tornado is violent, its walls will be burst outward by the sudden expansion of the air within, (C. 509.) Just as a sealed bottle of thin glass, under the exhausted receiver of an air-pump, is shivered by the elastic force of the enclosed air. 148. EFFECTS OF EXPANSION. In the tornado that happened at Natchez, in 1840, the houses exploded wherever the doors and windows were shut; the roofs shooting up into the air, and the walls, even of the strongest brick buildings, bursting outward with great Are rain, hail, and electricity necessary to the production of awhirlwind? Why are buildings burst outward by the action of tornadoes? force; but no such destruction occurred when a free outlet was afforded to the air within. One gentleman as the storm approached, caused all the windows and doors of his house to be opened, and though its structure was frail it experienced no injury; not even a single pane of glass being broken. 149. On the 18th of June, 1839, a whirlwind (to which we have alluded) fell upon the village of Chatenay, near Paris. In the room of a house, over which it passed, several articles of needlework were lying upon a table: the next day some of them were found in a field, at a great distance from the house, together with a pillow-case taken from another room. They must have been carried up the chimney by the rush of air outwards, as every other means of exit was closed. Another singular illustration of the fact before us took place in the Shelbyville tornado. Soon after its occurrence, a lady missed a bonnet, which, the day before the storm, was lying enclosed in a bandbox in her chamber; some weeks afterwards, she accidentally observed a ribbon hanging from the chimney, which proved to be the string of her bonnet. The house had been closed during the storm, and the expansion of the air within the bandbox had forced off the lid—the lost article had been borne by the outward flowing current up the chimney, which afforded the only mode of egress, and there it had lodged. مامایی CHAPTER IV. WATER-SPOUTS. 150. A WATER-SPOUT is a whirlwind over an expanse of water, as the sea or a lake, differing from a land-whirl in no other respect than that water is subjected to its action, instead of the bodies upon the surface of the earth. 151. A water-spout usually presents the following Define a water-spout. successive appearances. At first it is seen as an inverted cone, either straight or slightly curved, extending downward from a dark cloud to which it seems to be attached. As the cone approaches the surface of the water, the latter becomes violently agitated, and, rising in spray or mist, is whirled round with a rapid motion. As the cone descends lower the spray rises higher and higher, until both unite, and a continuous column is formed extending from the water to the clouds. The spout is now complete, and appears as an immense tube, possessing both a rotary and progressive motion; bending and swaying under the action of the wind as it advances on its course. When the observer is near, a loud, hissing noise is heard, and the interior of the spout seems to be traversed by a rushing stream. After continuing a short time the column is disunited, and the dark cloud gradually drawn up; for a while a thin, transparent tube remains below, but this at last is also broken, and the whole phenomenon then disap pears. These successive changes are represented in figures 9, 10, 11, which are taken from sketches of waterspouts actually seen. |