round five times in the space of one hundred and seventeen hours, having an average velocity of eleven miles and seven-tenths per hour. The whole distance thus sailed by the vessel was thirteen hundred and seventythree miles; while her actual progress during this time in a south-westerly direction, was found to be only three hundred and fifty-four miles.

132. AXIS OF THE HURRICANE. The axis of the hurricane is not, necessarily, upright, but is usually inclined to the horizon; leaning in the direction which the tempest takes. This is owing to the friction of the base of the hurricane against the surface of the earth. Its velocity is thus checked, while the upper portion is driven forward, and overhangs the base.

This position of the axis is indicated by the circumstance that the tokens of the approaching tempest often appear in the higher regions of the atmosphere, before it is felt below. The navigators of the tropic seas sometimes behold, high in the ter, a small black cloud; rapidly it spreads down to the horizon, shrouding sea and sky, and the tempest then suddenly descends upon them in all its fury.

133. REMARKS. Such are the opinions entertained by Redfield, Reid, Dove, and others, in regard to storms and hurricanes; opinions based upon a vast assemblage of facts and observations, gathered from all points, within the track of a great number of these desolating gales. The numerous observations taken upon the American coast, commensurate with the extent of the Atlantic tempests, have been systematized by Mr. W. C. Redfield, of New York; while Col. Reid has investigated the West India hurricanes, and those of the southern hemisphere, with great success. The logbooks of the British navy, in which the phenomena of the weather are recorded every half hour, have been

What is the position of the axis of the hurricane?

How is it caused?

How is this position sometimes indicated?

Detail the labors of Redfield, Reid, and Dove.

placed at his disposal, and he has thus been furnished with an immense collection of valuable facts. Prof. Dove, of Berlin, has studied the laws of hurricanes in Europe, and gathered a large number of observations from every quarter of the globe. By noticing the time and place of each observation, storm-charts have been constructed for the use of mariners, and it is highly in favor of the rotary theory, that the conclusions result ing from these extensive and independent investigations are substantially the same.

134. ESPY'S THEORY. The rotary character of hurricanes, including tornadoes and water-spouts, is however denied by Mr. Espy, of Philadelphia, who maintains that the wind blows from every quarter towards the centre of the storm. Espy asserts, that this law obtains without a single exception, in seventeen storms which he has investigated. The influx of wind towards the centre, he supposes to be caused by the development of heat, which occurs whenever atmospheric vapor is condensed in the form of a cloud. The heat, thus disengaged, rarefies the surrounding air, and establishes an upward current; and so great an expansion is believed, at times, to result from this cause, that the velocity of the ascending current has been computed to exceed three hundred and fifty feet per second.

To this point of greatest rarefaction, the atmosphere rushes in from every side, just as the air of a room flows towards the heated current of the chimney; the violence of the wind depending upon the rate of speed in the ascending column. Most of the phenomena of meteorology are also explained by Mr. Espy in accordance with his peculiar views.

135. The centripetal theory has found many able supporters; but that of Redfield and Reid has been more generally adopted by men of science.

136. It may perhaps be found, when our investigations are multiplied and more extended, that both these

Detail Mr. Espy's theory.

Which theory has been more generally adopted?

May these two motions co-exist?

motions often co-exist; a circumstance which is by no means impossible. For when a whirlwind is once in motion, from any cause whatsoever, the great rarefaction of air that occurs at the centre, will create an influx of the atmosphere towards this point from all quarters, except where it is opposed by the centrifugal force. Now if the base of the whirl is above the surface of the earth, or when touching it, is inclined to it, (which is usually the case,) currents of air will flow beneath the base towards the vortex, and evidences of centripetal action will not be wanting.

CHAPTER III.

OF TORNADOES OR WHIRLWINDS.

137. Tornadoes may be regarded as hurricanes, differing chiefly in respect to their extent and continuance. They last only from fifteen to sixty or seventy seconds, their breadth varies from a few rods to several hundred yards, and it is probable that the length of their track rarely exceeds twenty-five miles.

138. FACTS. This phenomenon is usually preceded by a calm and sultry state of the atmosphere; when suddenly the whirlwind appears, traversing the earth with great velocity, and sweeping down by its tremendous power the mightiest products of nature, and the strongest works of man. Ponderous bodies are whirled aloft into the air; trees of large dimensions twisted off or torn up by the roots; buildings of the firmest construction prostrated, and streams whirled from their beds and their channels laid bare. A whirlwind that occurred in Silesia, in the year 1820, carried a mass weighing more than 650 lbs., fifty feet above the top of a house, and

What are tornadoes?
Describe their effects.

By what phenomena are they attended?

deposited it on the other side in a ditch, one hundred and fifty paces distant.

139. In 1755 a tornado fell upon the village of Mirabeau, in Burgundy, laying dry the channel of the small river by which it is traversed, and carrying the stream to the distance of sixty paces. In the New Haven whirlwind of 1839, and in that which occurred at Chatenay, near Paris, during the same year, trees eighteen inches in diameter were torn up by the roots. In one which happened at Maysville, Ohio, in 1842, a barn containing three tons of hay and four horses, was lifted entirely from its foundations. And such was the force of the wind during a tornado which occurred at Calcutta in 1833, that a bamboo was driven quite through a wall five feet thick, covered with masonry on both sides; an effect which was estimated, by a person on the spot, to be equal to that produced by a cannon carrying a six-pound ball.

By the action of a tornado, fowls are often entirely stripped of their feathers, and light substances carried to a distance varying from two to twenty miles.

140. The whirlwind is attended by all the usual phenomena of thunder-storms; showers of hail frequently occur, and, at times, it is the scene of very extraordinary electric appearances. In the one which happened at Morgan, Ohio, on the night of the 19th of June, 1823, a bright cloud of the color of a glowing oven, and apparently half an acre in extent, was seen moving below the dark canopy of the tempest. It shone with a splendor above that of the full moon, and ten minutes after its passage, the narrator of the phenomena was enabled to read his Bible by its light. Just before the Shelbyville tornado, which took place at midnight, on the 31st of May, 1830, two luminous clouds were seen approaching each other, of the color of red hot iron; for a moment they united above the town, extending over it like two fiery wings, and, at the next, rushed down to the earth: at this instant the whirlwind burst in all its fury upon the devoted spot. The writer

What extraordinary appearances are sometimes seen?

of this work was informed by an eye-witness, that, during the prevalence of the storm, so incessant was the play of the lightning, that the titles of books could be easily read, and the use of lamps was discarded in going to different parts of the house.

141. ORIGIN. Several theories have been advanced to explain the causes of whirlwinds, but they are supposed to be generally produced by the lateral action of opposing winds, or the influence of a brisk gale upon a portion of the atmosphere in repose; in a manner analogous to the eddies that arise at the junction of two streams, flowing with unequal velocities, or the airwhirls that occur, when a wind sweeps by the corner of a building, and strikes the calm air beyond it.

142. The existence of such opposing currents is fully proved by the observations of aëronauts, as well as by those of observers at the surface of the globe.

The whirl appears to originate in the higher regions of the atmosphere, and as it increases in violence, to descend; its base gradually approaching until it touches the earth.

Thus, when on the summit of the Rigi-a mountain in Switzerland-Kaemtz beheld two masses of fog approaching each other, in the valley of Goldan, while the air around him was calm, and the sky serene. As soon as they united, a gyratory motion was perceived, the fog rapidly extended, accompanied with violent gusts of rain and hail. At the same time, (as appeared from subsequent information,) a furious storm fell upon the lake of the Four Cantons, far below; in the midst of which a water-spout was seen. (Art. 150.)

Extensive

143. WHIRLWINDS EXCITED BY FIRES. conflagrations have been known also to produce whirlwinds, in consequence of the strong upward current, resulting from the great expansion of the heated air.

A remarkable instance of this kind occurred between

How do they originate, and where?
What did Kaemtz witness?

What is the effect of extensive fires?