We should consequently expect to find (in accordance with the laws of circular motion) the density of the air increasing from the centre to the circumference of the storm, and even for some distance beyond its boundary; and likewise, that when a hurricane passed diametrically over any region, the atmospheric pressure would decrease, and the barometer continue to sink, during the first half of the storm; but that the instrument would gradually rise, as the last half passed over. Such indeed is the case; for, amid all the phenomena of storms, no fact is better established than this, that an extraordinary depression of the barometer in tropical climates is a sure forerunner of a hurricane.

128. Before the tempest of Aug. 2d, 1837, the harbormaster of Porto Rico warned the shipping in port to prepare against a storm, as the barometer was falling in an unusual manner; having sunk one and a half inches since 8 o'clock in the evening of the preceding day. All precautions were however in vain; thirty-three vessels at anchor were destroyed, and, at St. Bartholomews, two hundred and fifty buildings levelled to the earth. The following table of observations, taken at St. Thomas, over which island this hurricane passed, is full of instruction in regard to this important point.

Is this the case? Relate the instances given.

130. In the case of the Water Witch, we have seen, that, when the centre of the tempest was past, and the gale abated, the barometer rose an inch.

131. CIRCUIT SAILING. The gyratory motion of hurricanes is strikingly evinced by vessels sailing on a circular course, when scudding before the wind. The most remarkable case is that of the Charles Heddle, related by Mr. Piddington, which occurred in a storm, near Mauritius, in Feb. 1845.

It appears from the log-book of this ship, that, in her course before the gale the wind changed completely

What example is given of circuit sailing?

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 air, 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?

an

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 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?