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The highest temperature takes place between one and two o'clock P.M. Radiation of heat influences the diurnal changes of the temperature. Leslie discovered that bodies possess very different powers of radiating heat; metals possess this quantity in an inferior degree to vitreous substances, and vegetable and fibrous substances are good radiators. A clear sky does not return any heat to the surface of the earth; it produces a chilling aspect, and this is the reason that clear nights are cold, and prolific of dews; a clouded sky does not produce any radiation, and very seldom dew. Humboldt and Bonpland enjoyed the freshness of the tropical nights on the grass-covered plains of the Lower Orinoco. Wells and Daniell found that the thermometer sunk on heath-covered plains 14° to 16° Fahr. Dr. Davy, Inspector-General of Hospitals (the brother of Sir Humphry Davy), has kindly favoured me with the numerical data of several observations respecting radiation in Barbados, which will be found in the Appendix.

This radiation continues during the whole night; it therefore becomes evident that the coldest moment is the period of sunrise (or more exactly, about fifteen minutes before sunrise). The temperature acquires at sunset about the mean height of the whole year.

The evaporation under the tropics is not so great as might have been supposed from the high temperature of these regions. The state of evaporation depends upon three elements, namely, the temperature, the expansive force of the vapours which the atmosphere contains, and the resistance which a greater or less density, or a more or less agitated current of air opposes to the dilatation of vapours. The quantity of water which evaporates in a given place bears an equal proportion to the masses of vapours which are really present in the atmosphere, and the quantity which the atmosphere in a state of saturation is able to support. From this it follows that the reason of the moderate evaporation under the tropics is to be ascribed to the presence of a great quantity of moisture in the air.

In consequence of the heat which is carried off by the water in a state of evaporation, cold is always generated by spontaneous evaporation; hence the freshness of the air in the neighbourhood of lakes, ponds, or rivers1.

It is assumed that the evaporation generally amounts to 36 inches in the year, or about one-tenth of an inch per day. Dalton has calculated that during a calm air, water loses in a minute about 0-9 of a grain in weight by evaporation, but a strong wind causes it to lose about 18 grain.

A wet towel or any other body is always cooled when exposed to dry air or a draft of wind. This is the reason why wine is wrapped under the tropics in wet cloths and exposed to the air. For the same reason the water in a porous jug is cooler than in one which is glazed.

The highest evaporation which I have observed according to Dalton's plan, amounted only to 30 grains per hour, or 0.5 per minute. This small quantity perfectly agrees with the assertion that the evaporation under the tropics is by no means greater than in Europe.

The quantity of aqueous vapours in the air, its variation in quantity, and its actual quantity existing in any given bulk of air, may be ascertained by a hygrometer. But the readiest means to ascertain these points, is the following:-Take two thermometers, previously tested to be alike under all circumstances; one having been covered with a wet cloth, let both be exposed to the air, and after a short time the height of the temperature of both be noted; the greater or less difference between the dry and the wet thermometer proves a greater or less quantity of aqueous vapours present in the air. If the air be very damp, the difference will be small; if very dry, the difference will be great1. The difference is least in the morning; it then gradually increases until an hour after the temperature has reached its highest point during the day, namely till about two o'clock, when it`gradually recedes again. The mean difference in the morning during fair weather amounts to 2° Fahr., and at two o'clock to about 8° Fahr. The greatest difference which I have noted in Barbados during my observations was on the 23rd of March 1846, at three o'clock P.M., and amounted to 13° Fahr.

On Hurricanes in general.-The causes of the most awful of nature's phænomena, and the most devastating in their effects, are unknown to us; theories have been formed which our present advance in meteorological science prove to be fallacious, and our feeble minds have not yet been able to divulge the secret workings of nature's destructive operations. Earthquakes, eruptions of volcanos, lightning, and in tropical climates the devastating effects of hurricanes, are covered with a veil which man has in vain attempted to remove.

The violence of severe storms was known in the earliest periods on record; but the first accounts of the tropical hurricane, that phænomenon in which all the fearful devastating powers of nature appear combined, were brought by Columbus after his discovery of the New World. His ships were lost during his second voyage in a tempest, which Peter Martyr informs us occurred in June 1494, commencing from the southwest; the wind blew with such violence "that it plucked up by the roots

1 A very easy method to ascertain the dew-point with sufficient accuracy for practical purposes is the following:-"Take the difference between the dry and wet thermometer after the latter has been swung in the air; multiply that difference by 103, and divide the result of this by the degrees of the wet thermometer; the result deducted from the degrees of the dry thermometer gives the dew-point. Supposing the dry thermometer stands at 60° Fahr., the wet thermometer at 514° Fahr., the difference is 84°, which being multiplied by 103 gives 8754; this being divided by 514 (the degrees of the wet thermometer) gives 17, which being deducted from the degrees of the dry thermometer, leaves 43° as the dew-point at that period.

whatsoever great trees were within the reach of the force thereof1." In the Appendix to this work will be found a list of hurricanes as far as I have been able to ascertain them from records. It proves how frequent they occur in the West Indian Archipelago; nevertheless their explanation has remained a mystery. Sir John Herschel makes the following observations in a note to his Astronomy:'-" It seems worth inquiry whether hurricanes in tropical climates may not arise from portions of the upper currents prematurely diverted downwards before their relative velocity has been sufficiently reduced by friction on, and gradually mixing with, the lower strata; and so dashing upon the earth with that tremendous velocity which gives them their destructive character, and of which hardly any rational account has yet been given. Their course, generally speaking, is in opposition to the regular trade-wind, as it ought to be, in conformity with this idea (Young's Lectures, vol. i. p. 704). But it by no means follows that this must always be the case. In general a rapid transfer either way, in latitude, of any mass of air which local or temporary causes might carry above the immediate reach of the friction of the earth's surface, would give a fearful exaggeration to its velocity. Whereever such a mass should strike the earth, a hurricane might arise; and should two such masses encounter in mid-air, a tornado of any degree of intensity on record might easily result from their combination"."

An approaching storm generally exercises some influence upon the barometer. Baron de Humboldt, in his excellent disquisition on the horary variations of the barometer in the tropics, alludes very pointedly to this effect. He also relates, that the north winds which are so impetuous in the Gulf of Mexico, cause the barometer to rise slightly, and produce afterwards most remarkable oscillations. "By inspecting the barometer, the proximity of the tempest, its force and duration, may be prognosticated with great probability." He adds in a note at page 749, "The hurricanes are not in general accompanied by such an extraordinary lowering of the barometer as is imagined in Europe. I possess fifty-six barometric observations made by the captain of a ship, Don Tomas de Ugarte, nearly from hour to hour at the Havannah during the terrible hurricane of the 27th and 28th of August 1794. When the tempest was most violent the column of mercury sunk only 5 lines. Kirwan asserts however that at the island of Saint Bartholomew, the barometer has been seen to lower in a hurricane (1792) 42 millimetres, 'equal to 1.65 English inch 4."

The observation that the barometer rises slightly before the violence of a hurricane is at its height, has been borne out by various observations.

1 Peter Martyr, Decades of the Ocean, 1st Decade, 4th book, p. 26.

2 Astronomy, by Sir John Herschel, in Lardner's Cabinet Cyclopædia, p. 132.

746.

3 Humboldt's Personal Narrative, English translation, vol. vi. p. Irish Transactions, vol. viii. p. 387.

While the originating cause is hidden in mystery, it appears to be beyond doubt that a hurricane is analogous to a whirlwind; and it is remarkable that the Indians in the interior of Guiana call the whirlwinds which so frequently arise on the Savannahs, Uranan'. We learn from Peter Martyr, that the dreadful hurricane which destroyed Columbus's vessels was called by the natives Furacanes 2. It is worthy of observation that these simple children of nature should have recognised the analogy between a whirlwind and a hurricane. The latter is no doubt a wind which gyrates around a centre. The generality of these masses of air revolve in the northern hemisphere from right to left, or they veer from east through north and north-west to south-west and south; but there are instances on record where the wind veered with the sun. Colonel Reid and Lieutenant Evans describe this motion by a comparison with the face of a watch; the dial-plate is the compass, and the hand the course of the wind; consequently the natural course of the wind would be with the motion of the hand from twelve to one, from one to two, and so on, or from east to south, from south to west, &c.; the unnatural or hurricane motion from twelve to eleven, from eleven to ten, or from east to north, from north to west, &c. Mr. Henry Davy, in his description of the voyage of H.M.S. Cornwallis from the West Indies in 1837, observes that the wind during the passage of the ship through the Gulf-stream veered in circles in a most extraordinary manner. The gale blowing from west to north-north-west, ceased at north; it then veered to north-east and east, with fine weather; blew strong at south-east-south and south-west, and commenced another gale as the wind completed its circle northward.

The extraordinary quantity of electricity in the air during these violent convulsions of nature, forcibly attract our attention as one of the causes, or as others pretend, as the effect of the contending elements. The accounts of the great hurricanes in Barbados prove in every instance upon record, the existence of large masses of electricity. Hughes, in describing the hurricane of the 31st of August 1675, observes that the lightning did not dart with its usual short-lived flashes, but in rapid flames skimming over the surface of the earth, as well as mounting to the upper regions. I could not employ better words to describe the scene I witnessed on the 12th and 13th of August 1830. I was then in the island of St. John's, and resided at Emaus, one of the Moravian stations in that island, when the gale commenced with great fury; it turned to the south-west, and a well-barricaded door of the house which was strongly built, was forced in by the blast. This gave me the opportunity of rushing on to the terrace, which faced Crux Bay and the ocean. The scene which presented itself to my eyes was awfully sublime. Black masses,—whether they were clouds, or of a more solid nature, I could form no idea,-rested

1 Journal of the Royal Geographical Society of London, vol. xiii. p.

67.

2

* Peter Martyr, 1st Decade, 4th book, p. 26.

on the bay; the sea, lashed into foam, seemed to strike against it; and flashes of vivid fire descended as it were from heaven and were instantly engulphed in the sea. The next moment they appeared from beneath the white foam, and apparently ascended towards the sky met by other masses hovering above. The howling of the storm, and a peculiar noise as if it were the rumbling of thousands of chariots, struck me with surprise and awe. The blast carried with it numerous small pebbles which struck with some force against my face. It is strange that during these moments Schiller's description of Charybdis flashed across my mind, and appeared realized before me. My kind friend the missionary forced me into the room: I am sure the time which elapsed from the moment the door was blown in, until the time when it was fixed again, was not ten minutes, nevertheless the quantity of water which was blown into the room had perfectly covered the floor. It must have been mostly sea-water, as the floor was covered with the efflorescence of salt next morning.

The height to which the foam of the sea is carried during a hurricane is astonishing; we must however remember that the rotatory motion of the blast would contribute in some measure towards this. It cannot be supposed that the gyrations act only on the surface of the water; they ascend, following their rotatory motion, and no doubt carry by gyration the sea-water in their course. During the severe gale which touched Tortola in 1831, I was residing with the late President Donovan at St. Bernard's, a hill the summit of which is about 1000 feet above the the dwelling-house, however, is at an elevation only of 920 feet. The day after the gale the leaves of the trees and plants in the garden which had remained became black, from the contact with the sea-water spray; indeed the trees appeared,

sea;

"As when heaven's fire

Has scathed the forest oaks, or mountain pines,

With singed top their stately growth, though bare,
Stands on the blasted heath ;"

and the rain-water in the cistern and vats, which was to be used for domestic purposes, was rendered brackish.

During my exploring expedition in Guiana, I observed in the valley of the river Wenamu, the ravages of a whirlwind which for the distance of several miles had perfectly cleared a belt 500 yards wide of all trees, and thrown them down with their heads towards all quarters of the compass; a rather steep hill and about 500 feet in height had opposed its course, but the belt continued in the same direction up the hill as in the valley. The course was from north-west to south-east, and the angle which that line formed with the hill-side was about 27°. It only continued for a short distance downwards on the opposite side, and from thence I could not trace it any further.

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