the poles, or the burning sands of the Great Desert; whether it belong in geological series to the youngest formation or to the primitive granite. Those only who have experienced that peculiar feeling which entirely destroys our faith in the stability of the earth, apparently so solid and compact, they alone can conceive the impression which even the slighter shocks cause within our breasts. The causes of earthquakes are merely conjectured, like those of thunder and volcanoes. They have been ascribed to a central fire, to a highly overcharged state of the electric fluid, to the sudden explosion of gases in the interior of the earth, and to various other theories. Giovanni Batista Beccaria, whose whole life was devoted to practical philosophy, ascribes the phænomenon to an accumulation of electricity in the crust of the earth which produces concussions with the clouds and developes the phænomenon of earthquakes. It has frequently occurred that severe shocks of earthquakes have accompanied hurricanes, and it is generally allowed that during the great hurricane in 1831 which almost destroyed Barbados, subterranean convulsions below the surface joined with the havoc above.

I considered the preceding general remarks on meteorological phænomena necessary in order to see how far they are borne out in Barbados. It will be requisite to observe the general changes which with certain modifications prevail in tropical regions. I have alluded to the theory which has been adopted for explaining the origin of the trade-wind. The change of the seasons under the tropics is closely connected with it. It has been assumed that a zone of variable breadth exists, situated within the regions of the south-eastern and north-eastern trade-wind, where calms and rains generally prevail, and which are only interrupted as if it were by striking contrasts; namely, by terrible thunder and lightning, by waterspouts, and such heavy rains, that the belt has been called by mariners "the rains." This rainy region lies between the equator and 5° north latitude; its extent is however liable to changes. Beyond its limits the winds become variable, and rain alternates with dry weather, according as the seasons of our terrestrial globe advance or

recede.

During the period that the north-eastern trade-wind prevails in the northern part of the tropics, the sky is serene, and the breeze generally prevents the atmosphere from acquiring the state of saturation. The stream of air from the north flows towards regions where the humid vapours which it carries are able to expand; they ascend, and are carried towards the poles, where their accumulation produces rain and changeable

'It must not be considered that constant calms or constant rains prevail within this region. The unfortunate M. La Perouse observes in this regard, "the tradewind left us in 14° north latitude, and the wind then constantly blew between west and west-south-west till we reached the line. We were not a day without wind, and once only had rain, when indeed it was so abundant as to fill twenty-five casks."

weather'. But when the sun approaches the northern limits of his annual course, the force of the trade-wind relaxes, and the streams of air coming from the north where now summer prevails, are warmer and charged with more humidity. The cessation of the breezes announces the season of rains; the sky loses its beautiful blue; a peculiar whitish haze veils distant objects; and if a change in the increase of the evaporation should take place, the vesicular vapours become visible. The hazy appearance vanishes, and distant objects are seen with a distinctness of which an inhabitant of a northern climate has no idea2; this distinctness of distant objects is the prelude to rain. Small clouds rise in the south-eastern horizon; they accumulate in size, and become opake in the middle. The slightest breeze dies suddenly away, and a perfect calm prevails. This calm attests the confluence of the trade-wind with variable winds, and its consequences are convulsions of the strata of air, electrical phanomena, storm and violent rains. That oppressive calm which for a short period intervenes during a violent hurricane, full of hope to those who are unacquainted with its nature, as a sign of the cessation of the conflict of the elements, is a warning of deep meaning to the experienced: it betokens that two contrary volumes of air have added new fuel to the hurricane which will then break loose from another quarter.

The rainy season may be considered as commencing in July, and it changes into drier weather in October. With the approach of the sun to the equinoxes, the trade-wind, which has been hitherto a point or a point and a half to the north of east, commences gradually to veer to the south. The effects of the sun's rays are more powerful when that luminary stands nearly in the zenith, than during the period when the rays reach the surface of the earth in an oblique position; the evaporation becomes therefore much stronger, and the atmosphere more humid; steel commences to rust; and leather-work, as shoes and boots, is covered with mildew.

The development of the causes which produce the ascent of warmer air from the surface to the upper regions, is much more rapid at the period when the sun reaches the meridian height than when only a few degrees above the horizon. The elasticity of every stratum of air, as it

1 The most evident proof of the existence of this counter-current has been given at the period of the eruption of the Souffrière in St. Vincent, when the ashes were carried through the upper air to Barbados, which lies nearly eighty miles to the eastward of St. Vincent.

2 I have seen on such an occasion the island of St. Crux from St. Thomas with such distinctness, that large buildings became visible, and the eye could distinguish what was land, and that which was under cultivation of the sugar-cane. St. Crux lies forty miles south of St. Thomas.

3 For some years past the regularity of the north-eastern breeze has very much changed. It is now more frequently to the south of east during periods where the breeze was never expected to come from such a direction.

rises from the surface, is increased by this circumstance in a much higher ratio than it is in the strata which rest upon the lower. The atmospheric pressure of the upper strata decreases rapidly; and as the air during the process of dilatation assumes a lower temperature, it becomes likewise more humid; it reaches the dew-point, and a precipitation of moisture or rain takes place. The sky is generally more cloudy during the hours when the sun passes the meridian than in the morning and evening. In Barbados a more rapid evaporation commences as soon as he has reached an altitude of ten or fifteen degrees, and it is seldom indeed that the sky is perfectly clear between the hours of eight in the morning and four in the afternoon. Every-day experience causes such observations to be lost; we become accustomed to see the sky clouded without further considering whether this was already the case when the sun was rising, or commenced at only a later period. It is different when an object is connected with a circumstance which obliges us to pay attention to the slightest change in the atmosphere; it will then become evident that the sky is seldom cloudless between the hours which I have stated. The general clouded state soon after seven o'clock in the morning, obliged me to take the altitudes of the sun for the determination of the hour angles as early as seven o'clock, if I wished to procure clear observations for astronomical purposes.

It is a very remarkable fact, that during that period the zone of “the rains" (as the calm interval between the trade-winds of the northern and southern tropics has been called) advances gradually further northward, and reaches its furthest northern extent in August. Purdy, in his 'Memoir of the Atlantic Ocean,' gives a table showing the equatorial limits of the north-east and south-east trade-wind, which rests upon the comparison of nearly four hundred nautical journals.

Table showing the Equinoctial Limits of the North-east and South-east Trade-wind between the meridians of 18° and 26° west.

North-east Trade-wind. South-east Trade-wind.

In August the extreme northern limit reaches the 15th° of north latitude, from thence it recedes towards the equator and reaches its least northern limit in December, when the extremes reach only the 7th°. The interval between the north-eastern and south-eastern trade-wind increases in breadth between the solstice and autumnal equinox. The month of August is the most sultry month, replete with thunder and heavy rains, and a great number of hurricanes have taken place during that interval. After the sun has crossed again the equator and moves towards the tropic of Capricorn, the zone of calms and rains decreases, and finer weather sets in. It is evident that this zone itself does not advance or recede, but the primary causes which lead to its existence. The summer in the temperate zone corresponds with the rainy season under the tropics. The air is more heated, and the flow from the northern regions towards the equator is not so constant: nor are these currents of air so low in temperature as during autumn and winter; the southern currents acquire therefore the ascendency in their passage from the antarctic they imbibe more humidity; and in consequence of their moistness and warmer nature, they are not qualified to act as counter-currents, or correctives to prevent the atmosphere under the equator from reaching the point of deposition or becoming saturated.

The surface of the soil becomes heated by radiation; much depends therefore upon the colour and nature of the soil. As a general mean, it has been assumed that the temperature of the surface, heated by the full effect of the sun's rays, amounts under the tropics to 126°5 Fahr. Humboldt observed the thermometer in the granitic sands at the cataracts of the Orinoco at 140°5. While descending the river Corentyne in small bark canoes in October 1843, I placed a thermometer of Troughton and Simms (the scale of which reached 142°) in the sun, on one of the thwarts of the canoe, but I was obliged to remove it as the quicksilver column reached the extent of the glass tube.

The calcareous nature of the soil in Barbados and its white colour operate negatively; the greatest heat which I have noted during my observations in Barbados, by placing the thermometer flat on the coral rock fully exposed to the sun, is 124° Fahr.-the temperature in the shade was at that time 88°.5. Fahr. Humboldt found the temperature of rocky islets composed of granitic gneiss in the midst of the tropical forest of the Orinoco during night, at 96°-8 Fahr., while the temperature of the air was only 78°.45 Fahr.

There are various circumstances which may contribute towards the formation of rain, and to which I have alluded in the preceding remarks. Temperature, pressure of the atmosphere, and its electrical state are chief agents; mountain-chains and forests form local causes. The effect which forests exercise upon the condensation of vapours has been ably treated by Daniell, in his 'Meteorological Essays '. (1827, pp. 230,

232, 278). Humboldt considers that forests exercise a triple influence upon climate: first, they protect the soil against the rays of the sun; secondly, they produce, by the vital activity of their leaves, a constant evaporation of aqueous vapours; thirdly, these leaves increase the radiation. These three simultaneous causes, as affording shade, evaporation and radiation, are so influential, that the knowledge of the extent of forests, compared with the naked savannahs, steppes and champaign ground, forms one of the most important elements in the climatology of a country. The active vitality of plants consists chiefly in the leaves; they are the organs of respiration, digestion, and nutrition. The great quantity of water which they perspire may be easily proved by placing a glass next the under-surface of a young vine-leaf in a hot day, and it will be found to perspire so copiously, that the glass will be in a short time covered with dew, which runs down in streams in half an hour. Hales computed the perspiration of plants to be seventeen times more than the human body; he calculated that the leaves of a single Helianthus three feet and a half in height covered 40 square feet; and comparing his former observation of the perspiration of leaves with this circumstance, Humboldt observes properly, if a plant of such a small size exercises influence upon evaporation, how much greater must be the perspiration of the forests of the Upper Orinoco, which cover 260,000 nautical square miles! The cloudy and misty sky of those regions and of the province of Las Esmeraldas, to the west of the volcano of Pichinche, the decrease of the temperature in the missions on the Rio Negro, and the streams of vapour which become visible on fixing the eyes on the top of the trees in the equatorial forest, must be alike ascribed to the aqueous exhalation of the leaves, and to their radiation towards the space of the atmosphere1.

"It is evident," says Dove, "that a vigorous vegetation produces its rain, which on the other hand nourishes again that vegetation; and that the senseless destruction of forests very often has destroyed the fertility of the soil. Previous to 1821, the Provence and the departement Varde possessed a superfluity of brooks and springs. In that year the olivetrees, which formed almost forests, were killed by frost, and they were cut down to the root in 1822, since which time the springs dried up, and agriculture suffered. In Upper Egypt, the rains, eighty years ago still abundant, have ceased since the Arabs cut down the trees along the Valley of the Nile towards Libya and Arabia. A contrary effect has been produced in Lower Egypt through the extensive plantations of trees by the present Pacha. In Alexandria and Cairo, where rain was formerly a great rarity, it has since that period become much more frequent."

1 Humboldt's Fragmens de Géologie et de Climatologie Asiatiques.