OBSERVATIONS ON THE CLIMATE.
THE Climate of ST. HELENA is pure and salubrious, and the temperature very moderate, for an island situated within the Torrid Zone. As its surface consists chiefly of rock, unsheltered with wood, which is exposed twice in the year to the perpendicular rays of the sun, and which is besides parched with long, continued droughts, we might easily suppose that it must be subject to great heat : Yet this never rises to excess and such is the peculiar felicity of the climate, that it is entirely exempted from all those severe agitations of Nature, which occasionally afflict and desolate so many other tropical islands.
From its great elevation and variety of surface, there is a diversity of climate at different heights - and at all seasons, the temperature of the high interior parts is from twelve to fifteen degrees below that of the valleys on the shore. The medium heat appears to be about 69, or probably a little lower ; for the thermometer on the heights sometimes sinks under 54, and in JAMES'S VALLEY it is said never to rise above 84. It rarely, and only for a short period, reaches this point in the valleys ; while on the heights, it frequently sinks to 54. It is difficult, however, to ascertain these matters accurately, without an opportunity of long observation; which as the writer of this wanted, he was obliged, to take some things from the report of the inhabitants', after comparing them with what he observed himself, in the months of July and August, while he stayed at ST. HELENA, the thermometer in JAMES'S VALLEY never fell below 68, nor rose above 72. This was the cool season ; and on the interior heights, during the same period, the temperature was fifteen degrees lower. On comparing the result of these observations with the information received from some of the inhabitants, who had paid attention to this subject, it seems probable, that the whole range of the thermometer here, taken at different heights, and for the period of a year, may be from 52 to 84.
Within these limits of temperature, which are found as favourable to the health and longevity of the inhabitants, as they are well adapted to cherish the growth of the various productions of remote climates, the hills and valleys of ST. HELENA are preserved by the influence of the south-east trade wind. This continually blows over the island ; and except for a short period, when the sun is vertical, blows with a steady and uniform current, overspreading the heights with light haze and vapour, and moderating the reflected heat of the subjacent valleys.
The land and sea breezes of tropical countries, arising from the variation which occurs every twenty-four hours in the temperature of the land and water, can have no place in an island, whose extent and influence are too inconsiderable to alter or modify the course of the trade wind, which keeps it so nearly attempered to the heat of the surrounding atmosphere and ocean. The refreshing effects of this steady gale are far more lasting and beneficial with respect to health, than those of the sea and land breezes of other warm countries: for even in situations, where these blow with the greatest regularity, they are preceded and followed by intervals of extreme closeness and sultriness ; besides that, the wind blowing from the land, is not of equal salubrity with, that which comes from the surface of the ocean.
There is no other wind so uniform and constant as that denominated the Trade. Deriving its motion from the diurnal revolution of the sun, and pervading a wide extent of sea, where it is not subject to disturbance from the influence of contiguous lands, it maintains an even and settled course, becoming lighter and stronger in different situations, without the intervention of storms or calms. In those parts of the ocean where it chiefly prevails the weather is mild, serene, and settled. There are no heavy gales or hurricanes, seldom rain ; and the ordinary phenomena of thunder and lightning rarely occur. The sky, which during the prevalence of close sultry heats was naked and torrid, or muffled up in a dead white haze, is now overspread with light fleecy clouds, which settling round the verge of the horizon, give a milder aspect to the rising and setting sun. Instead of those vast accumulations of clouds, which to frequently gather in a close and burning atmosphere, and burst forth in sudden and violent tempests, the season of the trade wind is only attended with small showers, light haze, and vapour. This settled and uniform state of things is somewhat disturbed by the approach of the sun to the zenith, which occasions temporary calms, during which, the clouds collect and give rise to storms, attended with violent and opposite gusts of wind. But the departure of the sun from the zenith restores the regular-current of the trade wind; and the serene weather of these latitudes, which had only suffered a short interruption, returns to its accustomed uniformity. It is, however, only meant here, to speak of those parts remote from the equator, where the southeast trade wind prevails.
Such are the general appearances of that benign and tranquil region, where, at a vast distance from every other land, ST. HELENA is descried in the solitude of the ocean. Being of an extent too inconsiderable to affect or modify the general course of the weather, which predominates in these latitudes, it enjoys the same settled serenity of climate, the same exemption from storms, and the same unvarying revolution of seasons, which prevail through all the interior parts of the Ęthiopic. It has no other wind besides that of the Trade ; it is never visited by hurricanes; and one may reside on it for several years, without observing the phenomena of thunder and lightning.
The principal inconvenience of this fine climate (for the most delightful retreats of Nature have their disadvantages) arises from a want of rain, which proves a great obstruction to the improvement of the soil, and not unfrequently a severe scourge : For the rains, which are always too scanty here, have been sometimes so deficient, that a continued drought , of three years has been known, which has destroyed the cattle, killed many of the trees, and withered every appearance of vegetation.
Rains are seldom wanting in those lands which are exposed to the influence of winds, blowing immediately from a great extent of sea - and in such situations, more inconvenience is often sustained from immoderately wet than from dry seasons. On this account, it seems at first surprising, that an island of such elevation, lying in a warm climate, where so much moisture is continually exhaled from the circumfluent ocean, should be parched with excessive drought. The circumstances of its situation, we should think, must prove a never-failing source of humidity; and that instead of being immoderately dry, it ought to be drenched with excessive rain, as every wind which can blow upon it, must come charged with the vapours and exhalations of the Ęthiopic. A mountainous rock, rising out of the waters to the height of near 2,700 feet, we might, a priori, imagine likely to become the centre of attraction to these exhalations, which, settling, and condensing round its summit, would burst in frequent showers and storms. The fact, however, is otherwise ; ST. HELENA presents the singular phenomenon of a land, embossomed in the ocean, and yet suffering as severely from drought as if it lay in the middle of a sandy desert.
Of this extraordinary dryness and deficiency of rain, occurring in a situation where so much moisture is continually exhaled, and where there are no parched or arid winds, if we inquire into the causes, we shall probably find them to be:
1. The great uniformity of the temperature, and the constancy of the trade wind.
2. The want of land and sea breezes, and of regular periodical winds.
3. The remoteness of other lands, and the inconsiderable size of the island itself
4. The nakedness of its surface.
The last cause only, it will be observed, is within our controul, and admits of a remedy. If utility were the sole object of our inquiry, we ought, perhaps, to confine ourselves to the consideration of this cause alone, and of the best means of improving and cultivating ST. HELENA. Yet it will be a matter of curiosity to consider, what influence the other causes have; and in this way we may be better able judge, how far this influence is likely to be counteracted by any change which can be produced on the surface of the island itself.
But it will be difficult to explain, in what manner some of the preceding causes operate in preventing rain, without a reference to what takes place in other tropical climates; and as the writer is best acquainted with that of the peninsula of INDIA, and of the adjacent islands, he thinks that light may be thrown on this subject, if some circumstances in the climate of INDIA and that of ST. HELENA are set in comparison. He hopes that he need not make any apology here for introducing some curious facts, which seem to him to illustrate this subject and which he may not otherwise have an opportunity of delivering to the public.
An equable temperature, exempted from the extremes of heat and closeness, and the uniform prevalence of the same wind, seem not favourable to the production of rain, which in INDIA, and probably every where else within the tropics, is observed to take place, in circumstances exactly opposite, that is, from the heat rising occasionally to great excess, and from, the effect, of periodical winds, which blow or succeed each other, in contrary directions. In the CARNATIC, which is remarkable for the equability of its temperature, five or six months sometimes pass without a shower; and during this period, the weather is serene, the winds steady and uniform; and so small is the difference between the temperature of the night and day, that there are no perceptible dews, for the atmosphere, in such a state, does not part with its latent moisture: Yet in this situation, extraordinary degrees of heat and closeness are always productive of occasional rain and such is the invariable course of things, in one of the most regular climates in the world, that the temperature never rises to very great excess, without being followed by storms or showers. Very often, in the midst of dead, close heat, while the sun strikes with insupportable intensity, a small dark, cloud makes its appearance, which suddenly increasing, seems to convert the whole face of the sky into an overwhelming tempest of rain, the dispersion of which through a burning atmosphere, by the opposite gusts of wind, which then take place, is the great instrument employed by Nature, to mitigate the fervour of the torrid zone: For in this sultry region, the winds, at particular periods, acquire a degree of heat and dryness, which would shortly prove fatal to every species of life, if such storms did not seasonably arise to disarm them of their malignity. But the excess of heat never fails to bring with is the means of counteracting its increase and continuance; and it is observable, that after heavy storms, the temperature continues moderate for many days. The wind too, which before was so parched and arid, now blowing over a surface, recently drenched with rain, becomes as mild and refreshing as the breeze of the ocean.
While Nature thus relieves the extraordinary heats of INDIA, by the gathering and dispersion of heavy storms, she mitigates and counteracts the sultriness, incident to some particular seasons, by the effects of frequent showers. It is surprizing how regular this course of things is, at some periods, especially in the close months of April and September, when it is not unusual for rains to, occur every afternoon, if the heat and sultriness of the day have been considerable. Yet rains, at this season, never take place in a morning, and very-rarely at night. The afternoon showers seem, to be the effect of each day's heat, and, proceed from clouds, which collect and discharge themselves within the visible horizon. For a considerable time after sun rise, no clouds are to be seen; but in the heat and closeness of the forenoon, small specks are observed to gather all round the lower sky, and not in the direction of any particular wind. These increase in size with the increasing heat of the day, and coalescing, form a continued belt or zone all round the horizon. This, in the afternoon or evening, blackens in different parts, and falls in rain. Sometimes the whole produces rain; though this, in general, is confined to particular quarters, from some one of which the lightning breaks forth, and the wind shortly after taking, its direction from the same point, blows delightfully coot and refreshing. After sun-set, these clouds subside beneath the horizon ; and the night is bright and starry. This succession of appearances frequently lasts for several weeks together, during which the mornings are always fair, the afternoons cloudy, the evenings showery, and the nights clear.
In these circumstances, the occasional, rains of INDIA usually take place. But such accidental storms and showers, arising out of extreme sultriness, and during the intermission of general winds, are seldom likely to occur in a situation where the temperature is so uniformly moderate, and where the Trade wind so rarely, and for so short a period, intermits its constancy. Yet, ST. HELENA is not entirely exempted from some occasional appearances of this sort ; for the inhabitants observe, that once in ten, twelve, or fourteen years, they are visited by a storm, attended with thunder and lightning, and such a deluge of rain, that it does great mischief, by loosening and dislodging the impending rocks, and sweeping away many of the little farms and gardens, which are situated on the declivities. When this happens, it is in the hot season, and during some transient intermission of the Trade wind; and it is only on occasion of such storms, which recur at very distant intervals, that there is any thunder and lightning. How rarely they have occurred, the state of the island seems a sufficient proof: For had such storms been frequent, and of long duration, all the lofty peaks and eminences, consisting of loose and broken rocks, intermixed with clays, which are fissured and friable, must have been levelled or washed away, and the whole probably reduced to an irregular mass of fragments and rubbish. Thus we observe in INDIA, that nothing, but the durability of the granite has been able to resist the sweeping ravages of the monsoons.
By its want of regular land and sea breezes, ST. HELENA is excluded from another source of occasional rain, which the opposition of these winds, is frequently observed to produce in warm climates. On the shores of INDIA, this circumstance is the more remarkable, as the sea and land breezes here occasion showers, during the months of January, February, and March; a period of the year, when no rain falls at any considerable distance from the coast. These showers usually happen in the evening and morning, and seldom at any other time : For the wind from the sea, blowing cool in the evening on the exhalations and vapours of the land, condenses and converts them into rain; and the land breeze, when it blows chill towards the morning, in like manner produces showers on the surface of the ocean. The influence of these breezes in generating rain, is not confined to any particular season of the year; they are observed at all times, to have some effect in this way. During the prevalence of the westerly winds of COROMANDEL, which blow with such force and constancy from May till September, overspreading the atmosphere with accumulations of clouds and vapour, brought from tile Western Ghauts and the coast of MALABAR, the air is in a highly parched and, and state, and there is no rain, except during the intermission of these winds. At this period, when the westerly winds happen at any time to die away on the coast, and the sea breeze sets in from the opposite quarter, its first effects are to blacken and condense the clouds and vapours, and to convert them into, rain. But further, it may frequently be observed, when the sky is perfectly serene and cloudless, that the sea breeze, blowing very cool from a clear horizon, suddenly fills the air with vapours and exhalations, which are evidently not brought from a distance, but evolved in the manner of a precipitate from an atmosphere uniformly clear.
It seems therefore evident, that a tropical island, by having no land and sea breezes, is shut out from one source of occasional rain. That ST. HELENA has no diurnal breezes of this kind cannot appear extraordinary, when we consider its small extent, its mild temperature, and the constancy of the Trade wind.
While its exclusion from the effects of alternate land and sea breezes, necessarily deprives it of many occasional showers, the circumstance of its having the same wind the whole year round, instead of monsoons or periodical winds blowing from opposite points, seems to be the principal cause of its having no stated or regular rains. Over all the climates of the EAST INDIES, the general rains which occur, are connected with the change of the monsoons; and are only observed to take place in the different temperature of the air, which accompanies the setting in of these periodical winds. For example, on the coasts of MALABAR, and COROMANDEL, the first access of the south west and north east winds immediately produces a milder temperature, and the atmosphere, which before was parched with drying winds and without a shower, is filled with watery exhalations, which arc light at first, but gradually thicken and accumulate; and at last breath in heavy and long continued rains. The same monsoon winds, which give occasion to this rain, continue to blow for a considerable time after it is over, and without producing a shower, any more than the Trade wind of ST. HELENA; and probably from the same cause:-the rainy monsoon having for a time established an equilibrium of temperature between the atmosphere of the ocean and land. But the heat very quickly increases, the winds become dry and parched, and the dews disappearing, the coasts of MALABAR and COROMANDEL are again languishing under the effects of excessive heat, when the access of their respective monsoons, blowing from the neighbouring ocean on a fervid atmosphere, restores the season of coolness and general rains.
It may be said, that the analogy of an extensive land, like that of INDIA, will not apply to a small island, whose surface and atmosphere can never acquire a degree of heat much exceeding that of the surrounding ocean ; that therefore an opposition of monsoons could not here have the same effect as in INDIA, and that every wind which blows upon it, as it must needs come over a vast extent of sea, would have the same quality with respect to heat and moisture. It must, however, be remembered, that the INDIAN monsoons are not confined to the peninsula; they produce the same effects in the islands of the ASIATIC seas, where the period of their setting in is the season of the rains. That a wind blowing from any other point at, ST. HELENA, besides the south, east, would have the same quality and temperature, though it may seem probable, is however contrary to fact : For, during the short suspension of the Trade wind, a north-west wind sometimes blows for a little while; and when this happens, it, proves very disagreeable to the inhabitants, who complain of the effects of this wind, as depressing and unhealthy. It seems probable, that the feelings of the inhabitants, accustomed to the uniformity of the Trade wind, would be sensibly affected by any other which differed from it in heat or moisture. But the wind from the north-west, when it takes place, only lasts for a little time, and is soon overcome by the more permanent current of the south-east. It cannot therefore have any influence in producing general rains, which are observed to depend on the effect of winds that blow for a fixed period from the same quarter. That winds, differing in temperature, and succeeding each other from opposite points, will produce rain, independently of other circumstances, seems evident from the effects which have been stated to arise from the land and sea breezes of INDIA. It is equally certain that monsoon winds, after their storms are exhausted, and when they have restored an uniformity of temperature and moisture to the regions through which they take their course, continue afterwards to blow for a considerable time, without occasioning any rain. This will perhaps, in some measure, explain the effects of the Trade wind at ST. HELENA, at those periods when its course is attended with showers. Although the rain here has no stated season, it most commonly takes place in the hottest or coolest time of the year; during which the temperature of this little spot varies most considerably from that of the surrounding sea. In the former situation, the greater coolness of the Trade wind seems to evolve the latent moisture from its heated atmosphere. In the latter case, the coldness of the summits of the island condenses the exhalations, borne hither by this wind, as evidently happens during July and August. Here, however, somewhat is no doubt to be ascribed to the effects of mechanical pressure and impulse: For the light vapours of the Trade wind being stopped in their course by the lofty summits of the island, are impelled and forced against each other, till they accumulate and acquire the density necessary to the formation of rain. It is observable of this rain, which falls during the cool season, that while on the interior mountains it is often considerable it becomes lighter and lighter at every descent, till among the valleys on the shore, it dwindles into a fog, or drizzling mist. The lighter showers of this season are often confined altogether to the high cultivated parts, where they keep up a fine verdure, and feed the springs; while not a drop of rain falls on the naked hills, which border on the sea, notwithstanding their elevation.
Of the third cause which has been assigned for the unusual dryness of this climate, viz. the small size of the island, and its distance from other lands, very little need be said here : For it will be obvious, at first view, that an inconsiderable spot of land, encompassed with so wide an extent of water, can have but little influence on the temperature of its surrounding atmosphere, which is regulated by the invariable course of the Trade wind. It will be equally obvious, that by its remoteness from the shore of any continent, or of any other island, it cannot, like many of the islands in the Asiatic seas, participate in the rainy seasons of other, countries.
Thus situated, and necessarily excluded by the circumstances of its situation from several sources of regular and occasional rain, the natural deficiency of moisture in its climate is still further increased by the nakedness of its surface, which in its present bleak and unsheltered state, cannot have that effect in favouring the descent of showers and dews, which the elevation of the island might otherwise produce. The influence which wood, when growing in lofty situations, has upon clouds and exhalations, will not be questioned ; and this seems less remarkable in cold than in warm climates where a deficiency of rain is frequently observed to be connected with the heat and nakedness of the surface. The well-wooded tops of mountains preserve a coolness which stops and condenses the passing vapours; while the clouds are often observed to glide over the naked and heated surfaces of rocky summits without producing rain. Something of this kind we see take place in ST. HELENA, where the light showers of the cool season are chiefly confined to the wooded and cultivated parts of the interior, and the clouds and vapours which occasion these showers .ire observed to glide over the naked hills on the shore without wetting them. From this circumstance, it seems probable, that the climate here has already undergone some change in respect of humidity ; and that long after the formation of this volcanic island, its atmosphere was seldomer refreshed with showers than it is at present. Placed in a latitude where no extremes or sudden chances of temperature, dead variation of winds, give rise to any unusual aggregation of clouds, the mere elevation of the island must have had less influence on the vapours that passed over it, before the interior hills were covered with shrubs and plants. The first vegetation that arose on a burnt and scorified surface must necessarily have been very slow and difficult ; yet, after the trees and shrubs (in whatever way they came here) had taken root among the rocks, they appear to have attracted sufficiency of moisture, to promote their growth and increase ; for they had overspread some of the hills which are now quite naked. But since a settlement has been made here, their farther growth has been checked by the depredations of the goats that have been introduced, and by the inhabitants who have destroyed them for fire-wood. Had the island remained undiscovered, its indigenous gum trees and shrubs would probably have spread from the midland heights, where they first began to grow, over ail the surrounding hills and valleys, to the margin of the sea. But while Nature was employed in subduing the crude volcanic mass, and gradually investing its surface with plants, the intrusion of mankind surprized her in one of her remotest recesses, before she completed the work which she had begun. What she has left deficient, as our possession has interrupted her operations, can only now be supplied by human care. It is unnecessary to enter more largely into this subject now, as a farther opportunity will be afforded of touching upon it in another place; when the soil and productions of ST. HELENA, and the best means of improving it, come to be considered.
In describing this climate, it is necessary to speak more particularly of its salubrity; and in this respect, probably, no country in the world can exceed ST. HELENA. Removed from the extremes of heat and cold ; exempted from all sudden changes of temperature, and freed from the inconvenience of an excessively humid or an immoderately dry atmosphere, (for though the rains are very scanty and deficient, the air is never parched with and winds) it may easily be supposed, that such a situation must be favourable to health and longevity. We accordingly find, that the natives in general arrive at a good old age; and what is of still more consequence, that they escape from most of those diseases which oppress the inhabitants of less temperate and more variable: climates. The sickly crews of ships that touch here, very shortly recover ; and of the invalids, who are discharged from the different regiments of INDIA, and sent home as incurable and unfit for service, many during their stay at ST. HELENA recover so fast, that they again enlist here, and continue to enjoy good health. Of this the writer saw some remarkable instances, in men whom, he had known in INDIA, and who were there reduced to such a state of weakness, or were afflicted with such diseases, that their recovery and even their existence in that climate, seemed impossible: Yet these men had recovered completely, and appeared sufficiently strong and vigorous for any military duty. This fact deserves the attention of the EAST INDIA Company, as it shews how the military part of this settlement may be recruited with more convenience, and at far less expense, than by sending soldiers immediately from Europe. Besides, as the troops of the EAST INDIES, instead of knowing their duty, merely from the exercises of parades and field days, are trained up in a scene of perpetual warfare, and under the eye of officers, inured to the most arduous services, there would be a farther advantage in recruiting the military, establishment of ST. HELENA, from the regiments of INDIA.
It is a matter of great importance to the fleets of the company to have such a convenient station, abounding with excellent water, and where the climate is so favourable to the sickly and convalescent. Here the ships' crews run no risk by sleeping on shore, or by any unguarded exposure to the night air; and the vessels themselves are never liable to suffer from storms or hurricanes. There are few places, indeed, which unite so many advantages as this volcanic rock, whose rude and naked aspect seems to promise so little. Its full value and importance, as a convenient station for the shipping of the company, will be yet more apparent, when we consider how few places there are (if any besides this) in the route to INDIA, or the Oriental seas, where vessels can, at all seasons, touch with safety as they do here. In several of those lands, on whose surface Nature has so profusely lavished her bounties, malignant distempers are caught by remaining on shore, during night; and at particular seasons, by being merely exposed, during night, to the wind blowing from the land, while ships are passing near, or lying to at sea. It is with great propriety, therefore, that the Court of Directors have expressly enjoined the commanders of their ships not to touch at JOANNA, unless in cases of certified necessity; and that in such cases, no part of the crew shall, on any account, be permitted to remain on shore during night. There is the same danger of sickness by touching at several other islands, and in many of those in the Oriental seas, at particular seasons. It is observable, that many of the most unhealthy of them are green and luxuriant to the water's edge; that they are generally nearer the equator than ST. HELENA, and consequently, subject to the storms, calms, and oppressive sultriness of that malignant region. It is not, however, to its distance from those equatorial latitudes, where Nature relieves the dead calms and sickly intemperature of the climate by elemental commotion, that ST. HELENA owes its exemption from storms, its unruffled atmosphere, and perpetual salubrity. Nor can its freedom from noxious damps amid vapours be ascribed to the nakedness of its surface. These effects more probably arise from the constant prevalence of the Trade wind, blowing over a wide tract of sea, where there are no neighbouring lands to disturb or interrupt its course.
In tropical countries, the open plains and the sea shore are in general the only situations which are not sickly. The neighbourhood of hills is found unhealthy; and Europeans can seldom live in confined valleys, which are always infested with noxious damps. Nothing therefore can be a stronger proof of the salubrity of ST. HELENA, than the, healthfulness of those confined situations, where the inhabitants have fixed their dwellings. Most of their houses are in the bottoms of very narrow valleys, or rather chasms, shut up with lofty hills and steep precipices. Such situations on the island of JAVA, or even on the more salubrious Continent of INDIA: would be altogether uninhabitable. What makes the salubrity of these confined places the more remarkable is, that though this climate is in general very dry, and hardly ever refreshed with a sufficiency of rain, it is, however, at particular seasons, very damp; and the mornings and evenings are very raw and chill. Yet neither the natives nor the recent settlers from EUROPE ever experience any farther inconvenience from this, than that, at such periods, they are rather more subject than usual to colds and rheumatisms. But they have no malignant or contagious fevers ; and from many of the severest diseases of other countries they are wholly exempted. One cruel distemper, which, in its natural and unmitigated course, is perhaps the most deadly pest that ever lighted upon earth, has hitherto not found its way to ST. HELENA; and as its ravages in EUROPE seem now about to disappear altogether, by one of the happiest discoveries recorded in the Annals of Mankind, it is probable that the natives of this fortunate isle may never know the smallpox, except from description. That the contagion of this disease has hitherto not reached them, will naturally be attributed to the remoteness of their situation. But it can be ascribed to the benignity of the climate alone, that it exempts part of the brute creation from one of its most severe and irremediable sufferings : It is a singular fact, that the dog here has never been affected with the hydrophobia.
It has been observed that the heat of this climate never rises to great excess, and that the temperature is always mild and agreeable. This must be understood. Of the island in general for in some of the deep and narrow valleys, the reverberation from the naked rocks, at one season, renders the temperature close and uncomfortable : Yet the thermometer here never rises above 84, and rarely reaches that extreme. We might easily suppose, that the heat in such circumstances ought to rise much higher, from the power of a clear and vertical sun, increased by reflection from the surrounding rocks. On the heights and declivities, the temperature is always pleasant to the feelings; and from the, constant purity and agitation of the atmosphere, it is probable, that a much greater degree of heat would not, in this, situation, be felt oppressive. To make this matter more evident, it is to be observed, that the degree of absolute beat existing in the atmosphere, as it is measured by the thermometer, forms no criterion of the sensation of heat - which is felt by the human body. This last is altogether a relative feeling, depending on the particular state of the air, and on the circumstances of the habit in which it is excited. This fact, which is conformable to universal experience, and may easily be verified, especially by, those who live within the tropics, was first, as-far as the writer knows, taken notice of by the historian of ANSON'S Voyage : and although that able and eloquent author has been led into some oversights concerning it, he has, however, stated and explained the general principle with his accustomed clearness and energy. He observes justly, (page 256) that as the presence and perpetual succession of fresh air is necessary to our respiration, so there is a species of tainted and stagnated air, often produced by the continuance of great heats, which never falls to excite in us the idea of sultriness and suffocating warmth, much beyond what the mere heat of the air alone, supposing it pure and agitated, would occasion. Hence continues he, it is evident that the mere inspection of thermometer will never determine the heat which the human body feels from this cause. So far, we must entirely agree with this author, whose admirable narrative no person who has derived from it the pleasure which the writer of this has done, would willingly depreciate by trivial objections. But when this narrator affirms that the equability and continuance of the tropical heats render them, with respect to the human body, more intense and insufferable than the occasional heats of higher latitudes and that the same degree of absolute heat in most places between the tropics, is felt more troublesome and uneasy than in the temperate zone, he appears to be completely mistaken, both in his facts and in his reasoning: For, in the first place, the equability of the tropical heats is the very circumstance which renders them more tolerable than they otherwise would be, from that power in the constitution which, however irksome to our feelings the first impressions of extraordinary heat may prove, easily accommodates itself to a new situation. People after their arrival in warm climates find, that the disagreeable sensations of heat shortly wear off; and that as long -,is they continue to enjoy health, they suffer no inconvenience or uneasiness from the ordinary temperature of the climate, whatever uneasiness its unusual heats, and the closeness of a warm atmosphere, may sometimes occasion. But, in the next place, the same degree of absolute heat, within the tropics, is so far from being more oppressive than it is in higher latitudes, that the contrary is actually the case. The heats of GREAT BRITAIN, for instance, are in proportion to their degree, much more sultry and oppressive than those of the EAST INDIES which will not be disputed by any one who has had the experience of both ; for it is not unusual to hear persons complaining of the heat of ENGLAND, When the thermometer is little above 76, who have passe twenty years in a climate whose medium temperature is nearly ten degrees higher, and where the thermometer in the shade, during a great part of the year, is above 90.
But the author already quoted, as timing it as a fact, that the same degree of absolute heat is attended with greater oppression and sultriness within the tropics than in the higher latitudes, goes on to account for it in the following manner:- "The equability and duration of the tropical heats, contribute to impregnate the air with a multitude of steams and vapours from the soil and water, and these being many of them of an impure and noxious kind, and being not easily removed by reason of the regularity of the winds in those parts, which only shift the exhalations from place to place, without dispersing the them, the atmosphere is, by this means, rendered less proper for respiration, And mankind are consequently affected with what they stile a most intense and stifling heat: whereas, in the higher latitudes, those vapours are probably raised in smaller quantities, and the irregularity and violence of the winds frequently disperse them, so that in, general, the air being pure and less stagnant, the same degree of absolute heat is not attended with that uneasy and suffocating sensation." Page 257.
It seems surprising, that a writer so profound and acute as Mr. ROBINS, should have been inadvertently betrayed into this view of things, which is so contrary to fact and experience. A mind so enlightened as his, could hardly have fallen into this error, if, previously to the writing, of his narrative, he had had an opportunity of visiting the scenes which he describes. The very reverse of what be supposes actually takes place: For, the uniformity and duration of the tropical heats, instead of filling the air with noxious steams and vapours, contribute more than any thing else to volatilize and disperse them ; and these heats, with the more constant agency of the winds, which they never fail to excite, produce a greater lightness and purity in the state of the atmosphere, than takes place during the warm weather of higher latitudes. This seems to be the true reason, why a much smaller degree of absolute heat is felt more oppressive in the latter situation than in the former. From what cause besides the greater purity and lightness of the air, is a person able, to live and breathe with ease, in a temperature equal to that of the human blood ? How does it happen- that some of the hottest countries within the torrid zone are uniformly healthful; while the long continued heats of higher latitudes seldom fail to generate distempers ? How comes it in tropical climates, that the exhalations from crowded, hospitals, and from all receptacles of stench and corruption, are observed to taint the surrounding air less perceptibly with any offensive effluvia than, ever happens in similar situations, during the warm weather in northern latitudes? All this can only arise from the purifying effects of extreme and unremitting heat, and the, constancy of the winds which it excites. If the heat of those climates which are exposed to the perpendicular rays of the sun, were not productive of greater purity, lightness, and agitation, in the state of the atmosphere, no human being could ever have existed in the torrid zone. Were the conclusions of the writer of ANSON'S Voyage well founded, we might easily suppose with the ancients, that this region of the globe must be uninhabitable: For a heat that is continually impregnating the air with noxious steams and vapours, which the winds, from their regularity, can only shift from place to place, without dispersing, must, in its inevitable consequences, produce an insupportable and pestilent climate.
The desire of correcting an important error, has perhaps led the writer further than the mere view of the climate of ST. HELENA, where the heat is never excessive, seemed to require: Yet what has been said may serve to explain, why the temperature here is always so healthful and agreeable, notwithstanding the power of the vertical sun, aided by the reflection of his rays from a rocky surface.