Continuing our course to the northward, nothing remarkable occurred for two or three days, though we spread our ships in such a manner that it was not probable any vessel of the enemy should escape us. During our voyage along this coast, we generally observed that a current set us to the northward, at the rate of ten or twelve miles every day. When in about the latitude of 8° S. we began to be attended by vast numbers of flying fish and bonitos, which were the first we had seen after leaving the coast of Brazil. It is remarkable that these fish extend to a much higher latitude on the east side of America than on the west, as we did not lose them on the coast of Brazil till near the southern tropic. The reason, doubtless, of this diversity, is owing to the different degrees of heat obtaining on different sides of the continent in the same latitude; and, on this occasion, I use the freedom to make a short digression on the heat and cold of different climates, and on the variations which occur in the same places at different times of the year, and in different places in the same degree of latitude.
The ancients conceived that of the five zones into which they divided the surface of the globe, two only were habitable; supposing that the heat between the tropics, and the cold within the polar circles, were too intense to be supported by mankind. The falsehood of this idea has been long established; but the particular comparison of the heat and cold of these various climates have as yet been very imperfectly considered. Enough is known, however, safely to determine this position, that all the places within the tropics are far from being the hottest on the globe, as many within the polar circle are far from enduring that extreme degree of cold to which their situation seems to subject them; that is to say, that the temperature of a place depends much more upon other circumstances, than upon its distance from the pole, or its proximity to the equinoctial line.
This proposition relates to the general temperature of places taking the whole year round, and, in this sense, it cannot be denied that the city of London, for instance, enjoys much warmer seasons than the bottom of Hudson's Bay, which is nearly in the same latitude, but where the severity of the winter is so great as scarcely to permit the hardiest of our garden plants to live. If the comparison be made between the coast of Brazil and the western shore of South America, as, for example, between Bahia and Lima, the difference will be found still more considerable; for, though the coast of Brazil is extremely sultry, yet the coast of the South Sea, in the same latitude, is perhaps as temperate and tolerable as any part of the globe; since we, in ranging it along, did not once meet with such warm weather as is frequently felt in a summer day in England, which was still the more remarkable, as there never fell any rain to refresh and cool the air.
The causes of this lower temperature in the South Sea are not difficult to be assigned, and shall be mentioned hereafter. I am now only solicitous to establish the truth of this assertion, that the latitude of a place alone is no rule by which to judge of the degree of heat and cold which obtains there. Perhaps this position might be more briefly confirmed by observing that on the tops of the Andes, though under the equator, the snow never melts the whole year round; a criterion of cold stronger than is known to take place in many parts far within the polar circle.
Hitherto I have considered the temperature of the air all the year through, and the gross estimations of heat and cold which every one makes from his own sensations. But if this matter be examined by means of thermometers, which are doubtless the most unerring evidences in respect to the absolute degrees of heat and cold, the result will be indeed most wonderful; since it will appear that the heat in very high latitudes, as at Petersburgh for instance, is, at particular times, much greater than any that has been hitherto observed between the tropics. Even at London in the year 1746, there was a part of one day considerably hotter than was at any time felt in one of the ships of our squadron in the whole voyage out and home, though four times passing under the equator; for, in the summer of that year, the thermometer in London, graduated according to the scale of Fahrenheit, stood at 78°, and the greatest observed heat, by a thermometer of the same kind in the same ship, was 76°, which was at St Catharines in the latter end of December, when the sun was within about 3° of the vertex. At St Petersburgh, I find by the acts of the Academy, in the year 1734, on the 20th and 25th of July, that the thermometer rose to 98° in the shade, or 22° higher than it was found to be at St Catharines; which extraordinary degree of heat, were it not authenticated by the regularity and circumspection with which the observations appear to have been conducted, would appear altogether incredible.
If it should be asked, how it comes then to pass, that the heat, in many places between the tropics, is esteemed so violent and insufferable, when it appears, by these instances, that it is sometimes rivalled, and even exceeded, in very high latitudes, not far from the polar circle? I shall answer, That the estimation of heat, in any particular place, ought not to be founded upon that particular degree of it which may now and then obtain there; but is rather to be deduced from the medium observed during a whole season, or perhaps in a whole year; and in this light, it will easily appear how much more intense the same degree of heat may prove, by being long continued without remarkable variation. For instance, in comparing together St Catharines and St Petersburg, we shall suppose the summer heat at St Catharines to be 76°, and the winter heat to be only 56°. I do not make this last supposition upon sufficient authority, but am apt to suspect the allowance is full large. Upon this supposition, therefore, the medium heat all the year round will be 66°; and this perhaps by night as well as by day, with no great variation. Now, those who have attended to thermometrical observation will readily allow, that a continuance of this degree of heat for a length of time, would be found violent and suffocating by the generality of mankind. But at Petersburg, though the heat, as measured by the thermometer, may happen to be a few times in the year considerably higher than at St Catharines, yet, at other times, the cold is intensely sharper, and the medium for a year, or even for one season only, would be far short of 60°. For I find, that the variation of the thermometer at Petersburgh, is at least five times greater, from its highest to its lowest point, than I have supposed it to be at St Catherines.[2]
Besides this estimation of the heat of a place, by taking the medium for a considerable time together, there is another circumstance which will still farther augment the apparent heat of the warmer climates, and diminish that of the colder, though I do not remember to have seen it remarked by any author. To explain myself more distinctly upon this head, I must observe, that the measure of absolute heat, marked by the thermometer, is not the certain criterion of the sensation of heat with which human bodies are affected; for, as the presence and perpetual succession of fresh air is necessary to our respiration, so there is a species of tainted or stagnated air often produced by the continuance of great heats, which, being less proper for respiration, never fails to excite in us an idea of sultriness and suffocating warmth, much beyond what the heat of the air alone would occasion, supposing it pure and agitated. Hence it follows, that the mere inspection of the thermometer will never determine the heat which the human body feels from this cause; and hence also, the heat, in most places between the tropics, must be much more troublesome and uneasy, than the same degree of absolute heat in a high latitude. For the equability and duration of the tropical heat contribute to impregnate the air with a multitude of steams and vapours from the soil and water; and many of these being 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 them, the atmosphere is by this means rendered less capable of supporting the animal functions, and mankind are consequently affected by what they call a most intense and stifling heat. Whereas, in the higher latitudes, these vapours are probably raised in smaller quantities, and are frequently dispersed by the irregularity and violence of the winds; so that the air, being in general more pure and less stagnant, the same degree of absolute heat is not attended by that uneasy and suffocating sensation.
This may suffice, in general, with respect to the present speculation; but I cannot help wishing, as it is a subject in which mankind are very much interested, especially travellers of all sorts, that it were more thoroughly and accurately examined, and that all ships bound to the warmer climates were furnished with thermometers of a known fabric, and would observe them daily, and register their observations. For, considering the turn to philosophical enquiries which has obtained in Europe since the beginning of the eighteenth century, it is incredible how very rarely any thing of this kind has been attended to. For my own part, I do not remember to have ever seen any observations of the heat and cold, either in the East or West Indies, which were made by marines or officers of vessels, excepting those made by order of Commodore Anson on board the Centurion, and those by Captain Legg on board the Severn, another ship of our squadron.
I have been in some measure drawn into this digression, by the consideration of the fine weather we experienced on the coast of Peru, even under the equinoctial, but I have not yet described the particularities of this weather. I shall now therefore observe, that every circumstance concurred, in this climate, that could render the open air and the day-light desirable: For, in other countries, the scorching heat of the sun in summer renders the greater part of the day unapt either for labour or amusement, and the frequent rains are not less troublesome in the more temperate parts of the year: But, in this happy climate, the sun rarely appears. Not that the heavens have at any time a dark or gloomy aspect; for there is constantly a cheerful gray sky, just sufficient to screen the sun, and to mitigate the violence of its perpendicular rays, without obscuring the air, or tinging the light of day with an unpleasant or melancholy hue. By this means, all parts of the day are proper for labour or exercise in the open air; nor is there wanting that refreshing and pleasing refrigeration of the air which is sometimes produced by rains in other climates; for here the same effect is brought about by the fresh breezes from the cooler regions to the southward. It is reasonable to suppose, that this fortunate complexion of the heavens is principally owing to the neighbourhood of those vast mountains called the Andes, which, running nearly parallel to the shore, and at a small distance from it, and extending immensely higher than any other mountains upon the globe, form upon their sides and declivities a prodigious tract of country, where, according to the different approaches to the summit, all kinds of climates may be found at all seasons of the year.
These mountains, by intercepting great part of the eastern winds, which generally blow over the continent of South America, and by cooling that part of the air which forces its way over their tops, and by keeping besides a large portion of the atmosphere perpetually cool, from its contiguity to the snows by which they are always covered, and thus spreading the influence of their frozen crests to the neighbouring coasts and seas of Peru, are doubtless the cause of the temperature and equability which constantly prevail there. For, when we had advanced beyond the equinoctial to the north, where these mountains left us, and had nothing to screen us to the eastward but the high lands on the Isthmus of Darien, which are mere mole-hills compared to the Andes, we then found that we had totally changed our climate in a short run; passing, in two or three days, from the temperate air of Peru, to the sultry and burning atmosphere of the West Indies.