Notes of a naturalist in South America. Ball John
whose visits are so often predicted from the western continent, but which so often fortunately lose their way or get dissipated before they approach our shores. It would seem that little progress has been made in forecasting the direction in which these great aërial eddies traverse the ocean, or the conditions under which they expend their force. It seems allowable to suppose that the most important of the causes influencing their direction depend upon the general movements of the great currents of the atmosphere; and that, as these are constantly modified by the changing position of the earth in her orbit, the element of season is primarily to be considered. It being admitted that the origin of these disturbances is to be sought in the abnormal heating or cooling of some considerable portion of the earth’s surface, it would seem that, in the case of the Atlantic, local causes can have little effect, unless we suppose that the heating of the surface of the Azores in summer, or the annual descent of icebergs from the polar seas, are adequate to influence the march of a travelling cyclone.
On the evening of the 20th the barometer had risen again to its former position, rather over 30·40 inches; the mean of the four following days was 30·55, and that of the entire run from Southampton to Barbadoes was 30·36. This fact of the continuance of high or low pressures at the sea-level at certain seasons in some parts of the world has scarcely been sufficiently noted in connection with the ordinary rules for the measurement of heights by means of the barometer. The tables supplied to travellers are all calculated on the assumption that the pressure at the sea-level is constant – the English tables fixing the amount at 30·00 inches of mercury, those calculated on the continent starting from a pressure of 760 millimetres, or about 29·921 inches. It is admitted that this mode of determining heights, when comparative observations at a known station are not available, is subject to serious unavoidable error. With regard, however, to mountains not remote from the sea-coast, it may be possible to lessen this inconvenience in many parts of the world by substituting for the assumed uniform pressure that higher or lower amount which is known to prevail at given seasons. Such a correction could not, of course, be made available in very variable climates, such as that of the British Islands, but might be applied in many parts of the broad zone lying within 40° of the equator.
ATLANTIC SPRING TEMPERATURE.
Soon after ten p.m. on the 21st we were abreast of the bright light which marks the harbour of St. Michael’s, but, the night being dark, we saw very little of that or any other of the Azores group. The spring temperature of these islands is about the same as that of places in the same latitude in Portugal; but it appears that the cooling effect of the east and north-east winds prevailing at that season must in the mid-Atlantic extend even much farther south. With generally fair settled weather, the thermometer rose very slowly as we advanced towards the tropics. Between the 18th and 24th of March, in passing from 50° to 29° north latitude, the mean daily temperature rose only from about 55° to about 65° Fahr. – the thermometer never rising to 70°, nor falling below 52°. Notwithstanding the relatively low temperature, a few flying-fish were seen on the 24th – rare, it is said, outside the tropics so early in the year, though sometimes seen in summer as far north as the Azores.
On March 25th we, for the first time, became conscious of a decided though moderate change of climate. The thermometer at noon stood at 71°, and was not seen to fall below 70° until, some three weeks later, off the Peruvian coast, we met the cold antarctic current which plays so great a part in the meteorology of that region. We were now in the regular track of the north-east trade-wind, and my mind was somewhat exercised to account for the circumstance, said to be of usual occurrence, that the breeze increases in strength from sunrise during the day, and falls off, though it does not die away, towards nightfall. It is easy to understand the cause of this intermittence in breezes on shore, whether near the sea-coast or in the neighbourhood of mountain ranges, inasmuch as their direction and strength are determined by the unequal heating of the surface; but the trade-winds form a main part of the general system of aërial circulation over the surface of our planet, and, supposing the phenomenon to be of a normal character, the explanation is not quite simple. Regarding the trade-wind as a great current set up in the atmosphere, it is conceivable that the heating and consequent expansion which must occur as the sun acts upon it, tends to increase the rate of flow at the bottom of the aërial stream, while the cooling which ensues as the sun’s heat is withdrawn, has the contrary effect.
On this and the next day or two my attention was called to the frequent recurrence of masses of yellow seaweed, sometimes in irregular patches, but more frequently arranged in regular bands, two or three yards in width, and extending in a straight line as far as the eye could reach. We were here at no great distance from the great sargassum fields of the Northern Atlantic, but I was unable to satisfy myself that the species seen from the steamer was that which mainly forms the sargassum beds; and, whatever it might be, this arrangement in long straight strips seemed deserving of further inquiry. More flying-fish were now seen, and two or three small whales of the species called by seamen “black-fish” were sighted during this part of the voyage.
ENTERING THE TROPICS.
On the afternoon of the 26th we entered the tropics, and this and the following day were thoroughly enjoyable, but did not offer much of novelty. The colour of the sea was here of a much deeper and purer blue (rivalling that of the Mediterranean) than we had hitherto found it, while that of the sky was much paler. The light cumuli with ill-defined edges were such as we are used to in British summer weather; and, excepting that the interval of twilight was sensibly shorter, the sunsets were devoid of special interest. At this season the Southern Cross was above the horizon about nightfall, and was made out by the practised eyes of some of the officers; but, in truth, it remains a somewhat insignificant object when seen from the northern side of the equator, and to enjoy the full splendour of that stellar hemisphere one must reach high southern latitudes.
Although the thermometer never quite reached 80° Fahr. in the shade until we touched land, the weather on the 28th and 29th was hot and close, and few passengers kept up the wholesome practice of a constitutional walk on the long deck of the Don. Of the rain which constantly seemed impending very little fell.
ARRIVAL AT BARBADOES.
At daybreak on the morning of the 30th, in twelve days and seventeen hours, we completed the run of about 3340 nautical miles which separates Southampton from Barbadoes, and found ourselves in the roads of Bridgetown, about a mile from the shore. Being somewhat prepared, I was not altogether surprised to find that this first view of a tropical island forcibly reminded me of the last land I had beheld at home – the northern shores of the Isle of Wight. Long swelling hills, on which well-grown trees intervene between tracts of tillage, present much the same general outline, and at this distance the only marked difference was the intense dark-green colour of the large trees that embower the town and nearly conceal all but a few of the chief buildings. The appearance of things as the morning advanced quite confirmed the reputation of this small island as the most prosperous, and, in proportion to its extent, the most productive of the West Indian Islands. With an area not greater than that of the Isle of Wight, and a population of about sixty thousand whites and rather more than a hundred thousand negroes, the value of the exports and imports surpasses a million sterling under each head; and, besides this, it is the centre of a considerable transit trade with the other islands. Under local representative institutions, which have subsisted since the island was first occupied by the English early in the seventeenth century, the finances are flourishing, and the colonial government is free from debt. The average annual produce of sugar is reckoned at forty-four thousand hogsheads, but varies with the amount of rainfall. This averages from fifty-eight to fifty-nine inches annually, but any considerable deficiency, such as occurred in the year 1873, leads to a proportionate diminution in the sugar crop.
Among other tokens of civilization, the harbour police at Bridgetown appeared to be thoroughly efficient. As, about nine o’clock, we prepared to go ashore, we found on deck two privates – black men in plain uniform – who seemed to have no difficulty in keeping perfect order amid the crowd of boatmen that swarmed round the big ship. We had already learned the event of the hour – the fall of three inches of rain during the day and night preceding our arrival. This is more than usually falls during the entire month of March, and seemed to be welcomed by the entire population. On landing we encountered a good deal of greasy grey mud in the streets, but all was nearly dry