John Ball - Notes of a naturalist in South America

ATLANTIC CYCLONES.

On the 19th the barometer, which, in spite of a gentle breeze from south-west, had stood as high as 30·40, fell about a quarter of an inch between sunrise and sunset; and in the night, on the only occasion during the entire voyage, remained for some hours below 30·00. A moderate breeze from the north brought with it a disproportionately heavy sea, and although there was no sensible pitching, the ship rolled so heavily as to send many of the passengers to solitary confinement in their berths. This continued throughout the 20th, afterwards styled Black Monday by the sufferers from sea-sickness, and we escaped into smoother water only on the evening of the following day. The discomfort which I felt from fancying that I had “lost my sea legs” was entirely relieved by fortunately coming across a distinguished naval officer, on his way to take a command on the West Indian station, who like myself was forced to hold on with both hands during the rolling of the ship.

It was clear that we had passed at no great distance from a cyclone in the North Atlantic – one of those disturbances 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.