Pendulum measurements were made in the Strait of Magellan, at Punta Arenas.
The sketch of Belgica Strait was drawn by taking, as principal points, twelve stations whose co-ordinates were astronomically determined. The other stations were obtained either by the method of sufficient segments or by that of magnetic bearings. We employed also Admiral Mouchez’s method.
While drifting, the positions of the ship were observed and calculated either by Marcy Saint-Hilaire’s method or Borda’s, when the latitude had been determined beforehand by a culmination or a circummeridian.
METEOROLOGY
The only notions we had about the climate of the antarctic were based upon the very inadequate observations made during the three summer months. The Belgian Antarctic Expedition is the first which enables us to furnish a series of observations taken hourly during a full year. These observations were made during the year of the imprisonment of the Belgica in the ice-pack between latitudes 70° and 71° 36′ south, and from longitude 85° to 103° west. In order to appreciate thoroughly the conclusions which can be derived from these observations, we must not forget that the Belgica, during her wintering in the ice-pack, was in the neighbourhood of free waters; in consequence, the climate studied is a coast climate, influenced partly by the neighbourhood of the sea, partly by that of the continental antarctic land mass covered with eternal snow. The definite corrections of figures obtained have not as yet been made; still, we are able to present the general results with an adequate approximation.
The minimum temperature was observed in September; it was -43°. The maximum is remarkably low: +2° (in February). The month of July is the coldest of the year, with an average of -22.5°. The warmest month is February: average, -1°. The mean temperature of the year is -9.6°, an extraordinarily low figure for that latitude.
North of the Spitsbergen, at latitude 80° north, we have -8.9°. The mean temperature in summer is -1.5°, a figure just as remarkable for its latitude, considering that the expedition of the Fram obtained for a summer average -1.2° by latitude 84° north. This low temperature can only be explained by the absence of land towards the north, and the presence of an antarctic continent entirely covered with ice. This hypothesis is based upon a fact which was observed by the expedition. Every time the wind blew from the north the temperature rose, even in midwinter, to 0°, but did not ascend higher. As soon as the wind shifted and blew from the south the thermometer descended abruptly, even in the middle of summer, to a very low temperature.
In the interior of the antarctic continent there must be a pole whose temperature is much lower than the frigidity of the arctic poles of cold; the frozen surface of the antarctic continent is in effect much larger than that of Greenland, northern Siberia, or North America. The zone explored by the Belgica lies in a cyclonic region; yet the mean barometric pressure of the year, 744 mm. .7, obtained by a direct observation, is superior by 6 mm. to the theoretical figure obtained by Ferrel for that latitude, and demonstrates that the pressure does not decrease progressively towards the pole, where, on the contrary, there must reign an anticyclone. The absolute minimum was 711 mm. .74, one of the lowest pressures observed on the level of the sea. The maximum pressure was 772 mm. .14. The maximum average monthly variations of the barometer height is 34 mm. .30—a very high figure, which indicates that the tempestuous region extends beyond the polar circle. The barometer height is in the average maximum at the solstices, and minimum at the equinoxes, which shows that in the antarctic there is a direct and very simple relation between the barometric pressure and the sun’s altitude. Winds are frequent and generally violent. Only fifty-five days of calm or very feeble wind were reckoned for a whole year. In the summer, breezes blow mostly from eastern regions; in winter from the western. It is probable that our region is already freed from the direct influence of the circular antarctic zone of western winds. The air is almost constantly saturated with watery vapour, and humidity settles down in the form of fog and snow with remarkable facility. Hoarfrost accumulates in enormous quantities upon every object—upon the ice-pack, the new ice, and even upon the falling snowflakes. During the year we counted two hundred and fifty-seven days of snowfall, and fourteen days of drizzling rain. The sky is almost constantly obscured by a cloak of grayish and low mists, which, when they sometimes happen to disappear, allow a pure sky to be seen, upon which only a few high clouds and very elongated cirri may be noticed. It would not do to generalise these observations and come to the conclusion that the whole antarctic is subject to the climatic régime which we have just described. It is very probable, on the contrary, that in the interior of the antarctic continent the sky must be very often pure, humidity lighter, and snowfalls less frequent. The Belgica was, in fact, imprisoned in a littoral zone, that is, in a zone where came, to be condensed itself, all the humidity brought forth by the winds of the vast regions of a free sea situated farther north. The south wind, or land wind, always had the effect of driving the clouds away and bringing on a dry cold. Optical phenomena were very often noticed. Splendid sunrises and sunsets, parhelia, paraselenæ, and mirage phenomena were remarkable and varied. During the whole winter austral auroras were frequent, but not remarkably vivid. One single drape-like aurora was seen; the others looked like luminous clouds traversed by moving rays.
Insolation during the summer months is considerable. On the 30th of December the thermometer with a black ball marked +41°, while the temperature of the air was at -1°. The effect of that insolation is, however, but little felt upon the ice-pack; the upper layer of snow hardly melts in summer.