Page:Physical Geography of the Sea and its Meteorology.djvu/302

276 air above and occupy the space it held. Nor is this all. This austral vapour, rising up, is cooled and condensed. Thus a vast amount of heat is liberated in the upper regions, which goes to heat the air there, expand it, and thus, by altering the level, causing it to flow off. This unequal distribution of atmosphere between the two halves of the globe is imperfectly represented in barometric profile on Plate I. (§ 215)—the shading around the periphery of the circle being intended to represent the relative height, and the scales standing up in it, the barometric column.

513. Influence of Antarctic icebergs in expelling the air from austral regions.—This part of the southern ocean where the barometer shows diminished pressure is frequented by icebergs, many of them very large and high, and some of them sending up towers, minarets, and steeples, which give them the appearance in the distance of beautiful cities afloat. Each one of them is a centre of condensation. Could an eye from aloft look down upon the scene, the upper side of the cloud stratum would present somewhat the appearance of an immense caldron, boiling, and bubbling, and intumescing in the upper air. These huge bergs condense the vapour, and the liberated heat causes the air above them to swell out, and to stand like so many curiously-shaped fungi above the general cloud level. And thus, where the icebergs are thick, the clouds are formed low down. Icebergs, like islands, facilitate the formation of clouds and promote precipitation.

514. The horse latitudes—the doldrums.—Turn we now to the equatorial cloud-ring. Seafaring people have, as if by common consent, divided the ocean off into regions, and characterized them according to the winds; e.g., there are the "trade-wind regions," the "variables," the "horse latitudes," the "doldrums," .etc. The "horse latitudes" are the belts of calms and light airs (§ 210) which border the polar edge of the north-east trades. They were so called from the circumstance that vessels formerly bound from New England to the West Indies, with a deck-load of horses, were often so delayed in this calm belt of Cancer, that for the want of water for their animals, they were compelled to throw a portion of them overboard. The "equatorial doldrums" is another of these calm places (§ 212). Besides being a region of calms and baffling winds, it is a region noted for its rains and clouds, which make it one of the most oppressive and disagreeable places at sea. The emigrant ships from Europe for Australia