Page:Physical Geography of the Sea and its Meteorology.djvu/473

Rh precise locality has not been determined, but I suppose it to be a band or disc—an area—within the polar circle, which, could it be explored, would be found, like the equatorial calm belt, a place of light airs and calms, of ascending columns of air,—a region of clouds, of variable winds, and constant precipitation.

833. Also of a low barometer.—But, be that as it may, the air which these vapour-bearing winds—vapour-bearing because they blow over such an immense tract of ocean—pour into this stopping-place has to ascend and flow off as an upper current, to make room for that which is continually flowing in below. In ascending it expands and grows cool, and, as it grows cool, condensation of its vapour commences; with this, vast quantities of latent heat, which converted the water out at sea into vapour for these winds, are set free in the upper air. There it reacts by warming the ascending columns, causing them still farther to expand, and so to rise higher and higher, while the barometer sinks lower and lower. This reasoning is suggested not only by the facts and circumstances already stated as well known, but it derives additional plausibility for correctness by the low barometer of these regions. In the equatorial calm belts the mean barometric pressure is about 0.25 inch less than it is in the trade-winds, and this diminution of pressure is enough to create a perpetual influx of the air from either side, and to produce the trade-winds. Off Cape Horn the mean barometric pressure is 0.75 inch less than in the trade-wind regions. This is for the parallel say of 57°—8° S. According to the mean of 2,472 barometric observations made along that part only of the route to Australia which lies between the meridians of the Cape of Good Hope and Melbourne, the mean barometric pressure on the polar side of 42° S. has been shown by Lieutenant Van Gough, of the Dutch Navy, to be 0.33 inch less than it is in the trade-winds. The mean pressure in this part of the South Indian Ocean is, under winds with easting in them, 29.8 inches: ditto, under winds with westing, 29.6 inches. Plate I. shows a supposed mean pressure in the polar calms of not more than 28.75 inches. The barometric curve, page 468, shows with a higher degree of probability that the mean pressure there is about 28.14 inches.

834. Aqueous vapour the cause of both.—To what, if not to the