Page:Physical Geography of the Sea and its Meteorology.djvu/368

342 the north-east trade-winds in September and February, that the average passage through them from New York to the line is 26.4 days in the winter against 38.8 in the fall month.

640. A thermal adjustment.—Thus it appears that the equatorial calm belt is made to shift its place with the seasons, not by reason of the greater intensity of the solar ray in the latitude where the calm belt may be at that season, but by reason of the annual variations in the energy of each system of trades; which variations (§ G38) depend upon the changes in the temperature and barometric weight of the air which each system puts in motion. This calm zone, therefore, may be considered as a thermal adjustment—the dynamical null-belt—between the trade-winds of the two hemispheres. 641. The barometer in the trade-winds and equatorial calms.—The observations on the barometer at sea (§ 858) shed light on this subject. According to the Dutch, that instrument stands higher by 0.055 inch in the south-east than it does in the north-east trade-winds. According to the observations of American navigators, it stands 0.050 inch higher. The former determination is derived from 80,873, the latter from 1899 observations; therefore 0.055 inch is entitled to most weight. The trade-winds are best developed between the parallels of 5° and 20°. The mean barometric pressure between these parallels is 29.968 inches for the north-east, and 30.023 inches for the south-east trade-winds; while for the calm belt it is 29.915 inches. The pressure, therefore, upon the air in each of the trade-winds is greater than it is in the calm belt; and it is this difference of pressure, from whatever cause arising, that gives the wind in each system of trades its velocity. The difference between the calm belt and trade-wind pressure is 0.108 for the south-east and 0.053 for the north-east. According to the barometer, then, the south-east should be stronger than the north-east trade-winds, and according to actual observations they are.

642. Experiments in the French Navy.—Now if we liken the equatorial calm belt with its diminished pressure to a furnace, the north-east and the south-east trade-winds may be not inaptly compared to a pair of double bellows that are blowing into it. In excess of barometric pressure, the former is a bellows with a weight of 3.8 lbs., the latter with a weight of 7.8 lbs. to the