254. The two systems of trade-winds unequal both in force, duration, and stability.—By examining the log-books of vessels while sailing through the north-east and south-east trade-wind belts, and comparing their rate of sailing, it has been ascertained that ships sail faster with the south-east than they do with the north-east trade-winds, and that the south-east blow more days during the year than do the north-east trades.[1] The logs of vessels that spent no less than 166,000 days in sailing through these two belts of wind show that the average sailing speed through the south-east trade-wind belt, which lies between the equator and 30° S., is about eight miles an hour, and the average number of uninterrupted south-east trade-wind days in the year is 227. For the north-east it is 183 days, with strength enough to give ships an average speed of only 5.6 miles an hour. Hence it appears that the two systems of trade-winds are very unequal both as to force and stability, the south-east surpassing in each case.
255. Effects of heat and vapour.—Moreover, the hottest place within the trade-wind regions is not at the equator: it is where these two winds meet (§ 253). Lieutenant Warley has collated from the abstract logs the observations on the temperature of the air made by 100 vessels, indiscriminately taken, during their passage across the trade-wind and equatorial calm belts of the Atlantic. The observations were noted at each edge of the calm belt, in the middle of it, and 5° from each edge in the trade-winds, with the following averages: In the north-east trades, 5° north of the north edge of the equatorial calm belt, say in latitude 14° N, air 78°.69. North edge calm belt, say 9° N., air 80°.90. Middle of calm belt, say 4½° N., air 82°. South edge, say 0°, air 82°.30; and 5° S. (in south-east trades), air 81°.14. These thermometers had not all been compared with standards, but their differences are probably correct, notwithstanding the means themselves may not be so. Hence we infer the south edge of the calm belt is 1°.4 warmer than the north. The extreme difference between the annual isotherms that lie between the parallels of 30° N. and 30° S.—between which the trade-wind belts are included—does not probably exceed 12°. According to the experiments of Gay-Lussac and Dalton, the dilatation of atmospheric air due to a change of 12° in temperature is
- ↑ See Maury's Wind and Current Charts, vol. ii., 8th edition. Sailing Directions.
