98 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



254. Tlie two systems of trade-winds unequal both in force, dura- 

 tion, and stability. — By examining tlie log-books of vessels while 

 sailing: throTi<2;li 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.* 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 da3''S, 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 b}^ 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 lati- 

 tude 14° K, air 78°.69. North edge calm belt, say 9° N., air 

 80°.90. Middle of calm belt, say 4^ N., air 82°. South edge, 

 sayO°, 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 

 eda'c of the calm belt is 1°.4 warmer than the north. The ex- 

 treme difference between the annual isothenns that lie between 

 the parallels of 30° K. and 30° S. — between which the trade- 

 wind belts are included — does not probably exceed 12°. Ac- 

 cording to the experiments of Gay-Lussac and Dalton, the dila- 

 tation of atmospheric air due to a change of 1 2° in temperature is 



* See Maury's Wind and Current Charts, vol. ii., Sth edition. Sailing Dii-ec- 

 tioris. 



