94 PHYSICAL GEOGEAPHY OF THE SEA, AND ITS IIETEOEOLOGY. 



the south-east blow more clays 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 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-wdnds are very 

 unequal both as to force and stabiHty, the south-east surpassing in 

 each case. 



255. Moreover, the hottest place within the trade-wind regions 

 Effects of heat and IS not at the cquator I it is where these two winds 

 vapour. j^qqI- ^^ 253). Lieutenant Warley has collated fi^om 



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 

 Ibllowing 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 cahn belt, say 9"^ N., air 80''.90. Middle of calm belt, 

 say 4i-^ 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 dila- 

 tation of atmospheric air due a change of 12° in temperature is 2J- 

 per cent. ; that is, a column of atmosphere 100 feet high will, after 

 its temperature has been raised 12°, be 102-J feet high. However, 

 only about one thhd of the direct heat of the sun is absorbed in its 

 passage down through the atmosphere. The other two thirds are 

 employed in lifting vapour up from the sea, or in warming the 

 crust of the earth, thence to be radiated off again, or to raise the 

 temperatm'e of sea and air by conduction. The ah at the surface 

 of the earth receives most heat directly from ih.Q sun; as you 



* See Maury's Wind and Current Charts, vol. ii., 8th edition, Sailing Directions. 



