154 rnYSicAL geography or the sea, and its meteorology. 



Between the equator and 5'^ north, the annual average duration 

 of the trades is 67 days for the north-east, and 199 fur the south- 

 east, with a mean direction for the latter — which are the prevail- 

 ing winds between those parallels — of S. 47° 30' E. According 

 to the ITalleyan theory these should be south-west winds. 



343. Velocities of the trade-winds. — In the Atlantic the average 

 velocity of the south-east is greater than the average velocity Oi 

 the north-east trades.* I estimate one to be from 14 to 18, the 

 other from about 25 to 30 miles an hour. Assuming their velo- 

 city to be 14 and 25 respectively^ the following dejjartiires show 

 the miles of easting which the trade-winds average per hour 

 throua'h each of the above-named belts : 



34-i. Difference between observation and tJieory. — That diurnal 

 rotation does impart easting to these winds there is no doubt; 

 but the path suggested by the table does not conform to that 

 which, according to any reasonable hypothesis, the trade-winds 

 would follow if left to obey the forces of diurnal rotation alone, 

 as they would do were diurnal rotation the sole cause of their 

 easting. As these wdnds approach the equator, the effect of 

 diurnal rotation becomes more and more feeble. But the table 

 shows no such diminution of effect. They have as much easting 

 between 5° and 0° as they have between 30° and 25°. Kay, the 

 south-east trades between the equator and 5° N. — where, by the 

 Halleyan theory, they sJiould have ivesting — have as much easting 

 (§ 342) as they have between 30° and 25° south. We cannot tell 

 how much the air is checked in its easterly tendency by resisting 

 agents, b}^ friction, etc., but we know that that tendency is about 

 ten times stronger between 30° and 25° than it is between 5° and 

 0°, and jet actual observations show no difference in their 

 course. This table reminds us that diumal rotation should not, 



* "Average Force of the Trade-winds," p. 857, vol. ii., Maury's Sailing 

 Directions, 1859. 



