148 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS IIETEOEOLOGT. 



■with a mean direction for the latter — which are the prevaiHng 

 winds hetween those parallels — of S. 47° 30' E. According to the 

 Halleyan theory these should be south-w^est mnds. 



343. In the Atlantic the average velocity of the south-east is. 

 Velocities of the greater than the average velocity of the north-east 

 trade-winds. tradcs.* I estimate one to be from 14 to 18, the 



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

 to be 14 and 25 respectively, the following departures show the 

 miles of easting which the trade-winds average per horn* through 

 each of the above-named belts : 



Hourly Eate of Depaetl-re of the Teade-winds aceoss the Belts. 



344. That dim-nal rotation does impart easting to these wiads 

 Difference between thoro is uo doubt : but the path Suggested by the 



observation and iiii i e j.xlj.1' 1 T i. 



theory. table docs not coniorm to that wdiicn, accorcimg to 



any reasonable h3rpothesis, the trade-winds w^ould follow if left to- 

 obey the forces of dim^nal rotation alone, as they would do w-ere 

 dimiial rotation the sole cause of their easting. As these winds 

 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 liave be- 

 tw^een 30^ and 25°. Nay, the south-east trades between the equa- 

 tor and 5° N. — where, by the Halleyan theoiy, they sJiouId have 

 westing — 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 resistmg agents, by 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 yet actual observations show^ no 

 difference in their course. This table reminds us that diimial rota- 

 tion should not, until more numerous and accm^ate observations 



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

 tions, 1859 



