§ 815, 81G. THE WINDS OF THE SOUTHERN HEMISPHERE. 435 



while through the northeast trades the average speed is 6J knots 

 an hour one way (N.W. J W.), with the wind just abaft the beam, 

 and of the other (S.S.E.), with the wind at a point not so favor- 

 able for speed. Indeed, most of the ships which average a S.S.E. 

 course through this part of the northeast trade-wind belt are close 

 hauled ; therefore the average strength of the trades here can not 

 be fairly compared with the average strength where the fleet have 

 free winds. What is the difference in the strength of such winds, 

 which, impinging upon the sails, each at the particular angle indi- 

 cated above, imparts the aforesaid velocities? Moderate winds, 

 such as these are, give a ship her highest speed generally when 

 they are just abaft the beam, as they are for a northwest course 

 through the northeast trades of the N. Atlantic. So, to treat these 

 ships as anemometers which will really enable us to measure the 

 comparative strength of the winds, we should reduce the aver- 

 age knots per hour to the average speed of a mean ship sailing 

 through average " trades" in each ocean, with the wind imping- 

 ing upon her sails at the same angle for all three, as, for example, 

 just abaft the beam, as in the North Atlantic. 



815. Let us apply to the average speed through the South Atlan- 

 veiocity of the trade- ^^^ ^^^ Indian Occaus such a correction. Through 

 ^'■^°^'- the former the wind is aft ; through the latter, quar- 

 tering. If we allow two knots as a correction for the one, and 

 one knot for the other, we shall not be greatly out. Applying 

 this correction, we may state the speed of a mean ship sailing 

 with average trades just abaft the beam to be as follows: 



Through the N.E. of the Atlantic 6i knots per hour.* 



u " S E " " 8 " " " 



" " S.E. " Indian Ocean.. 8 " " " 



I do not take into this comparison the force of the N.E. trades on 

 a S.S.E. course (§ 813), because ^the winds along this route are 

 known not to be as steady as they are farther away from the Af- 

 rican coast. Thus it is clearly established that the S.E. trades are 

 stronger than the N.E., and so they should be if there he a crossing 

 of winds in the calm belt of Capricorn. 



816. The counter-trades of the southern hemisphere move, as 



* The correctness of this estimate is sustained by experiments which Admiral 

 Chabannes, in command of the French fleet on the coast of Brazil, has been so kind 

 as to institute. See note, p. 343-5. 



