418 PHYSICAL GEOGRAPHY OF THE SEA, AXD ITS METEOROLOGY. 



814. It is well to observe, that on each of these three oceans, 

 Ships used as anemo- though the direction of the wind is the same, the 

 ™^*"'^^- course steered by each fleet is different; conse- 

 quently, these sailing anemometers are at different angles with the 

 wind ; through the south-east trades, the wind is nearly aft in the 

 Atlantic, and quartermg in the Indian Ocean, giving an average 

 sailing speed of 7 knots an hour in the latter, and of 6 in the 

 former ; while through the north-east trades the average speed is 6 J 

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

 the beam, and 5J the other (S.S.E.), with the wind at a point not 

 so favourable for speed. Indeed, most of the ships which average 

 a S.S.E. course through this part of the north-east trade-wind belt 

 are close hauled ; therefore the average strength of the trades here 

 cannot 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 

 indicated! above, imparts the afores^d 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 north-west course 

 through the north-east trades of the North Atlantic. So, to 

 treat these ships as anemometers that wiU reaUy enable us to 

 measure the comparative strength of the winds, we should reduce 

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

 sailing through average " trades " in each ocean, T\dth the wind 

 impinging 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- tic and Indian Occans such a correction. Through 

 ^^•^s- the former the wind is aft ; through the latter, quar- 

 tering. K we aUow two knots as a correction for the one and one 

 as a correction for the other, we shall not be greatly out. Applying 

 such corrections, we may state the speed of a mean ship saihiig with 

 average trades just abaft the beam to be as foUows : 



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

 S.E. „ S. Atlantic . . 8 

 S.E. „ S. Indian Ocean .8 . „ 



* That this correction is not too large is indicated not only by the experiments 

 (note pp. 343-5) which Admiral Chabannes, in command of the French fleet on 

 the coast of BrazO, has kindly caused to be made by the brig " Zebra," but by 

 experiments whichjhe has subsequently made with the frigate *' Alceste," as per 



