434 PHYSICAL GEOGllAPHY OF THE SEA, AND ITS METEOROLOGY. 



average strength where the fleet have free winds. What is the 

 diiference in the strength of snch winds, which impinging upon 

 the sails, each at the particular angle indicated 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 tliey are for a north-west course through the north-east 

 trades of the North Atlantic. So, to treat these ships as anemo- 

 meters that will really 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, with the wind impinging upon her sails 

 at the same angle for all three, as, for example, just abaft the 

 beam, as in the JNTorth Atlantic. 



815. Velocity of the trade-winds. — Let us apply to the average speed 

 through tire South Atlantic and Indian Oceans such a correction. 

 Through the former the wind is aft ; through the latter, quarter- 

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

 as a correction for the other, w^e shall not be greatly out. Apply- 

 ing such corrections, we may state the speed of a mean yhip 

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



Through the N.E. of the N. Atlantic . . Oj knots per hour.* 



S.E. „ S. Atlantic . . 8 



,, S.E. „ S. Indian Ocean . 8 „ 



* TLiat this correction is not too large is indicated not only by the experi- 

 ments wliicb Admiral Chabaunes, in command of the Frencli fleet on tlie 

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

 experiments wliich he has subsequently made with the frigate "Alceste," 

 as per tlie following extracts of a letter, just received from that distinguished 

 officer, dated — 



" Frigate ' L'Alceste,' at Monte Video, May 10, 1860. 

 " Dear Sir,— I am replying to your letter of the 30th of January last, which 

 I have received with the duplicates of that dated January 15th, 1851), wliich 

 you have had the goodness to send me through M. de Montholon, as well as 

 the interesting nautical monograph No. 1, and, finally, the manuscript passage 

 from an article relative to the force of the winds to tiie height and velocity of 

 clouds and waves. I regret exceedingly that I have not under my orders a 

 squadron of sailing vessels with which I could put to sea, and devote myself 

 to all these resoarclies. Unfortunately, I have only steamers in my division, 

 with the exception of tlie frigate which carries my flag, and political circum- 

 stances have retained me almost constantly in the La Plata. I am now ap- 

 proaching the end of my command, and in a few days must leave Monte Video 

 to engage in some liydrographical works on tlie coast of Brazil, and afterwards 

 I shall return to Fiance, where I expect to arrive in the beginning of September. 

 I shall not fail during this voyage to make a series of experiments such as you 



