THE WINDS OF THE SOUTHERN HEMISPHERE. 4dO 



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

 ii 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 

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

 are stronger than the N.E., and so they should he if there he a cross- 

 imj of winds in the calm helt of Capricorn. 



816. Ditto of the counter-trades. — The counter-trades of the 

 southern hemisphere move, as before stated, towards their pole 

 more steadily and briskly than do the counter-trades of the north- 

 ern hemisphere. To give an idea of the difference of the strength 

 of these two winds, I cite the fact that vessels sailing through 

 the latter, as from jS'ew York to England, average 150 miles a 

 day. Along the corresponding latitudes through the former, as 

 on a voyage to Australia, the average speed is U23wards of 200 

 miles a day. Consequently, the counter-trades of the southern 

 hemisphere transport in given times larger volumes of air towards 

 the south than our counter-trades do towards the north. This 

 air returns to the tropical calm belts as an upper current. If, 

 descending there, it feeds the trade-winds, then, the supply being 

 more abundant for the S.E. trades than for the N.E., the S.E. 

 trades must be the stronger ; and so they are ; observations prove 

 them so to be. Thus the crossing of the air at the tropical calm 

 belts, though it may not be proved, yet it is shown to be so very 

 probable that the onus of proof is shifted. It now rests with 

 those who dispute the crossing to prove their theory the true one. 



have indicated, in order to establish the relative force of the trade-winds in the 

 .southern and in the northern hemisphere — determining the differences of velo- 

 city of the frigate with wind aft and wind abeam. 



" In my last letter, of tlie 25th of Januarj-, I gave you the results obtained 

 by the brig ' Zebra,' but they are very incomplete ; and I wish, with the 

 * Alceste,' to determine the comparative velocities obtained with a perfectly 

 regular wind in the different rates of going from the nearest the wind to wind 

 full aft. It is evident that the differences observed in this manner will bo 

 dependent on the special qualities of each ves.<el used in the experiments, and 

 that it will be also necessary to take full account of the state of tlie sea ; but 

 by multiplying experiments, one will certainly arrive at an average in wliich 

 confidence may be placed. I think with you, that for a ship sailing G knots, 

 with wind full att, there will be an increase of speed of 2-5 to 3 knots, with 

 the wind a little abaft the beam. I liave already liad occasion to remark that 

 for the 'Alceste' this difference sometimes exceeds 3 knots. My next letter 

 will give you the details of the experiments which I shall have made in this 

 respect, both south and north of the Line, which will necessarily lengthen my 

 voyage a little. I am too anxious to co-opei-ate in my feeble way in the great 

 works you are carrying on, not to sacritice willingly fcome hours daily whenever 

 circumstance s will permit." 



