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 African coast. Thus it is clearly established that the S.E. trades are stronger than the N.E., and so they should be if there be a crossing of winds in the calm belt 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 northern 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 New 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 upwards 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 velocity of the frigate with wind aft and wind abeam.
"In my last letter, of the 25th of January, 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 be dependent on the special qualities of each vessel used in the experiments, and that it will be also necessary to take full account of the state of the sea; but by multiplying experiments, one will certainly arrive at an average in which confidence may be placed. I think with you, that for a ship sailing 6 knots, with wind full aft, there will be an increase of speed of 2-5 to 3 knots, with the wind a little abaft the beam. I have already had 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-operate in my feeble way in the great works you are carrying on, not to sacrifice willingly some hours daily whenever circumstances will permit."
