SEA BOUTES, CALM BELTS, AND VARIABLE WINDS. 343 



square foot. It is this pressure which, like the weight upon the 

 real bellows in the smithy, keeps up the steady blast ; and as the 

 eftective weight upon the one system of trades is about double 

 that upon the other, the one under the greatest pressure should 

 blow with nearly double the strength of the other, and this 

 appears, both from actual observations and calculations, as well 

 as from direct experiments ordered in the French brig of war 

 " Zebra," by Admiral Chabannes, to be the case.* 



* Letter to Admiral Chabannes, with extracts from his reply thereto : — 



'' Observatory, Washington, 8th April, 1859. 



" My dear Admiral, — My last was dated loth January ultimo. I hope the 

 charts and vol, i., 8th ed. Sailing Dhections, and jjart of vol. ii. in the sheet;^, 

 came safely to hand. Vol, ii, is just out, and I hasten, in homage of my 

 respect, and as a token of good-will, to lay a copy before you, 



" Permit me, if you please, to call your attention to the chapter on the ' Ave- 

 rage Force of the Trade-winds,' p, 857, and especially to the table of compara- 

 tive speed (of sailing vessels) through the north-east and south-east trade-winds 

 of the Atlantic, p. 865, The average speed, you observe, is nearly the same, 

 notwithstanding that through the south-east trades the wind is aft, thi-ough 

 the north-east just abaft the beam, 



" In order to treat this question thoroughly, it is very desirable to know the 

 difference in the speed of vessels when sailing with the same wind aft, with it 

 quartering, with it a point or two abaft the beam, and with it close hauled, 

 AVith a good series of experiments upon this subject, we should be able to arrive 

 at definite conclusions with regard to the average ditference in force not only ol 

 the two systems of trade-winds, but of the winds generally in various parts of 

 the ocean. 



" If we assume that a wind which, being dead aft, drives a vessel at the rate 

 of six knots, will, when brought nearly abeam, drive her eight knots — as in 

 this chapter I have supposed — and then if we apply the dynamical law of the 

 resistance increasing as the squares of the velocity of the ship, we should be 

 led to the remarkable conclusion that the average velocity of the north-east 

 to south-east trades of the Atlantic is as 36 to 64. Therefore, in comiucting 

 these experiments, it would be very desirabie to know the area of canvas that 

 fcihly feels the wind when it is aft, and the area uj)on which the wind blows 

 when the ship is hauled up. Suffice it to say, that the facts which we 

 already have, indicate that the south-east trades, both of the Atlantic and 

 Indian Oceans are fresher than the north-east trades of the Atlantic, May we 

 infer from this that the south-east trades of the Pacific are also fresher than the 

 north-east trades of that ocean ? If we may so infer, and be right, then tliere 

 is another step which we may take with boldness, and pronounce the atmosphe- 

 rical circulation of the southern hemisphere to be much more active than tliat 

 of the northern. And having reached this round in the ladder uj) which I am 

 soUeitiug you to accompany me, we are prepared to pause and take a view of 

 some of the new pliy.sical aspects which these facts and this reasoning spread 

 out before us, 



" That 



