§ 0^2. SEA ROUTES, CALM BELTS, ETC. 843 



to a pair of double bellows that are blowing into it. In excess 

 of barometric pressure, tire former is a bellows with a weight of 

 3.8 lbs., the latter with a weight of 7.8 lbs. to the 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 effective weight 

 upon the one system of trades is about double that upon the oth- 

 er, the one under the greatest pressure should blow with nearly 

 double the strength of the other, and this appears, both from ac- 

 tual observations and calculations, as well as from direct experi- 

 ments ordered in the French brig of war " Zebra," by Admiral 

 Chabannes, to be the case.^ 



* Letter to Admiral Chabannes, with extract of his reply thereto : 



" Observatory, Washington, 8th AprU, 1S59. 



"My dear Admiral, — My last was dated 15th Januar}^, ultimo. I hope the 

 charts and vol. i., 8th ed. Sailing Directions, and part of vol. ii. in the sheets, 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 ^ Averar/e 

 Force of the Trade-winds,' ]). 857, and especially to the table of comparative speed 

 (of sailing vessels) through the northeast and southeast trade-winds of the Atlantic, p. 

 SGo. The average speed, you observe, is nearly the same, notwithstanding that through 

 the southeast trades the wind is aft, through the northeast just abaft the beam. 



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

 ference in the speed of vessels when sailing with the same wind aft, with it quarter- 

 ing, with it a point or two abaft the beam, and with it close hauled. "With a good 

 series of experiments upon this subject, we should be able to arrive at definite con- 

 clusions with regard to the average difference in force not only of tlie 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 conclu- 

 sion that the average velocity of the northeast to southeast trades of the Atlantic is 

 as 3G to Qi. Therefore, in conducting these experiments, it would be very desirable 

 to know the area of canvas that fairly feels ^he wind when it is aft, and the area 

 upon which the wind blows when the ship is hauled up. SuflSce it to say, that the 

 facts which we already have indicate that the southeast trades, both of the Atlantic 

 and Indian Oceans, are fresher than the northeast trades of the Atlantic. May we 

 infer from this that the southeast trades of the Pacific are also fresher than the north- 

 east trades of that ocean ? If we may so infer and be right, then there is another 

 step which we may take with great boldness, and pronounce the atmospherical cir- 

 culation of the southern hemisphere to be much more active than that of the north- 

 ern. And having reached this round in the ladder up which I am soliciting you to 

 accompany me, we are prepared to pause and take a view of some of the new phys- 

 ical ap])ccts v.'hifh these facts and this reasoning spread out before us. 



