Page:Physical Geography of the Sea and its Meteorology.djvu/369

SEA ROUTES, CALM BELTS, AND VARIABLE WINDS. 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 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 10th January 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 Average Force of the Trade-winds, p, 857, and especially to the table of comparative 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, through 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, With a good series of experiments upon this subject, we should be able to arrive at definite conclusions with regard to the average difference in force not only of 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 conducting these experiments, it would be very desirable to know the area of canvas that fairly feels the wind when it is aft, and the area upon 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 there is another step which we may take with boldness, and pronounce the atmospherical circulation of the southern hemisphere to be much more active than that of the northern. 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 physical aspects which these facts and this reasoning spread out before us.