Page:Physical Geography of the Sea and its Meteorology.djvu/371

east trade-winds is 1,235,250 millions of tons. This is the super-incumbent weight or pressure which is urging the south-east trade-winds forward faster than the north-east. It is inconceivably great; and to bring it within comprehensible terms, the mariner will be astonished to hear that the weight of atmosphere which is bearing down upon the deck of a first-class clipper ship is 15 or 20 tons greater when he is sailing in her through the south-east than it is when he is sailing in her through the north-east trade-winds.

644. Why the barometer should stand higher in the south-east than in the north-east trade-winds.—The question now suggests itself. Why-should the barometer stand higher in the south-east than it does in the north-east trade-winds? The theory of a crossing at the calm belts affords the answer. The air which the north-east trade-winds deliver into the calm belt is not as heavily laden with moisture as that of the south-east trades. It is not as heavily laden for two reasons; one is, the south-east trade-wind belt is broader than the north-east; consequently, in the former there is more air in contact with the evaporating surface. In the

 pray ascribe it to my desire to get by actual experiment an expression in the average speed of ships for the actual force and velocity of the winds.

"Wishing you all success and good luck in the investigation which you have in hand, pray believe me, my dear admiral, yours very truly,

"M. F. Maury.

"Admiral C. de Chabaness, Commander-in-chief of the French Naval Division of Brazil and La Plata, Rio de Janeiro."

Extract from a letter in reply to the foregoing:—
" Montevideo, January 25, 1860.
"My dear Sir,— * * * * As you have indicated to me in your letter of April, I have caused to be made, by a brig of my division, experiments upon the comparative velocities, wind abaft and wind abeam with a given force of wind, but I have not yet been able to deduce any positive rule, the experiments not having been sufficiently multiplied. I can, however, give as a result that the increase of headway given by wind abeam over the headway with wind aft has been a little less than two knots; when the velocity with wind aft was from 6 to 8 knots, the force of the wind aft might be expressed by 4, and of the wind abeam by 6. * * * *
"C. de Chabannes."
London, November 26, 1860.

Just in time for insertion here, I receive from the gallant admiral the subjoined very valuable and interesting series of experiments on the speed of his ship:—first, before the wind, i.e. sixteen points, and so on for every two points to six, and close hauled. At eight points the wind is abeam and at right angles with the course of the ship; at ten it is two points abaft the beam. From these experiments we infer that an average sailer that goes six knots before the wind.