SEA ROUTES, CALM BELTS, AND VARIABLE WINDS. 345 



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 incon- 

 ceivably great ; and to bring it within comprehensible terms, the 

 marmer 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 lie 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-imnds. — 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 aifords 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 

 heavil}^ 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 beheve me, my dear admiral, yours very truly, 



"M.F. Maury. , 



•"' Admiral C. de CHABAxyES, 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, 1S60. 



" My dear Sir, — * * * * ^s 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 

 Avind, but I have not yet been able to deduce any positive rule, the experiments 

 not having been sutRciently multiplied. I can, however, give as a result tliat 

 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 (J 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, 1S60. 



Just in time for insertion here, I receive from the gallant admiral the sub- 

 joined 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 sliip ; at ten it is two points abaft tlie beam. From these 

 •experiments we infer tliat an average sailer that goes six knots before the wind. 



