SEA ROUTES, CALM BELTS, AND VARIABLE WINDS. 329 



east than it does in the north-east trade-winds. According to tlie 

 observations of American na^dgators, it stands 0.050 inch higher.* 

 The former determination is derived from 30,873, the latter from 

 1899 observations; therefore 0.055 inch is entitled to most weight. 

 The trade-winds are best developed between the parallels of 5^ and 

 20*^. The mean barometric pressure between these parallels is 

 29.968 inches for the north-east, and 30.023 inches for the south- 

 east trade-winds ; while for the calm belt it is 29.915 inches. The 

 pressure, therefore, upon the air in each of the trade-winds is 

 greater than it is in the calm belt; and it is this difference of 

 pressure, from whatever cause arising, that gives the wind in each 

 system of trades its velocity. The diiference between the calm belt 

 and trade-wind pressure is 0.108 for the south-east and 0.053 for 

 the north-east. According to the barometer, then, the south-east 

 should be stronger than the north-east trade-'s\inds, and according to 

 actual observations they are.j 



642. Now if we liken the equatorial calm belt ^dth its diminished 

 Experiments in the pressuTC to a fm'naco, the north-east and the south- 

 French :savy. g^g^ trade- wmds may be not inaptly compared to a pair 

 of double beUows that are blowing into it. In excess of barometric 

 pressure, the 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 

 pressm-e 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 observa- 

 tions and calculations, as well as from du-ect experiments ordered 

 in the French brig of war " Zebra," by Admiral Chabannes, to be 

 the case.J 



* Maury's Sailing Directions : " Barometric Anomalies off Cape Horn." 



t Nautical Monograph, No. 1. 



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



" Observatory, Washington, 8th April, 1859. 



*' My dear Admiral, — My last was dated 15th 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, notwith- 

 standing that through the south-east trades the wind is aft, tlirough the north-east 

 just abaft the beam, [" In order 



