342 niYSICAL GEOGRAPnY OF THE SEA, AND ITS METEOROLOGY. 



the nortli-cast trade-winds in September and Febniary, that the 

 average passage through them from New York to the line is 2G.4 

 days in the winter against 38.8 in the fall month. 



G40. A thermal adjustment — Thus it appears that the equatorial 

 calm belt is made to shift its place wdth the seasons, not by 

 reason of the greater intensity of the solar ray in the latitude 

 where the calm belt may be at that season, but by reason of the 

 annual variations in the energy of each system of trades ; which 

 variations (§ G38) depend upon the changes in the temperature 

 and barometric weight of the air which each system puts in 

 motion. This calm zone, therefore, may be considered as a 

 thermal adjustment — the dynamical mdl-helt — between the trade-winds 

 oftlie tioo hemispheres. 



641. The harometer in the trade-ioinds and equatorial calms. — The 

 observations on the barometer at sea (§ 858) shed light on this 

 subject. According to the Dutch, that instrument stands higher 

 by 0.055 inch in the south-east than it does in the north-east 

 trade-winds. According to the observations of American navi- 

 gators, it stands 0.050 inch higher.* The former determination 

 is derived from 80,873, the latter from 1899 observations ; there- 

 fore 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, there- 

 fore, 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 difference 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-winds, and according to actual 

 observations they are.f 



642. Experiments in the French Navy. — Now if we liken the 

 equatorial calm belt with its diminished pressure to a furnace, 

 the north-east and the south-east trade-winds may be not inaptly 

 compared to a pair of double bellows 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 



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

 t Nautical Monograpli, No. 1. 



