34:2 '^^^^^ PHYSICAL GEOGKAFHY OF THE SEA. 



Sailing throufrh them ^^^o coiiiirmecl bj observatioDS ; for such is the dif' 

 in fuif and wrnter. fgrence as to Strength and regularity of the north- 

 east trade- winds in September and February, that the average 

 passage through them from New York to the line is 26.4 days in 

 the winter against 38.8 in the fall month. 



640. Thus it appears that the equatorial calm belt is made to 

 A thermal adjust- ^^^^^ ^^^ placc with the scasons, 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 (§ 638) depend upon the changes in the temperature 

 and barometric weight of the air which each system puts in mo- 

 tion. This calm belt, therefore, may be considered as a thermal 

 adjustment — the dynamical null-belt — between the trade-winds of the 

 two hemispheres. 



641. The observations on the barometer at sea (§ 855) shed 



The barometer in the light OU this SubjCCt. AcCOrding tO the Dutch, 

 trade-winds and equa- . ^ ^ • ^ ^ r\ r\- 1- - ^ ' ^ 



toriai calms. that mstrumcnt stands higher by O.Ooo mch in the 



southeast than it does in the northeast trade -winds. Accord- 

 ing to the observations of American navigators, 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 be- 

 tween these parallels is 29.968 inches for the northeast, and 30.023 

 inches for the southeast 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 difference be- 

 tween the calm belt and trade- wind pressure is 0.108 for the south- 

 east and 0.053 for the northeast. According to the barometer, 

 'then, the southeast should be stronger than the northeast trade- 

 winds, and according to actual observations they are.f 



642. ISTow if we liken the equatorial calm belt with its dimin- 

 Experiments in the ishcd prcssurc to a fumacc, the northeast and the 

 French Navy. southcast trade-wiuds may be not inaptly likened 



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

 t Nautical Monograph, No. 1. 



