§ 644. SEA ROUTES, CALM BELTS, ETC. 345 



Difference in tons of that the Hieaii piessuie of the atmosphere is 15 lbs. 



the barometric press- ^ , 



lire upon the north- uDOii the SQuaie mch, we mav readily determine in 

 trade-winds. tons the total bj which the superincumbent press- 



ure upon the southeast trade-winds between the parallels of 5° 

 and 20° exceeds that upon the northeast between corresponding 

 parallels. For the whole girdle of the earth, the excess of pressure 

 upon the southeast trade-winds is 1,235,250 millions of tons. 

 This is the superincumbent weight or pressure which is urging 

 the southeast trade-winds forward faster than the northeast. It 

 is inconceivably great; and to bring it within comprehensible 

 terms, the mariner will be still 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 southeast than it is when he is sailing in her 

 through the northeast trade-winds, 



614. The question now suggests itself, Why should the ba- 

 why the barometer romctcr staud his^hcr iu the southeast than it does 



Bhould stand higher . t • t o rm 



in the southeast m thc Horthcast tradc-wmds : The theory of a 



than in the north- . ■iiiiy'ti r^^^ 



east trade-winds. crossiug at the calm bclts auords the answer. The 

 air which the northeast trade-winds deliver into the calm belt is 

 not as heavily laden with moisture as that of the southeast trades. 

 It is not as heavily laden for two reasons ; one is, the southeast 

 trade-wind belt is broader than the northeast ; consequently, in 

 the former there is more air in contact with the evaporating sur- 

 face. In the next place, the northeast trade-wind belt includes 

 more land within it than the southeast ; consequently, when the 

 two winds arrive at the calm belt, they are, for this reason, also 

 unequally charged with moisture. Now, when they rise up and 

 precipitate this moisture, more heat is liberated from the soutli- 

 east than from the northeast trade-wind air ; the latter, therefoi e, 

 is the cooler and the more compact*,, and as, by the theory of tlie 

 crossings, it flows off to the south as an upper current, it presses 

 upon the barometer with more weight than the warmer and more 

 moist air that feeds the current which is above and counter to the 

 northeast trades. There is not in the whole range of marine me- 



way 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 G to 8 knots, the force of the 

 wind aft might be expressed by 4, and of the wind abeam by 6. * * * * 



" C. DE Chabannes." 



