§ 670-672. 



SEA ROUTES, CALM BELTS, ETC. 



357 



§ 670. Barometric Table. 



Mean Height of the Barometer as observed between 



671. These are the observed heights ; for the want of data, no 

 tjje corrections have been applied to them ; and for the 



Barometer 

 poles. 



at 



want of numbers sufficient to give correct means, 

 they lack that uniformity which larger numbers would doubtless 

 give. They show, however, most satisfactorily, that a low ba- 

 rometer is not peculiar to Cape Horn regions alone ; they show 

 that it is common to all high southern latitudes ; and other obser- 

 vations (§ 362) show that it is peculiar to these and not to north- 

 ern latitudes. Projecting on a diagram, A, with parallels of lati- 

 tude and the barometric scale as ordinates and abscissa, a curve 

 S, which will best represent the observations (§ 670), and continu- 

 ing it to the south pole — also projecting another curve, N, which 

 will best represent the observations (§ 362) on the polar side of 40"^ 

 IST., and continuing it to the north pole — we discover that if the 

 barometric pressure in polar latitudes continues to decrease for 

 the unknown as it does for the known regions, the mean height 

 of the barometer would be at the north pole about 29.6, at the 

 south about 28 inches. These lines, N and S, represent what 

 may be called the barometric descent of the counter-trades. 

 • 672. The rarefaction of the air in the polar calms is, as we 

 The ''brave west havc sccu (§ 667), sufficicut to crcatc an indraught all 

 metric descent. arouud to thc distaucc of fifty degrees of latitude 

 from the south pole ; also (§ 662) the rarefaction in the belt of 

 equatorial calms is sufficient to extend with its influence no far- 

 ther than thirty degrees of latitude. This fact also favors the 

 idea suggested by the diagram (§ 671), that the mean height of 

 the barometer in the polar calms is very much less than it is in 

 the equatorial. Moreover, the counter-trades of the southern 

 hemisphere are very much stronger (§ 626) than the counter- 



