82 Mr. H. H. Howorth on 



and outside the parallels named in both hemispheres we can 

 no longer trace the persistence of upper and lower currents 

 at all seasons, but the predominant direction of the air 

 motion is from the equator towards the pole ; and this 

 appears to increase in steadiness and force with the height 

 above the ground." {Elementary Meteorology, by R. H. Scott, 

 244-246.) 



This clearly proves that the circulation of the trades and 

 anti-trades is limited to the region between the equatorial 

 calms on the one hand, and the calms about the two tropics 

 of Cancer and Capricorn respectively. This we should in 

 fact expect from an examination of the tables of barometric 

 pressure. The mean pressure in the equatorial belt is only 

 about 29/84 inches, while between the 30th and 35th parallels 

 it reaches its maximum of 30 to 30 - o8 inches ; and as we 

 have seen, and as has been clearly proved, the lower winds 

 blow from situations of high pressure to those of low 

 pressure, while the return upper currents go the reverse 

 way. 



It is most important in the discussion we are engaged in 

 to remember this limitation in area of the trade winds, for 

 it is the trade winds alone with which we have virtually to 

 deal, since they give the equatorial current its principal 

 impulse. 



With the winds north and south of the two tropics we 

 have nothing to do in this discussion. They are much less 

 constant than the trades, their course being subject to many 

 influences, but in the main it is probable that there is a 

 general circulation between the temperate and the arctic 

 regions. This circulation, however, belongs to an entirely 

 different system, and does not, so far as we can see, interfere 

 with the trades and the anti-trades, which form a circuit of 

 their own. Dr. Croll has largely ignored this fact. He 

 argues as if the strength of the respective trade winds is 

 dependent on the fact that the difference in temperature 



