86 Mr. H. H. Howorth on 



o-ets more and more contracted and heavier. The former 

 current, therefore acquires, in addition to its horizontal move- 

 ment, first a tendency to rise, and then an actually upward 

 movement, and the latter a corresponding tendency to sink, 

 and eventually a downward movement ; and when the focus 

 of heat or cold is reached this upward and downward move- 

 ment respectively becomes dominant, and the currents of air 

 do not press against each other, but rise up almost vertically 

 and lose their lateral motion. It does not seem to me that 

 there can be any pressing of one set of winds against the 

 other, and both the north and south trades, on reaching their 

 furnace, must move upwards, and not laterally. The region 

 of calms merely marks the place where the vertical move- 

 ment upwards and downwards takes place. 



It is clear that the winds could not under any circum- 

 stances meet each other face to face, since, as we have seen, 

 the motion of the earth gives them a very decided motion 

 from east to west. If they collided at all, therefore, instead 

 of neutralizing each other, they would acquire a more direct 

 course westwards, as the corresponding currents of water 

 do ; and instead of a region of calms in the tropics we 

 should have a region of more or less violent east winds 

 along that zone, of which I know of no adequate evidence. 

 In every way the question is examined, it seems to me to 

 be impossible to attribute the equatorial calms to the 

 neutralizing tendency of a conflict of winds, as Dr. Croll 

 urges, and this is a most important factor in his argument. 



Let us proceed, however ; it is perfectly true, and an 

 elementary fact in physical geography, that the two trade 

 winds do not always blow between the same parallels, and 

 that the region of intervening equatorial calms is not sta- 

 tionary, but that the whole move together, to and fro, north 

 and south, as the sun moves from one equinox to the other. 

 This movement, Dr. Croll seems to argue, is due to the 

 greater strength of the north trade at one time of the 



