350 THE PHYSICAL GEOGRAPHY OF THE SEA. 



tension, rarely fail to help tlie cause of progress and of truth; 

 for, though wrong in themselves, they impart interest to the sub- 

 ject, set others to thinking, and often suggest what is right. 

 Moreover, by such a course discussion is encouraged ; and scien- 

 tific discussion, when philosophically directed and properly con- 

 ducted, is always profitable. 



It was in this spirit that this work has, from its commencement, 

 been conducted, and it was in this spirit that in treating of the 

 general circulation of the atmosphere, the crossings at the calm 

 belts, chapter vi., were suggested. Take, as an illustration of this 

 crossing, the calm belt of Capricorn : Near this tropic there is a 

 band encircling the earth, from which the wind on one side is 

 perpetually blowing toward the equator, and on the other almost 

 as perpetually blowing toward the pole. These winds are on the 

 surface, and to supply air for such winds there must be a regular 

 influx — a pouring in — at the top of this calm belt. As the sur- 

 face currents carry away this air both toward the north and the 

 south, it must return by counter currents both from the north and 

 the south to keep up the supply. These counter currents are 

 admitted to be upper currents. The same in-pouring and out- 

 flowing take place at the calm belt of the equator, and the calm 

 belt of Cancer ; only at the equator the in-pouring currents are at 

 the surface of the earth, while at the two tropical calm belts they 

 are in the regions of the upper air. 



In consequence of diurnal rotation these currents, as they come 

 from toward the pole and approach the equator, whether as upper 

 or as surface currents, have much easting in them ; and, in like 

 manner, they acquire westing as they return toward the poles. 



Arrived with my investigations at this stage in the construc- 

 tion of a theory, a question of this sort arose : Does the air which 

 is poured into these calm belts from the north, for instance, return 

 to the north as it flows out, or does it keep on its circuit toward 

 the south? 



There seemed to be reasons for supposing that the air which 

 enters the calm belts from the north flows out toward the south,* 

 and vice versa ; consequently, it was held that the construction of 

 the atmospherical machmery is such as to require a crossing of 

 air in those calm belts. 



* Sec chapter vi. 



