THE ATMOSPHERE. 89 



average of rain in the north temperate zone is, according to John- 

 ston, thirty-seven inches. He gives hut twenty-six in the south 

 temperate. The observations of mariners are also corroborative 

 of the same. Log-books, containing altogether the records for up- 

 ward of 260,000 days in the Atlantic Ocean north and south 

 (Plate XIII.), have been carefully examined for the purpose of 

 ascertaining, for comparison, the number of calms, rains, and gales 

 therein recorded for each hemisphere. Proportionally the number 

 of each is given as decidedly greater for the north than it is for 

 the south. The result of this examination is very instructive, for 

 it shows the status of the atmosphere to be much more unstable 

 in the northern hemisphere, with its excess of land, than in the 

 southern, with its excess of water. Rams, and fogs, and thunder, 

 and calms, and storms, all occur much more frequently, and are 

 more irregular also as to the time and place of their occurrence on 

 this side, than they are on the other side of the equator. 



176. Moisture is never extracted from the air by subjecting it 

 from a low to a higher temperature, but the reverse. Thus all 

 the air which comes loaded with moisture from the other hemi- 

 sphere, and is borne into this with the southeast trade-winds, trav- 

 els in the upper regions of the atmosphere (§ 130) until it reaches 

 the calms of Cancer ; here it becomes the surface wind that pre- 

 vails from the southward and westward. As it goes north it 

 grows cooler, and the process of condensation commences. 



177. We may now liken it to the wet sponge, and the decrease 

 of temperature to the hand that squeezes that sponge. Finally 

 reaching the cold latitudes, all the moisture that a dew-point of 

 zero, and even far below, can extract, is wrung from it ; and this 

 air then commences " to return according to his circuits" as dry 

 atmosphere. And here we can quote Scripture again : " The 

 north wind driveth away rain." This is a meteorological fact of 

 high authority and great importance in the study of the circula- 

 tion of the atmosphere. 



178. By reasoning in this manner and from such facts, we are 

 led to the conclusion that our rivers are supplied with their waters 

 principally from the trade-wind regions — the extra-tropical north- 

 ern rivers from the southern trades, and the extra-tropical south- 



