82 THE PHYSICAL GEOGRAPHY OF THE SEA. 



121. The great rivers of North America and North Africa, and 

 all the rivers of Europe and Asia, lie wholly within the northern 

 hemisphere. How is it, then, considering that the evaporating sur- 

 face lies mainly in the southern hemisphere — how is it, I say, that 

 we should have the evaporation to take place in one hemisphere 

 and the condensation in the other ? The total amount of rain 

 which falls in the northern hemisphere is much greater, meteorol- 

 ogists tell us, than that w^hich falls in the southern. The annual 

 amount of rain in the north temperate zone is half as much again 

 as that of the south temperate. 



122. How is it, then, that this vapor gets, as stated § 119, from 

 the southern into the northern hemisphere, and comes with such 

 regularity that our rivers never go dry and our springs fail not ? 

 It is because of the beautiful operations and the exquisite compen- 

 sation of this grand machine, the atmosphere. It is exquisitely 

 and wonderfully counterpoised. Late in the autumn of the north, 

 throughout its winter, and in* early spring, the sun is pouring his 

 rays with the greatest intensity down upon the seas of the south- 

 ern hemisphere, and this powerful engine which w^e are contem- 

 plating is pumping up the water there (§ 119) for our rivers with 

 the greatest activity. At this time, the mean temperature of the 

 entire southern hemisphere is said to be about 10° higher than the 

 northern. 



123. The heat which this heavy evaporation absorbs becomes 

 latent, and, with the moisture, is carried through the upper regions 

 of the atmosphere until it reaches our climates. Here the vapor 

 is formed into clouds, condensed, and precipitated. The heat 

 which held this water in the state of vapor is set free, it becomes 

 sensible heat, and it is that which contributes so much to temper 

 our winter climate. It clouds up in winter, turns warm, and we 

 say we are going to have falling weather. That is because the 

 process of condensation has already commenced, though no rain 

 or snow may have fallen : thus we feel this southern heat, that 

 has been collected from the rays of the sun by the sea, been bot- 

 tled away by the winds in the clouds of a southern summer, and 

 set free in the process of condensation in our northern winter. 



124. If the Plate at page 70 fairly represent the course of the 

 winds, the southeast trade-winds would enter the northern hemi- 

 sphere, and, as an upper current, bear into it all their moisture, 



