82 
THE PHYSICAL GEOGRAPHY OF THE SEA. 
r 
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 which 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 we 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, 
