THE CLIMATES OF THE OCEAN. 233 
to get cool. This brings the highest temperature to the sea in 
September, the lowest in March. Plate IV. is intended to show 
the extremes of heat and cold to which the waters- — not the ice — 
of the sea are annually subjected, and therefore the isotherms of 
40°, 50°, 60°, 70°, and 80° have been drawn for March and Sep- 
tember, the months of extreme heat and extreme cold to the in- 
habitants of the "great deep." Corresponding, isotherms for any 
other month will fall between these, taken by pairs. Thus the 
isotherm of 70° for July will fall nearly between the same iso- 
therms (70°) for March and September. 
497. A careful study of this plate, and the contemplation of the 
benign influences of the sea upon the climates which we enjoy, 
suggest many beautiful thoughts ; for by such study we get a 
glimpse into the arrangements and the details of that exquisite 
machinery in the ocean which enables it to perform all its offices, 
and to answer with fidelity its marvelous adaptations. 
498 How, let us inquire, does the isotherm of 80°, for instance, 
get from its position in March to its position in September ? Is it 
wafted along by currents, that is, by water which, after having 
been heated near the equator to 80°, then flows to the north with 
this temperature ? Or is it carried there simply by the rays of 
the sun, as the snow-line is carried up the mountain in summer ? 
We have reason to believe that it is carried from one parallel to 
another by each of these agents acting together, but mostly through 
the instrumentality of currents, for currents are the chief agents 
for distributing heat to the various parts of the ocean. The sun 
with his rays would, were it not for currents, raise the water in 
the torrid zone to blood heat ; but before that can be done, they 
run off with it to the poles, softening, and mitigating, and temper- 
ing climates by the way. The provision for this is as beautiful 
as it is benign ; for, to answer a physical adaptation, it is provided 
by a law of nature that when the temperature of water is raised, ■ 
it shall expand ; as it expands, it must become lighter, and just in 
proportion as its specific gravity is altered, just in that proportion 
is equilibrium in the s«a destroyed. Arrived at this condition, it 
is ordained that this hot water shall obey another law of nature, 
which requires it to run away, and hasten to restore that equilib- 
rium. Were these isothermal lines moved only by the rays of the 
sun, they would slide up and down the ocean like so many paral- 
