THE CLIMATES OF THE OCEAJN: 
237 
latitudes, we should have more clouds ; and therefore it would re- 
quire a longer time for the sun, with his feeble rays, to raise the 
temperature of the cold water, which, from September to January, 
has brought the isotherm of 60° from latitude 56° to 40°, than it 
did for these cool surface currents to float it down. After this 
southward motion of the isotherm of 60° has been checked in De- 
cember by the cold, and after the sources of the current which 
brought it down have been bound in fetters of ice, it pauses in the 
long nights of the northern winter, and scarcely commences its 
return till the sun recrosses the equator, and increases its power 
as well in intensity as in duration. 
Thus, in studying the physical geography of the sea, we have 
the effects of night and day, of clouds and sunshine, upon its cur- 
rents and its climates, beautifully, developed. These effects are 
modified by the operations of certain powerful agents which re- 
side upon the land ; nevertheless, feeble though those of the for- 
mer class may be, a close study of this plate will indicate that they 
surely exist. • > 
508. Now, returning toward the south : we may, on the other 
hand, infer that the mean atmospherical temperature for the par- 
allels between which the isotherm of 80° fluctuates is below 80°, 
at least for the nine months of its slow motion. This vibratory 
motion suggests the idea that there is, probably, somewhere be- 
tween the isotherm of 80° in August and the isotherm of 60° in 
January, a line or belt of invariable or nearly invariable temper- 
ature, which extends on the surface of the ocean from one side of 
the Atlantic to the other. This line or band may have its cycles 
also, but they are probably of long and uncertain periods. 
509. The fact has been pretty clearly established by the dis- 
coveries to which the wind and current charts have led, that the 
western half of the Atlantic Ocean is heated up, not by the Gulf 
Stream alone, as is generally supposed, but by the great equato- 
rial caldron to the west of longitude 35°, and to the north of Cape 
St. Roque, in Brazil. The lowest reach of the 80° isotherm for 
September — if we except the remarkable equatorial flexure (Plate 
IV.) which actually extends from 40° north to the line — to the 
west of the meridian of Cape St. Roque, is above its highest reach 
to the east of that meridian. And now that we have the fact, how 
obvious, beautiful, and "striking is the cause ! 
