318 On an Isothermal Oceanic Chart, illustrating 
The range of temperature is far greater in the Temperate zone 
than in the Torrid, it being 20° F. in the latter, and 33° F. in 
the former; and this should be a cause of a greater variety of 
genera in the latter for the same number of species. 
In the Torrid zone, the Subtorrid Region has nearly one-third 
the surface of the Torrid Region, and not one-fourth as much 
_ coast line, facts which should be regarded in comparing the num- 
er of species of the two. 
We add here, a few brief remarks, in a popular way, on the 
origin of the peculiar forms and positions presented by the iso- 
thermal lines of the ocean. The great currents of the globe are 
admitted to be the causes that produce the flexures and modify 
the courses of these lines. hese currents are usually of great 
depth, and consequently the deflecting land will be the deeply 
seated slopes off a coast, beyond ordinary soundings. 
The eastern coasts of the continents either side of the equator, 
feel the influence of a warm equatorial current, which flows west- 
ward over each ocean, and is diverted north and south by the 
coasts against which it impinges, and more or less according to 
the direction of the coast. 
The western coasts of the continents, on the contrary, receive a 
strong extra-tropical or polar current. In the southern oceans, it 
flows from the westward, or southward and westward, in latitudes 
45° to 65° south, and is brought to the surface by the submarine 
lands or the submarine slopes of islands or continents: reaching the 
continents of Africa and South America, it follows along the west- 
ern coasts towards the equator. The same current, being divided 
by the southern cape of America, flows also, with less volume up 
the eastern coast, either inside of the warmer tropical current, or 
else on both sides of it. In the Northern Seas, the system 0 polar 
currents is mainly the same, though less regular; their influence 
is felt on both eastern and western coasts, but more strongly on 
the eastern. In the Atlantic, the latter reduces the temperature 
of the waters three or four degrees along the north coast of South 
America, as far nearly as Cape St. Roque. 
The cold currents are most apparent along the coasts of con- 
tinents and about islands, because they are here brought to the 
surface, the submarine slopes lifting them upward, as they flow 
he limits of their influence towards the equator depends 
often on the bend of the coast; for a prominent cape ora ben 
in the outline will change the exposure of a coast from that favor- 
able to the polar current to that favorable to the tropical, or the 
reverse. Thus it is at Cape Hatteras, on the coast of the United 
States ; Cape Verde, on Western Africa ; Cape Blanco, on western 
South America, etc. 
ese are important principles modifying the courses of the 
Oceanic isothermal lines. We may now proceed to the application 
