16 
THE COTTOir ZONE OF THE WORLD. 
and cultivated to some extent ; but, so far as our observa- 
tions have extended in tbe last twenty years, the plant can- 
not be profitably cultivated north of 35-|^°. 
A correct view of the limits of this wide region may 
be obtained by an examination of the climatic chart and a 
close inspection of the isothermal lines. 
Isothermal lines are the lines of equal heat, extending 
around the globe. These lines do not coincide throughout 
with the parallels of latitude, but are always serpentine in 
their course. They have been determined by long-contin- 
ued observation with the thermometer, and we present 
them to the view of the reader as the result of scientific 
labor performed through a long series of years. 
The outside figures on the margin of this map indicate 
the degrees of latitude ; the inside figures show the degree 
of temperature by Fahrenheit's thermometer. Latitude is 
indicated by degrees of distance; — isothermal lines are 
designated by degrees of heat, regardless of distance. 
Thus the true equator, or zero parallel, is a line passing 
fi'om west to east without departing a hair's breadth from 
a direct course. It passes through the northern part of 
Brazil, S. A., thence across the Atlantic, and directly 
through the centre of Africa. The isothermal equator is 
a meandering line, which touches and crosses the true 
equator at different points. It passes entirely north of 
South America ; thence curving gently, it strikes the true 
equator in longitude 20° W. ; then curving northward, it 
passes through Guinea, Soudan, and Abyssinia, and pro 
ceeding eastward it makes its way through the southern 
part of Hindostan ; thence curving through the East In- 
dies, it crosses the true equator between Sumatra and 
Borneo. 
The cotton zone embraces all that portion of the earth 
