218 TROPICAL NATURE I 
The Three Climatal Zones of the Earth 
The three great divisions of the earth—the tropical, the 
temperate, and the frigid zones—may be briefly defined as 
the regions of uniform, of variable, and of extreme physical 
conditions respectively. They are primarily determined by 
the circumstance of the earth’s axis not being perpendicular 
to the plane in which it moves round the sun; whence it 
follows that during one half of its revolution the north pole, 
and during the other half the south pole, is turned at a con- 
siderable angle towards the source of light and heat. This 
inclination of the axis on which the earth rotates is usually 
defined by the inclination of the equator to the plane of the 
orbit, termed the obliquity of the ecliptic. The amount of 
this obliquity is 234 degrees, and this measures the extent on 
each side of the equator of what are called the tropics, because 
within these limits the sun becomes vertical at noon twice a 
year, and at the extreme limit once a year, while beyond this 
distance it is never vertical. It will be evident, however, 
from the nature of the case, that the two lines which mark 
the limits of the geographical “tropics” will not define any 
abrupt change of climate or physical conditions, such as 
characterise the tropical and temperate zones in their full 
development. ‘There will be a gradual transition from one to 
the other, and in order to study them separately and contrast 
their special features we must only take into account the 
portion of each in which these are most fully exhibited. For 
the temperate zone we may take all countries situated be- 
tween 35° and 60° of latitude, which in Europe will include 
every place between Christiana and Algiers, the districts 
farther south forming a transitional belt in which temperate 
and tropical features are combined. In order to study the 
special features of tropical nature, on the other hand, it will 
be advisable to confine our attention mainly to that portion 
of the globe which extends for about twelve degrees on each 
side of the equator, in which all the chief tropical phenomena 
dependent on astronomical causes are most fully manifested, 
and which we may distinguish as the “equatorial zone.” In 
the debatable ground between these two well-contrasted belts 
local causes have a preponderating influence; and it would 
