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218 TROPICAL NATUREThe Three Climatal Zones of the EarthThe three great divisions of the earth the tropical, thetemperate, and the frigid zones may be briefly defined asthe regions of uniform, of variable, and of extreme physicalconditions respectively. They are primarily determined bythe circumstance of the earth's axis not being perpendicularto the plane in which it moves round the sun ;whence itfollows that during one half of its revolution the north pole,and during the other half the south pole, is turned at a considerableangle towards the source of light and heat. Thisinclination of the axis on which the earth rotates is usuallydefined by the inclination of the equator to the plane of theorbit, termed the obliquity of the ecliptic.The amount ofthis obliquity is 23 degrees, and this measures the extent oneach side of the equator of what are called the tropics, becausewithin these limits the sun becomes vertical at noon twice ayear, and at the extreme limit once a year, while beyond thisdistance 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 anyabrupt change of climate or physical conditions, such ascharacterise the tropical and temperate zones in their fulldevelopment. There will be a gradual transition from one tothe other, and in order to study them separately and contrasttheir special features we must only take into account theportion of each in which these are most fully exhibited. Forthe temperate zone we may take all countries situated between35 and 60 of latitude, which in Europe will includeevery place between Christiana and Algiers, the districtsfarther south forming a transitional belt in which temperateand tropical features are combined. In order to study thespecial features of tropical nature, on the other hand, it willbe advisable to confine our attention mainly to that portionof the globe which extends for about twelve degrees on eachside of the equator, in which all the chief tropical phenomenadependent on astronomical causes are most fully manifested,and which we may distinguish as the "equatorial zone." Inthe debatable ground between these two well-contrasted beltslocal causes have a preponderating influence ;and it would