218 TROPICAL NATURE 



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 23 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 



