U TROPICAL NATURE, AND OTHER ESSAYS. 



a perpetual summer, and were it not for variations in 

 the quantity of rain, in the direction and strength of the 

 winds, and in the amount of sunshine, accompanied by 

 corresponding slight changes in the development of 

 vegetable and animal life, the monotony of nature would 

 be extreme. 



In the present chapter it is proposed to describe the 

 chief peculiarities which distinguish the equatorial from 

 the temperate climate, and to explain the causes of the 

 difference between them, causes which are by no means 

 of so simple a nature as are usually imagined. 



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 pri- 

 marily 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 considerable 

 angle towards the source of light and heat. This incli- 

 nation 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 J 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 



