CLIMATE. 



eastern division there is no mountain-chain thus to divide the 

 drainage. A slight and imperceptible elevation of the general 

 plain produces two opposite water-sheds, commencing in a low 

 range of hills separating the sources of the Dnieper from those 

 of the Vistula, and winding along the plain to the tableland of 

 Valdai, which forms its summit, 1200 feet above the level of the 

 sea. The ridge then turns northward towards Lake Onega, and, 

 following a winding course, terminates in the Ural Mountains, 

 about 62 N. lat. 



The drainage of the north side of this ridge forms the rivers 

 which flow into the Baltic and the White Sea, that of its southern 

 declivity those which flow into the Black Sea and the Caspian. 



148. General Plan of the Rivers of the "World. The prin- 

 cipal rivers of the world, with their tributaries, their embouchures, 

 and their sources, are exhibited in one general plan on the oppo- 

 site page, where their lengths ar3 indicated. 



CLIMATE. 



149. Since the prevailing character of the animal and vegetable 

 kingdoms, in each division of the earth's surface, depends chiefly 

 on climate, it is necessary, on that account alone, independent of 

 many other considerations, that the student in geography should 

 be rendered familiar with the conditions, which in each part of 

 the globe determine the varying vicissitudes and temperature of 

 the seasons. 



150. Dependence of Climate on Latitude. The first and 

 chief condition which determines the climate of a country, is its 

 position with respect to the equator. It may be stated, subject 

 to some special qualifications, that the nearer any country is to 

 the Line, or what is the same, the lower its latitude, the higher 

 will be the mean temperature of its seasons. 



The reason of this is partly geographical and partly astro- 

 nomical. 



The earth revolves diurnally upon an axis, so directed that 

 the equatorial parts are presented either exactly or nearly to 

 the sun. They are presented exactly to that luminary at the 

 epochs of the equinoxes, in March and September. From March 

 to June they are gradually more inclined from the sun towards 

 the south, the northern hemisphere inclining towards that 

 luminary, so as to receive its rays more directly, and in greater 

 quantity than the southern hemisphere. This inclination of 

 the globe increases constantly from March to June, and then 

 decreases from June to September. The northern hemisphere 

 is th\is more and more exposed to the light and warmth of the 



165 



