HISTORY AND GENERAL CONDITIONS n 



that the differences are unimportant, except in the 

 vicinity of the foot-hills of the Andes, where at the 

 higher elevation above sea-level lower temperatures 

 occur, especially at night-time. 



For practical purposes the year in the Amazon 

 Valley may be divided into the dry and the wet seasons. 

 The former covers the months of May to October ; the 

 latter extends from November to April. The rainfall 

 varies to a marked degree in different sections, owing 

 to the existence of the Tumuc Humac and other 

 mountain ranges on the southern borders of the 

 Guianas. These mountains precipitate the rainfall 

 before it reaches the north-eastern part of the Amazon 

 Valley, the effect being particularly noticeable through- 

 out a section of some 500 miles from east to west, of 

 which Manaos and Obidos are the two principal points. 

 Outside the influence of these northern mountain 

 ranges the rainfall is comparatively equable. Reliable 

 records of the meteorological conditions are available 

 at five points only. In 1911 these observations showed 

 the following precipitations : 



These figures give an annual average of 58 inches 

 for the dry sections affected by the Guiana Mountains, 

 and 99*33 inches for the districts free from that in- 

 fluence. From notes taken in connection with the 



