100 GENERAL REVIEW OF THE OKCHIDE.E. 



" When the sun is south of the equator the earth's surface north 

 of it is no longer under the same influence but under tliat of the 

 atmospheric currents flowing in from north or north-east to supply the 

 place of the ascending heated air. The moving air owing to tlie great 

 extent of land surface in the northern hemisphere is, at first, but slightly 

 charged with moisture, and as it travels from north to south becomes 

 warmer ; it is comiDaratively a dry wind and consequently its capacity 

 to contain vapour is continually augmenting. Similarly when the sun j 

 is north of the equator, the like phenomena occur on the south of it, 

 but not equally so owing to the greater extent of ocean surface in the 

 southern hemisphere. It is plain from these considerations that each 

 place between the tropics must have its dry and wet season ; dry when 

 the sun is on the opposite side of the equator, and wet when the sun 

 is overhead."* 



The trade winds and the general phenomena just described where 

 confined to the ocean are regular and constant^ but on the continents 

 they are subject to much variation, owing to the configuration of 

 the land, the trend and height of the mountain ranges, and to 

 many local causes ; but generally speaking within the region under 

 review the variations are periodical or seasonal. 



The most extensive of these periodical changes are the monsoons which 

 take the place of the regular trade winds in tlie Indo-Malayan region. 

 The south-west monsoon loaded with vapour raised from the Indian 

 Ocean when it meets the western Ghauts of India precipitates so much 

 rain along the coast districts stretching from Bombay to the extreme 

 south of India that some localities within it are the wettest spots 

 known ; similar phenomena occurs along the mountains of Aracan and 

 Lower Burmah ; and again along the lower Himalayan zone owing to 

 the enormous amount of vapour ascending from the Bay of Bengal 

 being drifted towards the mountains, and which being condensed by 

 contact with the higher and colder zone is precipitated into the lower 

 valleys, the precipitation increasing in amount in proceeding eastwards 

 to the Khasia Hills and Manipur where it attains its maximum. 



In the western hemisphere similar phenomena occur but on a smaller 

 scale. Thus, in southern Mexico and Guatemala south-easterly winds 

 prevail during the wet season from December to April, and north- 

 westerly winds during the remainder of tlie year. The vapour raised 

 in the south Atlantic Ocean during the sun's excursion between the 

 equator and the southern tropic is carried by the south-east trade wind 

 towards the Brazilian coast from Cape St. Roque to the Rio de la 

 Plata and thence across the continent to the Andes. A large amount 

 of this vapour is precipitated on and in the neighbourhood of the coast 



* Tyiidall, "Heat a Mode of Motiou," p. 212. 



