78 FLORA IN DIG A. 



after the beginning of April, it advances rapidly into the 

 northern hemisphere, and two defined regions of excessive 

 heat (86° Fahr.) occur, one in Africa, and a smaller one in 

 the peninsula of India. In May and June the equator of 

 heat lies in India considerably north of the tropic, and the 

 two regions of excessive heat, becoming united, extend unin- 

 terruptedly from North Africa, across Arabia and Persia, over 

 all India west of the Bay of Bengal. In July, a still hotter 

 area occurs in Nubia and Arabia, and Northern India is very 

 little inferior in temperature, whilst Southern India becomes 

 cooled ; the heat throughout India being modified by the ac- 

 cession of the rains. In this month the isotherms in all parts 

 of Asia are much curved, the convexity being towards the 

 north j and the amount of curve increases towards the north- 

 ern part of the continent. 



In August the equator of heat passes through Northern 

 India, which is still occupied by the rapidly contracting region 

 of excessive heat. In September and October the equator of 

 heat advances rapidly towards the south, and in November 

 it has entirely left India, and corresponds almost exactly with 

 the terrestrial equator, while the region of excessive heat lies 

 in the Indian Archipelago over Borneo and New Guinea. 



We see therefore that from the vernal to the autumnal 

 equinox a great part of India is preternaturally hot, but that 

 from October to February (inclusive) it is comparatively cool, 

 and at the same time the continents of Africa and Australia 

 become preternaturally hot. During the summer months 

 therefore, or the hot season as it is commonly called in India, 

 the wind blows from the south towards the north, while in 

 the winter or cold season it blows from north to south. At 

 both seasons these directions are often modified by local causes, 

 besides being uniformly affected by the earth's rotation, and 

 by the heating and cooling of the continent. 



The monsoons or periodical winds are known in the Indian 

 Ocean, and indeed generally throughout India, by the name 

 of the south-west and north-east monsoon, these being their 



