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TRANSACTIONS OF SECTION A. 449 
to corresponding feebleness of the south-east trades, or to unusual diversion to 
East Africa, or local feebleness in a part of India due to local conditions, or to 
abnormal diversion to other rainfall areas in South Asia. These conditions give 
rise in the areas affected to one or more of the following features :— 
(1) Prolonged delay in the commencement of the rains. 
(2) Scanty rainfall during the season, with prolonged periods of fine, clear, hot 
weather. 
(8) Early termination of the rains. 
These features areas a rule more marked in the drier districts of the interior 
than in the coast districts. The effect on crop production is greatest and most 
disastrous in the following areas :— 
(1) Central Burma. 
2) The Deccan, including the Bombay and Madras Deccan districts, and 
Hyderabad. 
(8) North-Western and Central India, more especially the South Punjab, East 
Rajputana, and the United Provinces. 
The following important inferences are based upon the preceding presentation 
of facts and the experience of the past thirty years :— 
(1) The lower air movement of the south-west monsoon is the northward 
extension of the lower movement of the south-east trades. The latter is a 
permanent feature of the Indo-oceanic region, and the former a periodic inya- 
sion of the Southern Asian seas and peninsulas initiated over equatorial regions 
and propagated northwards to the southern mountain barrier of the Central Asian 
lateau. 
fi (2) The primary factors determining this impulse across the equator (the first 
stage of the establishment of the south-west monsoon) are to be sought in the 
permanent field of the south-east trades, and are not due to actions in the heated 
areas of Southern or Central Asia, 2 
(8) The pressure conditions in the heated areas of Southern Asia and North- 
East Africa determine the direction, volume, and intensity of the advance over 
the Indian seas to what may be termed three competing areas for rainfall (viz., 
Abyssinia, India, and Burma), These conditions are hence important factors in 
the third stage of the advance of the south-west monsoon current. 
(4) The movement when fully established by these actions over the Southern 
Asian seas and peninsulas is continued—Ist, by the momentum of the lower 
circulation ; 2ndly, by the release of energy accompanying aqueous vapour condensa- 
tion ; and 3rdly, by thermal actions in Southern Asia, due to direct solar activity. 
The termination of the lower horizontal current by vertical movement occurs 
irregularly over the areas of frequent heavy rain in Southern Asia and Abyssinia, 
and not over a heated area in Central Asia. 
(5) The total volume of aqueous vapour brought up by this circulation not only 
varies in amount from month to month during the season, but also from year to 
year. The largest variations (seasonal and annual) depend chiefly, if not 
entirely, upon actions in the source of supply—viz., the Indian Ocean. If 
those actions determine an increased or diminished supply across the equator 
into the Indian seas, there is a corresponding variation in the total precipitation of 
the three competing areas. Amongst such causes and actions may be prolonged 
and untimely diversion of the south-east trades into East Africa, as in 1896, 
or general weakness of the air movement over the Indian Ocean, probably 
eying a displacement and decreased intensity of the southern anticyclone, 
as in 1899, 
(6) The relative distribution of the total rainfall in the three areas of discharge 
of the aqueous vapour of the monsoon currents probably depends upon the relative 
intensities of the pressure conditions established during the hot weather, 
which are continued for a part or the whole of the monsoon by actions depending 
1904, G@ 
