DISCUSSION AND ANALYSIS OF WINDS. 



669 



change took place ; how in all this region the winter was the dry, clear time of 

 the year, and summer the rainy season. The navigators of the sixteenth and 

 seventeenth centuries knew that the monsoons extended much further east than 

 India— to the Indo-Chinese Peninsula, the Sunda Islands, and Southern China. 



The cause of the monsoons is this : in our winter the continental regions of Asia, 

 are cooler than the surrounding seas, and pressure is higher. The air flows from 

 these towards the equatorial calm-belt in the Indian Ocean, and towards the region 

 of low pressure in the Northern Pacific, as a N. E., N., N. W. or W. wind. As the 

 pressure is continually lower on the seas than on land at this season, this flow of 

 air is very constant. As the air comes from the interior of the continent, and 

 generally also from higher latitudes, i. e., from colder regions, the season when 

 these winds prevail will be a dnj season, as the vapor contained in the air will be 

 further and further from its point of condensation the further south and east it 

 flows. 



In our summer, pressure is very low over a great part of the Asiatic continent, 

 owing to the heat and ascending current produced by it; therefore the air of all 

 surrounding regions will flow towards Asia, and the movement will be especially 

 rapid in and near Southern and Eastern Asia, as the greatest oceans of the world, 

 the Indian and the Pacific, approach Asia in this direction. 



Pressure is higher on the oceans in summer on account of the comparatively cool 

 temperature which prevails there. Thus the movement of air will be reversed, 

 and the wind in summer will blow from the S. W., S., S. E. and E. This summer 

 monsoon will also be very steady, as the diff"erence of pressure is nearly always in 

 one direction during the whole summer — lower on the land. 



Not only is the direction of the movement of air diff'erent in summer from that 

 prevailing in winter, the influence on the weather is also different. As the air 

 drawn towards Asia has to pass over a great extent of warm equatorial seas, it is 

 laden with vapor, and this vapor will be deposited in copious showers, especially 

 when it meets a mountain chain, which compels it to rise into higher and cooler 

 regions of the atmosphere. Thus the simimer monsoon is the time of cloud and 

 rain for all Eastern and Southern Asia, or the wet monsoon. There is no doubt 

 that the condensation of vapor, giving out its latent heat, is a new and powerful 

 cause for the continuance of the movement in the same direction. 



The influence exerted by the heated continent of Asia is so powerful that there 

 is no equatorial calm-belt in the Indian Ocean during our summer, but pressure 

 decreases steadily from about 25° S. L., the polar limit of the S. E. trade, till about 

 30° N. L. in Northern India, the S. E. trade crossing the equator, and being thus 

 converted into a S. and S. W. wind. On the eastern coast of Asia the tendency of 

 air to flow towards the continent similarly acts on the N. E. trade of the Pacific 

 Ocean, which is drawn in as an E., S. E. or S. wind. We see here the normal or 

 oceanic conditions very seriously disturbed by the influence of the great conti- 

 nental mass, Asia. 



I must correct here an error which is frequently made, i. e., limiting the mon- 

 soons to the tropical part of Asia, i. e., India, Indo-China, and Southern China. 

 Even on the new Pilot Chart published by the British Admiralty in 1872, this 



