370 



HYDROGRAPHY. 



land breeze in its turn prevails, the same thing occurs in reversed 

 order. It is well known that the Monsoons first begin near the inter- 

 section, or the separation of land and sea. 



The Southwest Monsoon is felt at the heads of the Arabian Gulf 

 and Bay of Bengal, and on the Coast of China, nearly a month pre- 

 vious to its commencement at the equator. As the heat of the land 

 increases, the air follows the receding heated areas ; while it flows in 

 from areas yet more and more remote, retrograding towards the south- 

 west or opposite side. 



The Northwest Monsoon, in like manner, begins at the south nearly 

 as much in advance as the Southwest Monsoon, though it retrogrades 

 in a contrary direction ; and it is well known that the northeast winds 

 between Portugal and Madeira are first felt at the latter place — while 

 the southwest winds are experienced on the coast of Portugal before 

 they are felt at Madeira or the Canary Islands. 



To make the progression of the winds more clearly understood, the 

 accompanying diagram will exhibit the 

 action of the currents of air. The cen- 

 tral space is supposed to be at the tem- 

 perature of 80° ; the next outer line 

 represents 70°; the next 60°, 50°, 40°. 

 Now, the flow of air into the rarefied 

 space of 80° will be from all sides — 

 first from 70°, then the 60°, and then 50°, 40° ; thus having a retro- 

 grade or backward motion from the centre. As soon as the tempera- 

 ture falls, the air will flow in with a less velocity, but it will continue 

 until the temperature attains the minimum, when there will be an 

 equilibrium, and consequently a calm. This is supposing the rarefied 

 space to occupy a central position, but it may be oblong, or the area of 

 heat extend over a whole country. In this case, as the temperature 

 falls, the area of heat will recede, and the current of air will follow 

 after the receding heat, whilst it flows from the quarter where it is 

 colder and denser retrograding. The annexed diagram will more 

 clearly exhibit my meaning : 



On the left the 



temperature diminishes, and the flow is first towards 



