280 THE PHYSICAL GEOGRAPHY OF THE SEA. 



centre or point like a circle on the water. According to the Pilot 

 Chart, which gives 11,800 observations for the five "fields" above, 

 the march of the southwest monsoons from Calcutta toward the 

 equator is at the rate of 15 or 20 miles a day. 



806. In other words, if a vessel in latitude 23° N., between the 

 meridians 85° and 90° E., were to commence about the first of 

 March to steer due south, and sailed 15 or 20 miles a day on that 

 course till she reached the equator, she would, at the end of each 

 day's sail, arrive with the regular setting in of the southwest mon- 

 soons at that place. 



807. We thus perceive how a desert land spreads its influence 

 through the distance upon the winds. The first effects of heating 

 up the plains are necessarily felt by the air nearest at hand, and 

 by that farther off at a later period, so that the southwest-mon- 

 soon influence is in this part of the ocean propagated from the land 

 out upon the sea at the rate above stated. 



808. Of course, the vast plains of Asia are not brought up to 

 monsoon heat ^er saltum^ or in a day. They require time both 

 to be heated up to this point and to be cooled down again. 



809. The monsoon season may be always known by referring 

 to the cause which produces these winds. Thus, by recollecting 

 where the thirsty and overheated plains are which cause the mon- 

 soons, we know at once that these winds are rushing with great- 

 est force toward these plains at the time that is the hottest season 

 of the year upon them. 



810. The influence of these heated plains upon the winds at 

 sea is felt for a thousand miles or more. Thus, though the Desert 

 of Cobi and the sun-burned plains of Asia are, for the most part, 

 north of latitude 30°, their influence in making monsoons (§ 797) is 

 felt south of the equator (Plate VIII.). So, too, with the great Des- 

 ert of Sahara and the African monsoons of the Atlantic ; also, with 

 the Salt Lake country and the Mexican monsoons on one side, 

 and those of Central America in the Pacific on the other. The 

 influence (§ 202) of the deserts of Arabia upon the winds is felt in 

 Austria and other parts of Europe, as the observations of Kriel, 

 Lament, and others show. 



811. So, also, do the islands, such as the Society and Sand- 



