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HYDROGRAPHY. 



come from the high land of the Isthmus, and rush with great violence 

 over the sea, though they are of short duration. 



Connected with this Monsoon, a southerly wind, an extension of 

 the Southwest Monsoon, prevails from the Gallipagos Islands to the 

 Bay of Panama. The cause here is also quite evident. The water of 

 the ocean surrounding the Gallipagos Islands is some fifteen degrees 

 colder than those of the Bay of Panama ; consequently, the air in 

 contact with it is cooler, and we should naturally expect, as we find 

 to be the case, the air constantly seeking this rarefied area. The 

 current of air, therefore, between these two points, prevails throughout 

 the year; although it is a region subject to calms and very light airs; 

 but when there is any wind, it is from the southwest. 



I conceive that these are strong and sufficient proofs of the theory 

 to which I wish to call attention. 



There are also Monsoons on the west coast of South America, 

 extending from Chili to Peru, which prevail from the northward from 

 May to August, and from September to April, from the south. The 

 former period is designated the rainy season. During the latter, the 

 winds are considered to be allied to the Southeast Trade. No rain 

 falls during this period. 



The extent of this northerly Monsoon is through 23 degrees of lati- 

 tude, from the 12th to the 35th degree south. This apparently is from 

 a warmer to a colder latitude ; but if we examine the temperature of 

 the land, we shall find that it is warmer than the sea to the north- 

 ward of it, and produces the same effect as in other cases. The north- 

 erly Monsoon takes place when the sun is in northern declination. 

 It is characterized by being light and inconstant, and as it approaches 

 the parts of Chili south of the rainless district which exists on this 

 coast, the condensation and precipitation are immense, evolving heat 

 and moderating the climate. The cold and dense atmosphere of the 

 tropical parts of Peru, which comprises the rainless district before 

 mentioned, is then induced towards the south ; the resulting current 

 of air is therefore from the northward, and nearly parallel to the 

 coast. 



This current of air is seldom experienced in the northern parts of 

 Peru. When it does take place, the vapor which exists in the atmo- 

 sphere is made evident in the form of mist, or the "garua" of Peru. 



The heat engendered by the sun when in southern declination, 

 causes the air to become rarefied on the coast of Peru; and the influx 



