THE ATMOSPHERE AND ITS CURRENTS. 439 



shore, these winds are not found at all, but contrary westerly breezes prevail. The 

 irregularity is easily explained. Owing to the rarefaction which the air undergoes over 

 the great hot desert of the Sahara, the colder air from the contiguous sea rushes in to 

 supply the partial vacuum created, and keep up the equilibrium of the atmosphere, 

 producing winds blowing towards the shore. 



In the Pacific Ocean, a similar zone is occupied by permanent north and south-easterly 

 breezes, or trade winds, though subject to a variety of interruptions. An instance of 

 irregularity occurs along the coasts of Peru and Chili, where the general direction of the 

 wind is south, and a steady south-easterly wind is only experienced at the distance of five 

 or six hundred miles from the shore. The numerous shoals and islands which are found 

 in the Pacific, prevent uniformity in the tropical movements of the atmosphere. That 

 intelligent hydrographer Captain Horsburgh has observed, that where shoal coral banks 

 shoot up out of the deep water in many places between the tropics, a decrease of the 

 prevailing wind is frequently experienced ; for when a steady wind is blowing over the 

 surface of the deep water, no sooner does a ship get upon the verge of a shoal coral bank, 

 than a sudden decrease of the wind is often perceived. This he supposes to be occasioned 

 by the atmosphere over these banks being less rarefied by the increased evaporation than 

 that over the deep water, and consequently not requiring so great a supply of air to 

 restore the equilibrium as the circumjacent parts, which are more rarefied and heated. It 

 would undoubtedly be the case, if the earth were entirely covered with a mantle of water 

 of uniform depth, that the trade winds would everywhere prevail, throughout a zone, 

 bounded by the parallels of from 25 to 32 on each side of the equator. But the large 

 masses of land of uneven surface which occur between the tropics, and the consequent 

 inequalities of temperature, check the tendency of the intertropical atmosphere to a 

 regular course, introduce derangement in its movements, so that it is only in the great 

 open seas that the trade winds are experienced. Still, it has been observed, that in some 

 countries under and near the equator, constant easterly winds are found, which are no 

 doubt identical in their cause with those that distinguish the equatorial regions of the 

 ocean. They are met with on lands which exhibit extensive level plains, where nothing 

 occurs to obstruct their passage and alter their direction. Thus, along the immense low 

 tract drained by the Amazon an easterly wind prevails, by the assistance of which, the 

 voyager is enabled to ascend rapidly against the strong current of the river. This wind 

 blows from the estuary of the Amazon, where it is moderate, to its sources at the foot of 

 the Andes, where it has gathered such strength, that Humboldt found it difficult to make 

 head against it. The plain traversed by the lower course of the Orinoco has a similar 

 easterly breeze, but of less force. 



We owe the discovery of the trade winds to Columbus, and this would have been 

 prominently connected with his name, had it riot been supplanted by the glory of a greater 

 achievement, the revelation of a new world to the knowledge of mankind. The ancients 

 were entirely unacquainted with these permanent breezes, and though maritime adventure 

 had been largely prosecuted by the Portuguese at the instigation of Prince Henry, they 

 had not penetrated into the region of the trades. Proceeding cautiously along the shores 

 of Barbary, they had explored the coasts of Africa to Cape de Verde, rescued the Azore 

 Islands from the " oblivious empire of the ocean," and afterwards under Vasco di Gama 

 doubled the Cape of Good Hope ; but these voyages carried them clear of the district of 

 the north and south-east trade winds. But soon after leaving the Canaries in the Santa 

 Maria, Columbus fell in with the former, which in the summer extend to the latitude of 

 those islands, and, for the first time, a sail from the Old World swelled before the steady 

 breath of the northern tropic. This circumstance, favourable to the success of his 

 expedition, speedily excited the apprehensions of his crew, who found themselves borne, 



