220 
THE PHYSICAL GEOGRAPHY OF THE SEA. 
blow a regular southwardly monsoon for several months. They 
bring the rains which divide the season in these parts of the African 
coast. The region of the ocean embraced by the monsoons is cu- 
neiform in its shape, having its base resting upon Africa, and its 
apex stretching over till within 10° or 15° of the mouth of the 
Amazon. 
468. Indeed, when we come to study the elfects of South Amer- 
ica and Africa (as developed by the Wind and Current Charts) 
upon the winds at sea, we should be led to the conclusion — had 
the foot of civilized man never trod the interior of these two con- 
tinents — that the climate of one is humid ; that its valleys are, 
for the most part, covered with vegetation, which protects its sur- 
face from the sun's rays ; while the plains of the other are arid 
and naked, and, for the most part, act like furnaces in drawing the 
winds from the sea to supply air for the ascending columns which 
rise from its overheated plains. 
469. Pushing these facts and arguments still farther, these 
beautiful and interesting researches seem already sufficient almost 
to justify the assertion that, were it not for the Great Desert of 
Sahara, and other arid plains of Africa, the western shores of that 
continent, within the trade-wind region, would be almost, if not 
altogether, as rainless and sterile as the desert itself. 
These investigations, with their beautiful developments, eagerly 
captivate the mind ; giving wings to the imagination, they teach 
us to regard the sandy deserts, and arid plains, and the inland ba- 
sins of the earth, as compensations in the great system of atmos- 
pherical circulation. Like counterpoises to the telescope, which 
the astronomer regards as incumbrances to his instrument, these 
wastes serve as make-weights, to give certainty and smoothness 
of motion — facility and accuracy to the workings of the machine. 
470. When we travel out upon the ocean, and get beyond the 
influence of the land upon the winds, we find ourselves in a field 
particularly favorable for studying the general laws of atmospher- 
ical circulation. Here, beyond the reach of the great equatorial 
and polar currents of the sea, there are no unduly heated surfaces, 
no mountain ranges, or other obstructions to the circulation of the 
atmosphere — nothing to disturb it in its natural courses. The sea, 
therefore, is the field for observing the operations of the general 
laws which govern the movements of the great aerial ocean. Ob- 
