4-84 THE POPULAR SCIENCE MONTHLY. 



Zone of Periodic Winds and Rains. It is to this chantifinoc 

 line of the greatest heat that the main currents of wind are directed. 

 Within a zone extending for about 30 on each side of the equator 

 the winds blow with great regularity. When they leave the polar 

 regions the tendency of the surface-currents is due north and south, 

 but in their course they become deflected longitudinally in con- 

 sequence of the earth's motion, and reach the line of greatest heat 

 as northeast and southeast currents. The tropic of Capricorn has 

 its air rarefied by heat in our winter, and this produces within the 

 torrid zone what is called the northeast trade-wind or monsoon. 

 The tropic of Cancer has its air still more rarefied by heat in our sum- 

 mer, and this produces the south monsoon. Through these causes 

 this central belt of the world has its winds and rains perfectly steady 

 and regular, and wdthin it there falls the greatest quantity of rain 

 which there is in any part of the world. The rainy season begins 

 some time before the sun reaches the zenith of a place, and continues 

 for some time afterward. In a belt near the equator there are two 

 rainy seasons, the main one, which lasts three or four months, begin- 

 ning when the sun, in its progress to a vertical position, has crossed 

 the equator, and a shorter one, which lasts four or six weeks, when the 

 sun is coming again from the tropic to the equator. Nearer the two 

 tropics the countries have only one rainy season, which begins when 

 the sun approaches the tropic, and one dry season, the year being 

 divided between the two. The rain pours down in torrents in a way 

 of which we can form no notion from our experience in temperate 

 countries. Our London rainfall is 2 inches a month, but in the tropics 

 an inch a day is not an uncommon average for the whole rainy season. 

 On the banks of the Rio Negro Humboldt collected as an ordinary 

 rain If inch in five hours. In Cayenne Admiral Roussin collected, 

 between the 1st and 24th of February, 12^ feet, and in one night, be- 

 tween 8 p. M. and 9 a. m., measured \^\ inches. In the Himalayas of 

 Khasia as much as 600 inches are said to fall in a single year. The 

 rain, however, does not commonly pour down without intermission 

 night and day, and day after day, as is sometimes the case in the Eng- 

 lish lake country. The ordinary succession of atmospheric phenomena 

 is as follows: The sun rises in a cloudless sky. Toward noon some 

 faint clouds appear on the horizon, which increase rapidly in density 

 and extent, and are soon followed by thunder and violent gusts of 

 wind, accompanied by heavy rains. Toward evening the rain abates, 

 the clouds disappear, the sun sets in a serene sky, and during the 

 iiight no rain falls. The annual quantity of rain which falls upon any 

 particular place depends greatly upon local circumstances, just as it 

 does in the temperate zones, and is gi'eatest where hill-ridges are placed 

 so as to catch the clouds, and smallest in tracts that lie to landward 

 of such ridges. To take our illustrations from India, where the south 

 monsoon blows laden with the copious vapors raised by the equatorial 



