154 FIRST YEAR SCIENCE 



east trade winds over the Indian Ocean. It thus happens 

 that near the southern coast of Asia there are strong sea- 

 sonal winds that blow toward the northeast in summer and 

 toward the southwest in winter. These winds are called 

 monsoons. In the early sailing voyages to India they were 

 very important, the trip to India being made so as to uti- 

 lize the summer monsoons and that from India so as to uti- 

 lize the winter monsoons. On this account these winds 

 had much to do with the conquest of India by the nations 

 of Europe. 



77. Rainfall and its Measurement. Experiment 81. Place a 

 dish with vertical sides in a large open space so that the rim is 

 horizontal and at a height of about one foot above the ground. 

 Fasten the dish so that it cannot be overturned by the wind. After 

 a rain, measure the water that has collected in the dish to the 

 smallest fraction of an inch possible. This will be the amount of 

 rainfall for this storm. 



The amount of rainfall during the year varies greatly 

 in different places. It amounts to nothing or only a few 

 inches over some regions, as in parts of Peru where rain 

 falls only on an average of once in five years. But in the 

 Khasi Hills region of India it has been known to be over 

 600 inches; and over 40 inches, or about the average 

 yearly rainfall for the eastern United States, has been 

 known to fall in 24 hours. This was in the season of 

 the southwestern monsoons. 



The rainfall in different parts of the earth has been 

 carefully measured and maps showing its average amount 

 prepared. As agriculture is largely dependent upon the 

 amount of rain and the season of the year in which it 

 falls, these maps tell much about the relative productivity 

 of different regions of the earth. An annual total of 

 eighteen or more inches is necessary for agriculture ; and 

 this must be properly distributed throughout the year. 



