SCIENCE OF COMMON THINGS. 109 



Ilain-gauge. "Wet and dry seasons of the tropics. 



693 Why does it rain more- frequently in the temperate zones than in 

 the tropics f 



Because the temperate zone is a region of variable 

 winds, and the temperature of the atmosphere changes 

 often ; while in the tropics the wind changes but rarely, 

 and the temperature is very constant throughout a great 

 part of the year. 



693 How is the amount of rain measured f 



By means of a rain-gauge. 



694: How is this constructed ? 



The best form consists of a cylindrical metal vessel 

 furnished with a float ; the rain falling into the vessel 

 raises the float, the stem of which is so graduated that 

 the increase in depth can be very accurately measured. 



695 Why does it rain more upon the sea-coast than in the interior of 

 a country ? 



Because the air adjacent to the ocean contains more 

 moisture than the air inland. 



696 What is the average yearly fall of rain in the tropics and tempe- 

 rate zones ? 



The average yearly fall of rain in the tropics is nine- 

 ty-five inches ; in the temperate zone only thirty-five. 



The greatest rain-fall, however, is precipitated in the shortest time. 

 Ninety -five inches fall in eighty days on the equator, while 'at St. Peters- 

 burg the yearly rain-fall is but seventeen inches, spread over one hundred 

 and sixty-nine days. Again, a tropical wet day is not continuously wet. 

 The morning is clear ; clouds form about ten o'clock ; the rain begins at 

 twelve, and pours till about half-past four; by sunset the clouds are 

 gone, and the nights are invariably fine. 



697* In the tropics, how are the seasons divided? 



Into the wet or rainy, and the dry season. 



698 Are there some countries entirely destitute of rain ? 



In some parts of Egypt it never rains ; in Peru it 

 rains once, perhaps, in a man's lifetime. 



Upon the table-land of Mexico, in parts of. Guatemala and California, 

 for the same reason, rain is very rare. But the grandest rainless districts 

 are those occupied by the great desert of Africa, extending eastward 

 over portions of Arabia and Persia to a desert province of the Belooches 

 districts. farther continued in the heart of Asia over the great desert of 

 Gobi, the table-land of Thibet, and part of Mongolia. In all these are 

 five or six millions of square miles of land that never taste a shower. 



