278 



WELLS'S NATURAL PHILOSOPHT. 



FlQ. 226. falling upon the top of 



the cylinder flows into 

 the interior through 

 the opening, and i 

 thus protected from 

 evaporation. From the 

 base of the apparar 

 tu3 a graduated glass 

 tube, A, ascends, in 

 which the water 

 rises to the same 

 height as in the in- 

 terior of the cylinder. 

 Supposing the apparatus to be placed in an exposed situation, and at the end 

 of a montli, for example, the height of the water in the tube is five inches: 

 this wotild indicate that the water in the cylinder had attained to an equal 

 elevation, and consequently that the rain which had fallen during this inter- 

 val, would, if not diminished by evaporation or infiltration, cover the earth to 

 the depth of five inches. 



, . . .» • 613. Rain falls most abundantly in countries near the equa- 



In what situa- •' ^ 



tions is rain tor, and decreases in quantity as we approach the poles, 

 most abundant . There are more rainy days, however, in the temperate zones 

 than in the tropics, although the yearly quantity of rain falling in the latter 

 districts is much greater than in the former. 



In the northern portions of the United States, there are on an average about 

 134 rainy days in a year ; in the Southern States the number is somewhat 

 less, being about 103. 



The reason why it rains more frequently in the temperate zones than in the 

 tropics is because, the former are regions of variable winds, and the tempe- 

 rature of the atmosphere changes often ; while in tlie tropics the wind changes 

 but rarely, and the temperature is very constant throughout a great part of 

 the year. In the tropics the year is divided into only two seasons, the wet, 

 or rainy, and the dry season. 



What is the The average yearly fall of rain in the tropics 

 ^If'ni'ndifcrent ^^ nincty-five inches ; in the temperate zone 

 countries? ^j^jy tliirty-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. Petersburg tha 

 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 morn- 

 ing 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. 



The depth of rain which falls yearly in London is about twenty-five inches ; 

 but at Vera Cruz, in the Gulf of Mexico, rain to the amount of two himdred 



