48 PHYSIOGRAPHY. [chap. 



heaviest rains occur. But the heavy tropical rains are usually 

 confined to definite periods — the raitiy season — and are not 

 spread over the entire year, as in the temperate zone. 



The Khasi Hills present the greatest rainfall in the world. 

 Sir J. Hooker recorded upwards of 500 inches during a 

 stay of nine months, and the total annual fall is about 524 

 inches. On the other hand, there are certain localities in 

 which little or no rain falls ; the chief of these rainless 

 regions being Upper Egypt, the Sahara, the desert of Gobi 

 in Central Asia, and the coast of Peru. As we recede 

 from the hotter regions of the earth, either to the north or 

 south, the rainfall, as a rule, diminishes, but the number of 

 rainy days in the year increases ; so that, speaking roughly, 

 it may be said that, where the rainy days are fewest, the 

 amount of rain is greatest. 



In temperate regions, the number of rainy days varies 

 greatly in different localities, and in different seasons. But 

 it is difficult to know exactly what is meant by so vague 

 a phrase as a " rainy day." To secure uniformity among 

 observers, Mr. Symons has therefore proposed that meteoro- 

 logists shall regard as a " rainy day," every day on which 

 the rainfall is not less than one-hundredth of an inch. 



It is matter of common observation that the rainfall 

 varies not only in different localities, but in the same 

 locality at different times. One year may be much wetter 

 than another. A remarkable instance of this was presented 

 by the exceptional rainfall of 1872, a fall which, in most 

 districts, was excessive, and in some was unprecedented. It 

 is believed that no such fall has been recorded since obser- 

 vations were first instituted, now two centuries ago. 



It is curious to compare the 1872 fall with that of the 

 following year, which was remarkably dry. Thus, Mr. 

 Symons has recorded the rainfall in Camden Square, 



