DISTRIBUTION OF RAINS. 129 



0.094 of an inch; March, 0.099; April, 0.1t9; May, 0.2f 9 ; June, 

 0.405 ; July, 0.495 ; August, 0.406 ; September, 0.415 ; October, 

 0.269; November, 0.162; December, 0.109. Thus we see that 

 the quantity of vapor in the atmosphere in the month of July was 

 about five times the quantity in the months of January, P'ebruary, 

 March or September. 



Another point that we should consider before comiyg- to the 

 direct laws of the distribution of rain, is the relation between 

 temperature and the .capacitj'- of air for moisture. As the temper- 

 ature of a quantity of air is increased, its capacity for moisture is 

 increased ; and not only increased with the temperatui-e, but at a 

 more rapid rate than the temperature. To illustrate : (and I will 

 iise the numbers only in the way of illustration) — suppose a 

 quantity of air at a temperature of fifty degrees is capable of con- 

 taining- 200 parts of moisture ; at seventy-five degrees, it would 

 be capable of containing 400 parts of moisture ; at one hundred 

 degrees, 800 parts of moisture ; — the ratio of increase of capacity 

 for moisture being greater than the ratio of increase in temper- 

 ature. 



Now, what results when such a quantity of air, charged with 

 vapor, is cooled ? When a reduction of temperature takes place,, 

 the air is no longer capable of retaining the moisture with which. 

 it has been charged, and a certain quantity must be thrown down 

 in the form of rain or snow ; in other words, precipitation takes 

 place, and upon this simple principle depends the entire rain-fall 

 for the globe. 



The quantity of rain-fall for the globe is undoubtedly very near- 

 ly uniform from year to j^ear. Its distribution over the earth's 

 surface is, however, subject to great varia^tion, but is dependent 

 upon fixed and definite laws. The general law .which we will first 

 notice, is known as the law of latitude ; that is, rain-fall is great- 

 est at the equator, and diminishes from the equator to the poles. 

 Rain-fall is greatest in the region where evaporation is greatest, 

 and evapoi-ation is greatest where the temperature is the highest. - 

 Hence, rainfall is greatest where there is the highest temperature, 

 and, as we should expect would be the case, the quantity of rain 

 that falls at the equator is in excess of that which falls in other 

 parts of the globe, and as the temperature diminishes the quantity 

 accordingly decreases from the equator to the poles. The annual 

 rainfall at the equator is 104 inches ; the amount of rainfall 10° 

 9 



