RAIN 19 



giving an average of about 78. These results apply to the 

 air 4 feet 6 inches above the ground ; nearer the ground the 

 humidity increases at night up to 100. 



At the Cinchona Plantation the humidity at 7 a.m. is 

 about 83, which is much the same as at Kingston ; but at 

 3 p.m. it increases to 88 probably due to afternoon showers, 

 which fall there nearly all the year round. 



Rain 



The amount of rain which falls at any place in any given 

 interval of time is measured by its depth in inches and 

 decimal parts of an inch. 



Thus, let us suppose that at anj^ time the depth of water 

 in an open tank was 5 feet, and that after an interval of 

 twenty-four hours, during which there were intermittent 

 showers, the depth of water was increased to 5 feet 2 inches ; 

 then the total amount of rain which fell at the place during 

 those twenty-four hours was 2 inches in depth ; or, more 

 shortly, the rainfall for that day was 2 inches. 



It is to be noticed that the size and form of the tank will 

 not affect the result, provided the sides of the tank are 

 perpendicular ; and, instead of a tank, we may use a small 

 cylindrical receiver, neatly made of thin metal. 



Again, in order to measure with accuracy the depth of 

 the water caught at any time in the receiver, we can pour the 

 water into a narrow cylindrical glass gauge, properly graduated, 

 and thus easily read the rainfall to the nearest hundredth 

 part of an inch. In graduating these glass gauges the scale 

 must be multiplied by the ratio of the square of the diameter 

 of the receiver to the square of the diameter of the gauge. 



Such an apparatus is called a rain-gauge ; and by register- 

 ing the rainfall daily we get the total rainfall for the month 

 or year. But on account of the extreme local irregularity in 

 the rainfall, it is necessary to read a large number of rain- 

 gauges in order to obtain the average rainfall for any district. 

 These rain-gauges should be placed so that the receiving 

 surfaces are all one foot above the surface of the ground ; 

 and, of course, they should be fully exposed to the sky, and 



