i8 THE METEOROLOGY OF JAMAICA 



Table XI. The explanation of this fact is very simple ; the 

 temperature falls at night until the dew-point is reached; 

 then dew is deposited, latent heat is given out, and the further 

 fall of temperature is arrested. 



Above the sea-level the tension of the aqueous vapour 

 present might be expected to decrease with the atmospheric 

 pressure ; but the dew-point at the Cinchona Plantation is 

 only 58,* giving a tension of only 0*482 inch ; and this 

 shows that we have to consider distance from the seashore 

 as well as elevation. 



But besides the amount of vapour, we require to know the 

 humidity of the air. Thus in the example given above the 

 dry and wet bulbs were 85 and 75, the dew-point was 68*5, 

 and the tension of vapour was 0*696 inch. On another day 

 the dry and wet bulbs may both read 68'5, and the tension 

 of vapour will be the same as before ; but the humidity is 

 different. On the first day the air was dry, and on the 

 second day the air was saturated with moisture. 



Humidity may be defined as the ratio of the vapour- 

 tension present at any time to the vapour-tension required 

 for saturation ; and humidity is recorded in whole numbers 

 from 0, when the air is perfectly dry, to 100, when the air is 

 perfectly saturated. 



Thus on the first day the temperature of the air was 85, 

 and a tension of 1"203 inch was required for saturation. But 

 the tension really was 0*696 inch ; hence the humidity was 

 58 which is found by multiplying 0*696 by 100 and by 

 dividing by 1*203. On the second day the humidity was, of 

 course, 100. 



Table X. gives the humidity by mere inspection. This 

 and the two preceding tables were taken from Glaisher's 

 Hygrometrical Tables, which have, of course, a far larger scope, 

 and to which further reference may be made if necessary. 



The diurnal variation of humidity in Kingston is fairly 

 large 



7 ;\.iii. ... ... ... ol 



3 p.m. ... ... ... ... ... ... ... 68 



11 p.m. ... ... ... ... ... ... 85 



* This is the mean of the 7 a.m. and 3 p m. readings, 55-5 and 60'5 respec- 

 tively ; the ojrrespondiDg dew-point at Kingston is 70-6. 



