VAPOUR 15 



understood ; the minimum occurs at sunrise ; the temperature 

 then rises rapidl}', the maximum occurring at noon. In the 

 evening the temperature falls until the dew-point is reached, 

 when the rate of fall is arrested, and then the temperature 

 slowly decreases to the minimum at sunrise. 



Table VII. gives the diurnal variation for Kingston in the 

 form of a correction to be applied to the temperature at any 

 hour to reduce it to the mean of the twenty-four hours. It 

 was deduced from hourly observations made during the years 

 1899, 1900, and 1901, at the United States Station, Halfway 

 Tree, by means of a self-recording instrument. 



Vapour 



Air consists of two gases, oxygen and nitrogen, which are 

 mixed together in the proportion of 23 parts of oxygen to 

 77 parts of nitrogen, with respect to weight. 



But the atmosphere also contains a little carbonic acid 

 gas, and a variable quantity of the vapour of water. The 

 amount of this aqueous vapour is measured by its pressure, 

 or tcusioiif as it is termed, in the same manner that the whole 

 pressure of the atmosphere is measured. Thus, while the 

 atmosphere at the sea-level exerts a pressure of about 30 

 inches of mercury, the amount of vapour generally present in 

 the atmosphere exerts a tension of about a quarter of an inch 

 in cool climates, and in warm insular climates about three 

 times as much. 



Now, while aqueous vapour resembles a true gas in most 

 respects, such as elasticity, invisibility, etc., yet it differs in 

 this important particular, that a given volume cannot contain 

 more than a certain amount of vapour depending on its 

 temperature. 



If a little water be poured into a glass jar, which is then 

 tightly closed, the jar will soon become filled with a certain 

 definite amount of vapour depending on the temperature, but 

 independent of the air which may be in the jar ; that is to 

 say, the amount of vapour will be the same whether the jar 

 was full of air or whether it had been previously exhausted. 



By increasing the temperature within the jar, the amount 

 of vapour will be increased ; by diminishing the temperature, 



