78 METEOROLOGICAL 



In finding the dew-point, or that degree of 

 temperature at which the aqueous vapour in the 

 atmosphere begins to be condensed into water, 

 I employ the method introduced by Le Roi, and 

 recommended by Mr. Dalton. An observation 

 was made every day, in the open air, usually 

 between nine and ten in the evening, but as this 

 hour is rather too late, except in summer, to 

 give the true daily mean, the monthly means 

 obtained from the observations are a little lower 

 than they should be, especially in the winter 

 months. It will be perceived, that on the ave- 

 rage of the five years, the quantity of aqueous 

 vapour in the atmosphere is at a minimum in 

 January, and that it goes on progressively in- 

 creasing till August, when it arrives at the maxi- 

 mum ; it then begins to diminish gradually, and 

 continues decreasing till the month of February. 

 The mean annual point of deposition is 43*^.4, 

 which is 5° lower than the mean annual tempe- 

 rature for the same period. The lowest state of 

 vapour in the atmosphere, observed in the course 

 of the five years, took place on the 18th of 

 January, 1823; when the dew-point was 13^, 

 corresponding to .1 of an inch of mercury in 

 force, which is equal to 1.4 inches of water, the 

 temperature of the air, at the time, being 17*^; 

 and the highest state occurred on the 18th of 

 July, 1825; when the dew-point was 68*^.5, 



