472 



MR J. Y. BUCHANAN ON THE 



an inch higher in clear than in foggy weather. The maximum excess recorded in the 

 table is , 6()2 in. at twenty-one hours in January, and the minimum is 0"296 in. at 

 fourteen hours in February. In the following small table the mean monthly and the 

 mean annual excesses of pressure in clear weather over that in foggy weather are given 

 in the first line, and the difference of the mean monthly excesses from the annual excess 



are given in 



the second line. 



















Jan. 



Feb. 



Mar. 



April. 



May. 



June. 



July. 



Aug. 



Sept. 



Oct. 



Nov. 



Dec. 



Mean. 



0-570 



0-320 



0-501 



0-353 



•0-372 



0-390 



0-414 



0-433 



0-519 



0-550 



0-537 



0-513 



0-456 inch 



+ •114 



-•126 



+ •045 



-•103 



- -084 



-•066 



-•042 



-•023 



+ •063 



+ •094 



+ •081 



+ •057 



o-ooo 



Roughly speaking, in spring and summer the excess is below the mean, excepting 

 the month of March, and in autumn and winter it is above the mean. The drop from 

 the maximum to the minimum value in one month, from January to February, is very 

 remarkable, but the recovery in March to a much higher value, and indeed to a second 

 maximum, suggests that there are special reasons for this irregularity which may be 

 accidental to the particular years under consideration. It is certainly difficult to 

 imagine, though it is well worth while to try to find out, what conditions vary so much 

 and so rapidly between January and April that the excess of pressure in question 

 should show the variations which we observe in the table. 



A general summary of the movements of the barometer during the year is presented 

 in the tables on the following page, in terms of the inch and the millimetre 

 respectively. 



Tension of Aqueous Vapour. — In foggy weather the vapour tension is that of 

 saturation at the temperature of the air. As the water particles which form the fog 

 permeate the whole of the air, any addition of heat has the effect of changing into 

 vapour a portion of the fog, and any loss of heat has the opposite effect, of condensing 

 some of the vapour of the air on the water (or ice) of the fog, both being accompanied by 

 a certain exchange of heat, In the clear weather the atmosphere is generally in a state 

 far removed from saturation, and the effect of addition or removal of heat is simply a 

 rise or fall of temperature. In itself the air in clear weather has no means of increasing 

 its supply of water vapour. 



The barometric pressure is the measure of the sum of the tensions of the various 

 gaseous constituents of the atmosphere. If this pressure is diminished all the con- 

 stituents expand proportionately, and their tensions diminish also proportionately. 

 If the pressure is increased they contract in the same proportion, and their tensions 

 increase proportionately. 



Therefore the determination of the tension of the aqueous vapour in the atmosphere, 

 combined with that of its barometric pressure, furnishes an exact analytical method 

 for the determination of the composition by volume of the air, in respect of water 

 vapour and permanent gas, 



