METEOROLOGY OF BEN" NEVIS IN CLEAR AND IN FOGGY WEATHER. 793 



of the constituents, then the volume at barometric pressure of this constituent is to 

 the total volume of the mixture as the observed tension of the constituent is to the 

 total tension of the mixture or the barometric pressure. It should be noted that in the 

 analysis of a gaseous mixture the volumes of the constituents are always given in terms 

 of the same temperature and pressure.* 



At all meteorological observatories the barometric pressure and the tension of 

 aqueous vapour in the atmosphere are among the chief objects of observation, and they 

 furnish at once the means of ascertaining the exact composition by volume of the air 

 in so far as it consists of permanent gas and aqueous vapour. 



In clear weather in winter on Ben Nevis the tension of aqueous vapour in the atmos- 

 phere is very low. The mean for January is 0*0892 in., with a mean diurnal range of 

 0*016 in. In July the mean value is 0*2362 in., with a mean diurnal range of 0"062 in. 

 In January the mean height of the barometer is 25 , 618, so that the air contains 0'35 

 per cent, by volume of aqueous vapour, and the range during the day is from 0*316 to 

 0*379 per cent. In July the mean barometric pressure is 25*628 ins., and the mean 

 vapour tension being 236 in., the air contains 0*92 per cent, by volume of aqueous 

 vapour. This ranges during the day from 0'80 to 1*04 per cent. The difference 

 between these figures is considerable, amounting to 24 per cent, of the larger amount, 

 and it indicates a considerable change of air. 



If we look at the tables or curves giving the monthly mean hourly values of the 

 vapour tension and of the barometric pressure in clear weather in each month, and if we 

 consider the range through which each of these elements varies in the mean monthly 

 day, we find that these ranges vary in the case of vapour tension from 16 to 30 per 

 cent, of the whole vapour tension, while the barometric pressure does not range through 

 more than a quarter of one per cent. 



In continuously clear weather, as we have specified it, the air at a height of 4000 

 feet has no means of changing its percentage of aqueous vapour except by mixture 

 with air from greater heights, which usually reduces the percentage, or with air from 

 lower levels, which usually increases the percentage. From the character of the vapour 

 tension curves it appears that, on the whole, at night drier air descends from above, 

 while during the day moister air rises from below. The changes in the composition of the 

 air, which we recognise by the accurate analytical method which we have thus in our 

 hands, afford us quantitative proof of the activity of changes of air in the atmosphere, 

 which on other grounds we are forced to conclude must exist, in order that it may 

 preserve unimpaired its life-supporting properties. 



The accompanying Table gives a summary of the movements of vapour tension in 

 the year, and combined with the atmospheric pressure the movements in percentage by 

 volume of the amount of aqueous vapour in the atmosphere : — 



* Thus, if H be the barometric pressure, and h the observed vapour tension, then the percentage by volume of 

 aqueous vapour in the air is given by the equation V= 100 =j . 



