METEOROLOGY OF BEN NEVIS IN CLEAR AND IN FOGGY WEATHER. 783 



clear and of foggy weather are clearly indicated, so that there is a valuable collection 

 of data available for any future work on the conditions prevailing in the two kinds of 

 weather specified. 



Note-books were then prepared for the tabulation of the sums and means of the 

 different meteorological elements in each month. These were ruled with the various 

 years (1885-1897) as the side argument, and the twenty-four hours of the day as the 

 top argument. 



Take the barometric pressure during clear weather for illustration. Under each 

 hour and opposite each year were recorded the sum of the barometric pressures, and the 

 number of observations involved at that particular hour of that particular year. Thus, 

 for each month we have thirteen longitudinal columns and forty-eight vertical columns: 

 twenty-four of the vertical columns being the record of the sums in the individual years, 

 and twenty-four being the record of the number of observations dealt with. On 

 summing the vertical columns and dividing the sums of the pressures by the sums of 

 the number of observations we find the mean pressures at each hour of each month for 

 the whole period of thirteen years. 



The same process was applied to the Barometric observations during foggy weather, 

 and to the temperature, rainfall, and tension of aqueous vapour for both clear and foggy 

 weather. In the case of the wind observations note-books were also prepared for the 

 tabulation of the number of winds in each month. The top argument in this case was 

 the sixteen points of the compass (N., N.N.E., N.E., E.N.E., etc.), and the side argument 

 the twenty-four hours of the day. Under each point and opposite each hour was recorded 

 the number of times that that particular direction of wind had been logged at the hour 

 under consideration in the whole thirteen j^ears. By summing the columns vertically the 

 total number of times that each particular direction of wind had been logged in each 

 month for the whole thirteen years was obtained without regard to the hour of the day. 



When the totals were obtained they were reduced to eight points, viz. : N., N.E., 

 E., S.E., S., S.W., W., N.W. This was done by distributing the N.N.E. winds equally 

 among N. and N.E., the E.N.E. equally among N.E. and E., and so on. 



A tabulation of the number of times the wind was observed to blow in a particular 

 direction in a particular month does not, however, give a true idea of its relative 

 importance, without also taking into consideration the number of observations 

 dealt with altogether in that particular month. Thus, we find in June that in our 

 clear weather and foggy weather we have westerly winds logged about an equal 

 number of times. It must be remembered, however, that in June we are dealing with a 

 very much larger number of observations of clear weather than of foggy weather ; 

 hence we find that the westerly winds are only 6 per cent, of the total dealt with in 

 June in clear weather, whereas they are 20 per cent, of those in foggy weather. A 

 table has, therefore, been made, showing for each month the percentage of each 

 direction. 



Although we have in the tables and curves all the data relating to clear and 



