DIURNAL RANGE OF THE BAROMETER IN FINE AND CLOUDY WEATHER. 449 



account was taken of whether rain was falling or not in selecting the days, nor of any- 

 other meteorological condition except clear or clouded skies. 



With each day was included the midnight of the previous day, so as to show by the 

 difference of the two midnights when the barometer was, on the whole, rising or falling 

 during the periods selected ; and also to give the means of eliminating this general rise 

 or fall in order to arrive at the strictly diurnal phenomena. The method used for this 

 ■elimination and a table for facilitating it are given on page 543. 



This adjustment of the hourly values is necessary to disentangle the diurnal changes 

 from those of longer periods ; otherwise the entries in the tables are simply arithmetical 

 means of the barometers reduced to 32° but not to sea level. 



The result may be broadly stated thus. The diurnal curve with double maximum 

 and minimum points is distorted in the same way at all the stations — namely, in fine 

 days, the forenoon maximum and afternoon minimum are increased while the evening 

 maximum and early morning minimum are diminished. On cloudy days the reverse 

 takes place, the evening maximum and early morning minimum being increased and the 

 forenoon maximum and afternoon minimum diminished. These effects are larger at 

 temperate and Arctic stations than in the tropics, although there the diurnal fluctuations 

 are so much greater. 



The numerical values for each station are given in the accompanying tables (pages 

 455 to 463), showing for each station — 



1st. The diurnal range of the barometer for each month of the year, without any 

 selection of special days. 



2nd. The diurnal range on days with little or no cloud, these days being grouped 

 and averaged in their respective months. 



3rd. The diurnal range on days when the sky was overcast, or nearly so, all day, 

 similarly grouped and averaged. 



Each table is arranged in the same manner. Each of the twenty-four hours of the day 

 occupy a column, and each of the twelve months a line. At the right hand is given the 

 mean value for each month of the barometer in inches and decimals, and under each hour 

 the difference between the mean at that hour, and the general mean of the month in 

 thousandths of an inch, heavy type figures indicating that the hourly mean exceeds the 

 general mean, and italic figures the. reverse. At the foot of each table is given the 

 values for the year, being the average of the twelve monthly values. In the tables for 

 fine and cloudy days the total number of selected days in each month is given in the 

 column headed "Days." 



It is well known that the diurnal range of the barometer is greatest near the Equator 

 .and diminishes almost to vanishing in the Arctic regions. Tins is well illustrated in the 



ROY. SOC. TRANS. EDIN. — VOL. XLII. 3 M 



