22 PHYSICAL GEOGRAPHY OF BRITAIN. 



jections which greatly detract from their value or utility 

 when brought together. Those made by register ther- 

 mometers assume the means of the daily extremes to give 

 the temperature of the place. Such are certainly better 

 than records at one or two fixed hours, and afford the 

 most convenient mode of comparing the temperatures of 

 different places ; but the true mean of the twenty-four 

 hours appears, on the average, to be higher than the means 

 of extremes, whether daily or yearly. A greater number 

 of records have been kept by noting the thermometer at 

 one, two, or more, fixed hours. If the different observers 

 had taken the same hours the value of their observations 

 would have been greatly increased, notwithstanding that 

 the hours adopted might not have been those which give 

 a mean nearest to the true mean of the whole twenty-four, 

 or of the extremes during the twenty-four hours. 



Observations were made at Leith Fort, near Edinburgh, 

 every half hour, for two whole years, during 1824 and 

 1825. The annual mean of each hour being calculated, 

 and also that of the whole twenty-four, we readily ascer- 

 tain how far the temperature at any given hour differs 

 from the mean of the whole twenty-four. The sub- 

 traction of the excess, or addition of the deficiency, forms 

 the corrected mean for the hour. This method and de- 

 gree of correction have been applied indiscriminately to 

 all places in Britain, and even to other countries ; but it 

 is very probable that in other places the annual mean of 

 any given hour will not bear the same proportion to that 

 deduced from the daily means of the whole twenty-four, 

 as it does in Scotland. However, it is likely to be suffi- 

 ciently near for practical purposes throughout Britain. By 

 the Leith Fort registry, on the average of the two years, 

 the difference between the mean temperature of each 

 hour and that of the day was calculated to be as in the 

 annexed table : — 



