24 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 90 



A full description is given in the above-mentioned " Memoirs," 

 of the methods employed to reduce the whole mass of data 

 up to the year 1900, to a homogeneous system. 



Pressure. 



The pressure means from 1769 to 1900 are taken from the above 

 mentioned work (Trans. Roy. Soc. Edinburgh, vols. 39 and 

 40). From 1901 to 1916 the monthly means are from the 

 values in inches that appear in the Journal of the Scottish 

 Meteorological Society and thereafter from the values in 

 millibars that are given in the Monthly Weather Reports 

 of the Meteorological Office, London. The values given in 

 inches (up to 1916) were converted to millibars, and certain 

 errors in the original tables were corrected. The hours of 

 observation were 9 a.m. and 9 p.m. and no correction has 

 been made for diurnal range. A correction of +.028 in. 

 was applied to the converted sea-level values in inches, to 

 reduce them to standard gravity at Lat. 45 °. 



Temperature. 



The monthly means are the mean of the daily maximum and 

 minimum temperature, and when, as from 1731 to 1736, 

 the means are those referring to fixed hours, the values 

 were corrected. Up to 1900 all the means were corrected 

 to a height of 250 ft. above sea* level, so that when the 

 Royal Observatory series, taken at a height of 441 ft., were 

 employed, it was necessary to reduce them to a height of 

 250 ft. After elaborate comparisons of the Royal Observa- 

 tory data with surrounding climatological stations in and 

 near the city, the following monthly corrections in degrees F. 

 were obtained, which not only include the correction for 

 difference of height, 191 ft. (to bring to 250 ft.), but also 

 those due to the exceptional position of the station, which 

 is near the crest of a hill, whereas all the other stations 

 were located on the flat or on gentle slopes. The monthly 

 corrections are : 



Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Year 

 °F. +0.3 +0.4 +0.G +0.8 +0.8 +0.9 +0.9 +0.8 +0.6 +0.3 +0.3 +0.2 +0.6 



For well-known physical reasons, in very severe and long- 

 continued frosts, of which the last experienced was in 1895, 

 the above positive corrections for the winter months may 

 well become negative. Several errors, typographical or other- 

 wise, that appear in the original " Memoirs," were corrected. 

 Of these, the most serious was the mean temperature of 



