REPORT ON ATMOSPHERIC CIRCULATION. 37 



in the lower limb of the instrument. This inquiry leaves little if any doubt that 

 these personal errors, in one form or other, are more general than might have been 

 supposed, and accordingly, particular attention was given, in extracting the monthly 

 means, to all changes made as regards observers, with a view to ascertain their personal 

 errors. 



There was no real difficulty in ascertaining the errors of particular barometers in 

 countries where stations are more or less numerous, and the meteorological system 

 under a competent control, if the ordinary sources of error are kept in view. But 

 over large portions of South America, Africa, and Oceania another method for the 

 detection of errors was required. In these regions the barometers have been controlled 

 by Baillie's Isobars for the ocean, recently published by the Meteorological Council. 

 In this work the annual chart is the mean of the four months, February, May, August, 

 and November. The mean pressure at 32° and sea level, for the four months, was 

 calculated for the stations of these regions, and the result being entered in its place 

 on the annual chart, the approximate error of the particular place was ascertained. 

 It may be added that Baillie's Isobars may well serve as a control, seeing they are 

 exclusively drawn from observations made only with properly compared barometers. 

 In Table V., the corrections which have been adopted are in every case entered in the 

 last column, from which, it need scarcely be added, the original uncorrected observations 

 may, if desired, be found. 



In the first place, the figures entered on the maps were restricted to those 

 stations from which observations for each of the fifteen years were available. It may 

 be said of no country that the number and distribution of its stations, furnishing 

 observations for the whole of this period, are sufficient for the purpose on hand. 

 Hence it was absolutely necessary in an inquiry where the same time must be dealt 

 with, to cover the ground in a more adequate manner with the means of other stations 

 at which observations have been made for other periods than the fifteen years, these 

 means being deduced by applying corrections to the monthly means arrived at by 

 differentiation with neighbouring stations. 



In differentiating, the* work was overtaken generally according to the length 

 of the times covered by the period of the observations of the stations, the means of 

 which were in the course of being rectified, beginning with the longest periods and 

 ending with the shortest. In a good many cases the same period for differentiation 

 was made to embrace a very wide area. Thus, over considerable portions of France, 

 Germany, Italy, and North Africa, observations were available only for the seven years 

 1878 to 1884. The following table was accordingly prepared, showing for forty-three 

 places the differences in thousandths of an inch of each month's average for these 

 seven years, compared with the averages of the same months for the fifteen years, which 

 may serve as an example of the method of differentiating employed : — 



