REPORT ON ATMOSPHERIC CIRCULATION. 39 



These differences were then entered on small maps of Europe, from which, by the 

 corrections thus found, the monthly means of the stations were brought to the means 

 of the fifteen years. In certain districts, where necessary, differences were found for 

 additional stations to these forty-three. Hence for all stations in the table for which 

 the period of observation is entered as fifteen years, 1870-84, the means are simply the 

 arithmetical averages of observations made during that period, or they are the 

 approximate means for the same years. An examination of the above table, or better 

 still, of a map on which the figures are entered, will show that the limit of error of any 

 deduced approximate mean is in each case small. 



In the United States, the term of years employed is not the fifteen years ending 

 with 1884, but the thirteen and one-fourth years extending from October 1871, when the 

 Signal Service of the War Department took charge of the Meteorological System of the 

 States, to December 1884. A comparison of the averages of these thirteen and one-fourth 

 years, with those of the fifteen years for about a dozen stations from which observations 

 have been obtained for the whole fifteen years, shows that the two sets of averages 

 closely agree. The isobars are therefore drawn from data virtually synchronous for the 

 greater portion of the land surfaces of the Northern Hemisphere. 



But generally over the Arctic Regions, South America, Africa, and Polynesia no 

 such full information is available ; the means of the observations actually made are alone 

 printed, except in such regions as Southern Africa, Australia, and Japan, where the 

 number and proximity of the stations seemed to warrant differentiation. 



Correction for Range. — The means in Table VI. are, in each case, for the hours 

 specified, no correction being applied here for variation due to diurnal range. But 

 in preparing the figures for the drawing of the isobars, corrections were applied with 

 the view of bringing the means for the hours observed to the daily means. In this part 

 of the work the corrections in each case were taken from the copious Tables III. and IV. 

 of hourly barometric range given in the Appendix, pp. 7 to 48. Care was taken in 

 correcting for range to use only data furnished by a station or stations similarly situated 

 geographically to the station the means of which were to be corrected. Thus Mullagh- 

 more and Belmullet were corrected from Valentia, Parsonstown from Armagh, Holyhead 

 from Liverpool, Cambridge from Oxford, Edinburgh from Makerston and Aberdeen, 

 and other places in a like manner. Here the results of the Challenger observations, 

 given in Table III, were of great service in correcting the means for small islands and 

 coast stations over large regions of the globe. It need scarcely be added that the 

 hourly variations for such stations as Cries and Klagenfurt in Austria, and Cordova 

 in the Argentine Republic, situated in deep narrow valleys, were in no case employed, 

 for the reasons already stated. Further, the means at places situated on plateaux more 

 or less elevated, were not corrected from the hourly variations of such high level 

 stations as Ben Nevis, Santis, and Hoch Obir, which, being placed on true peaks, have 



