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XVII. — The Mean Pressure of the Atmosphere and the Prevailing Winds over the 

 Globe, for the Months and for the Year. Part II. By Alexander Buch an, M. A., 

 Secretary of the Scottish Meteorological Society. (Plates XXV. to XXVII.) 



(Read 19th April 1869.) 



Charts, showing by Isobaric Lines the mean pressure of the atmosphere over 

 the globe during the months of the year, may be justly regarded as furnishing 

 the key to all questions of meteorological inquiry; for without the information 

 conveyed by such charts it is impossible to discuss satisfactorily those questions 

 which relate to prevailing winds, the varying temperature, and the rainfall 

 throughout the year in the different countries of the world. It is to meet this 

 desideratum that the Charts of Mean Atmospheric Pressure of the globe which 

 are given with this paper are offered as the first approximate solution of this 

 great physical problem. 



Since Part I.* was read in March 1868, valuable additional information has 

 been obtained from Australia, New Zealand, Tasmania, Africa, South America, 

 the west coast of North America, Iceland, Norway, and Sweden, and from 

 several isolated stations in different parts of Europe and Asia. The period for 

 the British Islands and a large portion of Europe has been extended so as to 

 include the eleven years from 1857 to 18G7. 



In this Part the complete set of Charts for the twelve months and for the 

 year are given, together with the data from .which the Charts have been con- 

 structed. 



As regards Pressure, the stations were selected with the view of representing 

 as well as possible the geographical distribution of the pressure. The first place 

 was assigned to those stations at which the barometric observations were known 

 to be, or presumably were, of the best quality ; and in drawing the isobaric 

 curves, the greatest weight was given to means deduced from these observations. 

 Since it is the mean pressure at sea-level which is here inquired into, and since 

 the manner of the geographical distribution of the pressure doubtless varies at 

 different heights, stations at low elevations were preferred to those at greater 

 heights. The pressures at a few elevated places, such as Great St Bernard and 

 Dodabetta, are given in the Tables; but they were not made use of in drawing 

 the curves, — their value consisting in the light they tend to throw on the 

 movements of the upper currents of the atmosphere. 



In an inquiry into the comparative distribution of atmospheric pressure, it h 



* Proceedings of the Society, vol. vi. p. 303. 

 VOL. XXV. PART II. 7 K 



