REPORT ON ATMOSPHERIC CIRCULATION. 



35 



over land, 8 P.M. to midnight; lightning over the open sea, 8 P.M. to 4 A.M.; and 

 thunderstorms over the open sea, 10 p.m. to 8 a.m. 



The evening maximum atmospheric pressure occurs at the time when the aurora 

 attains its diurnal maximum. Thirty years' observations at Christiania Observatory 

 give the number of times the aurora was observed each hour as under : — 



7 times. 



16 



46 

 105 

 133 

 156 

 529 

 130 



79 



Total, 



Of the 1320 instances recorded, it is seen that 529 occurred in the hour from 9 to 

 10 p.m., and in the four hours from 7 to 11 p.m. 948 cases were observed, a result 

 probably dependent in no small degree on the atmospheric conditions resulting in the 

 evening maximum of pressure, the more abundant ice spicules in the upper regions at 

 the time serving as a screen for the better presentation of any magneto-electric discharges 

 that may occur. 



Monthly, Annual, and Recurring Phenomena. 



Of the annual recurring phenomena of the atmosphere, the distribution of atmo- 

 spheric pressure, atmospheric temperature, and the prevailing winds of the globe, during 

 the months of the year have, as the more important, been thoroughly revised for this 

 article. The data on which the revision is based are given in Tables V. to IX., and 

 the results are graphically represented on Maps I. to LIL, which show the monthly 

 isothermals, isobars, and prevailing winds over the globe. These represent the average 

 temperature, pressure, and direction of wind over the larger portion of the land surfaces 

 of the globe based on the fifteen years' observations beginning with 1870 and ending 

 with 1884. 



Charts showing by isobaric lines the mean pressure of the atmosphere through 

 the months of the year, may be considered as furnishing the key to the fundamental 

 questions of meteorology, since it is only by the information thereby obtained that 

 questions relating to the prevailing winds, and the varying temperature, cloud, and 

 rainfall of different regions, can be satisfactorily handled. 



