The Time Factor in Correlation between Barometric Heights. 403 



" On the Influence of the Time Factor on the Con-elation between 

 the Barometric Heights at Stations more than 1000 Miles 

 apart." By F. E. CAVE-BROWNE-CAVE, Girton College, Cam- 

 bridge. Communicated by Professor KARL PEARSON, F.RS. 



Keceived June 3, and in revised form, December 20, 1904, 



Eead June 16, 1904. 



1. An investigation of the relationship of the daily barometric 

 heights on both sides of the Atlantic Ocean, has been in progress for 

 some years, and in a preliminary note by Professor Pearson and 

 myself* some account was given of the contemporaneous relationship 

 of a chain of stations from the extreme north of Norway down the 

 west coasts of Europe and Africa. Observations for this east side of 

 the Atlantic have now been copied for twenty years, as far as stations 

 are available from Norway to the Cape, and the only need here is more 

 aid in the very laborious reductions necessary before any inferences can 

 be drawn. A similar chain of stations from Nova Scotia to the 

 Falkland Isles has been completed, with the exception of Brazil, from 

 which, so far, we have been able to obtain no data whatever. 



2. It seemed desirable, before entering on the cross Atlantic corre- 

 lations, to obtain a better conception of how the time factor influences 

 the intensity of correlation. For this purpose two stations, Wilmington 

 and Halifax, seemed specially appropriate. They are both on the East 

 Atlantic coast, the one in North Carolina and the other in Nova Scotia. 

 The results obtained for these stations afford an illustration of the 

 manner in which the theory of correlation can be used for the purposes 

 of meteorological prediction. In a paper published in 1898,t Professor 

 Pearson and Dr. Lee indicated that the proper way to deal with the 

 pressure relationship between two or more stations was to proceed by the 

 method of correlation ; and they illustrated this by the simultaneous 

 correlations of various stations in the British Isles. The corresponding 

 simultaneous correlations for stations from Norway to Sierra Leone were 

 given by Professor Pearson and myself in the note already referred to. 

 The object of the present paper is to illustrate this method of correlation 

 as applied to prediction, the fundamental points being (a) the choice of 

 two stations which, although far apart, have a considerable correlation 

 with at least 12, and if possible, 24 hours' interval ; (b) the determina- 

 tion for any two stations of the interval for which the correlation is a 

 maximum. If the practical meteorologist is accustomed to predict 

 roughly the barometric condition at a station A, from stations B, C, 

 and D, then, if he has observations for twenty years or more for these 

 stations, modern statistical methods place him in a position to choose 



* ' Koy. Soc, Proc.,' vol. 70, pp. 465470. 

 f 'Phil. Trans.,' A, vol. 190, pp. 423169. 



