160 DALLAS— PRESSURE AND RAINFALL [April 13, 



four-year cycle in the Indian rainfall. The author believes that 

 these four-year oscillations form the basis of the weather changes 

 over the Indian monsoon area, though there occur at times violent 

 or spasmodic interruptions, the cause of which is not as yet appar- 

 ent, and that these interruptions are the cause of the great irregu- 

 larities in the course of the pressure cycle and in the occurrence of 

 the variations of rainfall. Since the history of these seven mon- 

 soon seasons was written, Professor Bigelow's " Contributions toCos- 

 mical Meteorology " ] has appeared. In it the following paragraph 

 occurs : 



" The increase of solar magnetic intensity is synchronous with a 

 diminution of temperature but with an increase of pressure and this 

 function persists throughout every phase of the research. In spite 

 of some irregularity there is a distinct conformity in the general 

 sweep of these curves and also in the tendency to describe crests 

 during the same years. Indeed the occurrence of four subordinate 

 crests in the n -year periods suggests strongly that a 2 f -year period 

 is superposed upon the long sweep of that period curve. Appar- 

 ently this minor period is the basis of the seasonal variations of the 

 weather conditions of the U. S. A. more than anything else, so 

 that in long range forecasting this period must be very carefully 

 considered. ' ' 



It will be noticed that the period of these minor oscillations as 

 then determined by Professor Bigelow was 2f years for the United 

 States. Subsequently Professor Bigelow produced his "Report on 

 the Barometry of the United States" and from the complete data 

 there employed he obtained an eight-year cycle of pressure which 

 is a simple multiple of the four year cycle determined for the Indian 

 Monsoon Area. Professor Bigelow's researches terminate with the 

 year 1899 but where they overlap the Indian series of observations 

 the principal characteristics of the two series agree. Thus Pro- 

 fessor Bigelow obtains a maximum in 1896 and a minimum in 1898 

 with pressure rising again to a maximum through 1899. In the 

 Indian equatorial area the period is approximately four years, and 

 the writer agrees in believing that these minor oscillations of 

 pressure are mainly influential in determining the seasonal varia- 

 tions of weather. On this point it appears probable that the ex- 



1 See Monthly Weather Review, July, 1902, and especially, Weather Bureau 

 Bulletin, No. 21, pp. 125-6, Washington, 1898. 



