162 DALLAS— PRESSURE AND RAINFALL [April 14. 



lations small; (6) these variations are superposed on the four- 

 year cycle of the tropical belt, and are spasmodic, occurring at 

 irregular intervals over irregular areas so that their influence oc- 

 casions irregular variations of rainfall and irregularities in the pres- 

 sure cycles. 



There appears to be no satisfactory explanation either of the four- 

 year cycle of pressure over the trades monsoon area or of the 

 irregular spasmodic disturbances of pressure referred to above. 

 With regard to the cycles it is possible that compensatory actions 

 are at work, so that when atmospheric pressure increases in one 

 part of the world it decreases in another, though the evidence of 

 the barometry of the United States is opposed to this and rather 

 suggests that the principal secular variations of pressure are of a 

 uniform character over the whole globe. It is impossible to believe 

 that the variations of pressure are a result of variations of rainfall. 

 For one thing, the variations are as marked in a dry area like Aden 

 as in a wet area like Bombay, and for another, the evidence, so far 

 as it can be sifted, shows that the variations of pressure precede the 

 variations of rainfall. Thus the increase of pressure which culmi- 

 nated in the large excess of pressure in the months of July, August 

 and September, 1899, commenced in February of that year, thus 

 preceding by some months and not succeeding the scanty rainfall of 

 that season. 



The memoir contains all the figures and data on which the 

 enquiry is founded. Some of the observed changes are at present 

 quite inexplicable, but the observations are given as recorded so 

 that though the author has not succeeded in obtaining any con- 

 clusive results, it may be possible for other students of meteorology 

 with more available leisure to work them into a more harmonious 

 scheme. 



In order to undertake this detailed examination the employment 

 of pressure or rainfall means of large areas has been abandoned, 

 and instead the actual monthly pressures and their variations for 

 certain selected stations, which it is believed represent fairly ade- 

 quately the whole monsoon area, have been used. The list of 

 stations includes : Batavia, Calcutta, Bombay, Aden, Cairo, Mad- 

 ras, Colombo, Seychelles, Zanzibar, Mauritius, Durban and Perth, 

 while in addition the marine observations of the Arabian sea and 

 the Equatorial belt have been utilized to obtain averages for those 



