( 318 ) 



variability and steadiness of the climate from year to year in so 

 far as this is determined by the oscillation of the atmospheric pres- 

 sure. By analogy to the secular variation of the elements of terrestrial 

 magnetism this instability might also be called secular variability. 



Assuming this criterion to be correct, it appears from Table I 

 that there is every reason to suppose that at Helder the deviations 

 follow the normal law, the average value of ^ not differing more 

 than 1.8 7o froiïi the real value. 



On comparing the climate at Helder, which is highly variable 

 from year to year, with the climate at Batavia (in so far as in this 

 case also the variability of atmospheric pressure may be taken as a 

 measure), we fmd totally different conditions. 



A period of about ten years for the Eastmonsoon, and of twenty 

 years for the Westmonsoon months is already sufficient to obtain 

 total monthly means of the barometric height with a probable error 

 of ± 0.1 mm. and for the di-y months the available series of 

 37 years is quite sufficient to obtain a degree of certitude twice as 

 great. 



TABLE II. Monthly means of barometric height, Batavia. 



M 



January. . . 

 February. . 

 March . . . . 



April 



May 



June 



July 



August , . . 

 September 

 October . . . 

 November. 

 December. 

 Mean. . , 



32 



26 



18 



8 



9 



7 



9 



10 



9 



12 

 19 

 17 



2.759 

 3.004 

 2.974 

 2.715 

 3.752 

 3.931 

 3.282 

 4.028 

 3.173 

 3.118 

 '2.999 

 3.086 



3.235 



Tiie application of the criterion as to whether the deviations follow 



