1S76.] Barometric Anomalies. 29 



adducing evidence to shew that abnormal variations of relative pressure 

 tend to be persistent ; and on the former head I shall only point out some 

 observed coincidences, with the caution that no satisfactory discussion of 

 this subj ect is practicable until the conditions are known over a very much 

 wider area than that treated in the present paper, and that for a sufficiently 

 long period. 



When the barometric traces of a number of stations in different parts 

 of India are compared together, it appears that, after eliminating the regu- 

 lar variation of the daily tides, they are all affected simultaneously or near- 

 ly so by irregular fluctuations of very various duration (from one to many 

 days), and that these are on the whole more intense at northerly than at 

 southerly stations. The amount by which the pressure at any place for a 

 given month or day deviates above or below its normal value for that period, 

 will necessarily be affected by these irregular fluctuations as well as by 

 those protracted anomalies of pressure with which I am now concerned. 

 The most ready way of exhibiting the latter is to take the barometric differ- 

 ence for the period in question of a pair of stations, not too distant from 

 each other, and from this to deduct their normal or average difference for 

 the corresponding period ; or, what comes to the same thing, to obtain the 

 total barometric anomaly* for each of the pair separately, by deducting 

 the corresponding normal values, and then to take the difference of these 

 anomalies. This latter method will be followed in the tables illustrating this 

 paper. If we find that as a general ride these final differences or, as I have 

 termed them, relative anomalies preserve the same sign + or — and not in- 

 frequently approximately similar values for many months together (the 

 comparison being sufficiently extensive fairly to test the whole mass of the 

 data) , the proposition that abnormal variations of relative pressure tend to be 

 protracted, may be considered as established. I must, however, remark by way 

 of precaution, that the above method of proceeding does not completely 

 eliminate the general irregular fluctuations. It would do so, only if all stations 

 were equally affected by them ; but actually this is not the case. A simple 

 inspection of the barometric curves (of which I have a series for several years) 

 shews that, as a rule, these fluctuations are more intense, the higher the lati- 

 tude ; and that, sometimes, stations in the interior seem to be more affected 

 than those on the coast. Hence the further two stations are apart (especially 

 in latitude), the less complete is the elimination. But for my present pur- 

 pose this rough method will suffice. I now proceed to the facts. 



In 1868 an abnormal barometric depression in the NW corner of the 

 Bay of Bengal and in part of Orissa characterised the whole of the SW 



* By this term, I designate the amount by which the mean pressure at any station 

 for a given day, month, or year, ranges above or below the corresponding average of 

 many years. 



