134 Sir Norman Lockyer and Dr. W. J. S. Lockyer. [Oct. 18, 



" On the Similarity of the Short-period Pressure Variation over 

 Large Areas." By Sir Norman Lockter, K.C.B., F.K.S., and 

 William J. S. Lockyer, M.A., Ph.D., F.E.A.S. Eeceived 

 October 18 — Eead December 4, 1902. 



[Plates 1 and 2.] 



In a paper presented in June last to the Society,* we pointed out 

 the existence of a short-period oscillation of barometric pressure over 

 the Indian area corresponding generally with a variation in the per- 

 centage number of prominences recorded on the sun's limb. This 

 oscillation was further shown not to be limited to the Indian area, but 

 to be marked at a far distant station, as Cordoba, in South America. 



The present paper, which is a continuation of this investigation, 

 was undertaken to extend the research over a larger area. 



The monthly means of the pressure variations for each station have 

 been divided as previously into two periods, namely, those months in 

 which the pressures are above and those in which they are below the 

 normal, the normal being the mean pressure for the whole period 

 under investigation in each locality. 



In dealing with large areas, it happens that during the same period 

 of time (that is generally but not invariably six months), the pressure 

 is above the normal in some places, and below the normal in others ; 

 the similarity of the curves representing the variation of the mean 

 for this period, from year to year, indicates therefore that, in one case, 

 a rise in the curve denotes that the pressure is higher, and, in the other, 

 that the pressure is not so low as usual. 



The accompanying curve (Plate 1) illustrates the variations of pres- 

 sure which have been analysed. Commencing with Indian pressures 

 (as represented by Bombay) the area was gradually extended to Ceylon 

 (Colombo), Java (Batavia), Mauritius, and finally to Australia (Perth, 

 Adelaide and Sydney). 



In this set of curves about the same months are in question, so that 

 the pressure variations refer in the northern hemisphere to the low 

 pressure (summer) months, and in the southern hemisphere to the 

 high pressure (winter) months. 



The striking similarity between these curves shows that over the 

 whole of this area, which includes both north and south latitudes, the 

 same kind of variations is in action, and that therefore the whole 

 region is intimately connected meteorologically. 



It was indicated in our previous paper that the pressure of Cordoba, 

 in South America, was the inverse of that of India for the same period. 

 Since the Indian pressure variations are seen now to extend over a 



* " On Some Phenomena which suggest a Short-Period of Solar and Meteoro- 

 logical Changes." ' Roy. Soc. Proc.,' vol. 70, p. 500. 



