1 904.] Short-Period Atmospheric Pressure Variation. 



457 



" The Behaviour of the Short-Period Atmospheric Pressure Varia- 

 tion over the Earth's Surface." By Sir Norman Lockyer, 

 K.C.B., LL.D, F.E.S., and William J. S. Lockyer, Chief 

 Assistant, Solar Physics Observatory, M.A. (Camb.), Ph.D. 

 (Gott), F.B.A.S. Eeceived April 1 3 — Eead April 28, 1904. 



[Plates 12 aitd 13.] 



In a paper which we communicated to the Society in June^ 1902,* 

 we drew attention to the fact that in our investigation dealing with 

 the percentage frequency of prominences and changes in atmospheric 

 pressures, we found that the pressures in India and Cordoba behaved 

 in an opposite manner, the short period variations of one being the 

 inverse of those of the other ; both, however, were closely associated 

 with the prominence frequency. 



In a subsequent paperf we showed that these two regions, in which 

 these inverse pressure-variation conditions were clearly distinguishable, 

 were, as far as had then been investigated, of considerable extent, the 

 Indian region extending to Ceylon, Java, Mauritius and Australia, 

 and that of Cordoba to the southern part of the United States. 



The facts there collected were stated to be so suggestive that the 

 inquiry was being continued by collecting and discussing observations 

 made in other' areas on the earth's surface, so as to note the extent of 

 these similar pressure areas. 



The present communication contains the results that have so far 

 been obtained. 



The greater portion of the facts here collected has been discussed 

 some time, but as it was considered desirable, before communicating 

 the present paper, to include as many regions on the earth's surface as 

 could be obtained, a longer delay than was anticipated has taken 

 place ■ even now there are many regions which we have been unable to 

 include. The regions for which further observations are desired 

 include the west coast of Africa, the northern part of South America, 

 and the north-western portion of North America, and Polynesia in the 

 South Pacific Ocean. 



In our previous papers we have pointed out the advisability of 

 dividing the year into groups of months according as the pressure is 

 above or below the mean value for the year. In this way the high or 

 the low pressure months can be dealt with separately, if necessary, 

 and any excess or deficiency from a mean value exhibited in either or 

 both of these from year to year can be closely followed. 



Such a division of the year can be accurately determined for places 



* ' Roy. Soc. Proc.,' vol. 70, p. 500. 

 f ' Roy. Soc. Proc.,' vol. 71, p. 134. 



