352 



NA TURE 



[August 9, 1906 



ATMOSPHERIC PRESSURE CHANGES OF 

 LONG DURATION. 



TN previous numbers of this Journal (vol. Ixvii., 

 ■•■ p. 224, and vol. Ixx., p. 177) I described a 

 barometric variation of short duration and world- 

 wide in extent which behaved in a see-saw manner 

 in an easterly and westerly direction between anti- 

 podal parts of the earth. The investigation, which 

 included the examination of pressure changes at 

 ninety-five stations scattered over the globe, indicated 

 that there was a transference of air from west to 

 east and from east to west alternately, a surge, in 

 fact, raising and lowering- the mean annual pressure 

 values. Thus, when the pressure in any year in India 



SUNSPOT CURVE 



MADRAS 



BOMBAY 

 BATAVIA 

 ADELAIDE 



the East Indies, and Australia behaved alike, while 

 the South American region behaved in an inverse 

 manner. The present inquiry was therefore limited 

 to these areas. For the first three a considerable 

 amount of data is available, but this is not the case 

 for the last-mentioned region ; to mitigate this de- 

 ficiency, curves for several separate stations have had 

 to be employed in order to determine over several 

 years the variation in operation there. 



The first step taken to prepare the data for this 

 comparison was to eliminate so far as possible the 

 variation of short duration. This was satisfactorily 

 accomplished by grouping the years in sets of four, 

 and employing the mean values of each of these 

 groups ; thus the means for the years 1873 to 1876, 



1900 



the Indian, East Indian : 

 iiiz'crted sun-spot c 



19000 



pared with the 



or Australia was in excess, that recorded in South 

 America showed a deficiency. 



During this inquiry it was noticed that there were 

 changes going on which extended over a longer period 

 of time than the short one (about 3.8 years) to which 

 reference above has been made. In order to find out 

 whether these long variations were similar all over 

 the earth, or whether they also were of an opposite 

 nature in different areas, several sets of long series 

 of pressure observations have been carefully examined. 

 The result of this limited survey recently formed 

 the subject ot a communication by the writer to the 

 Royal Society,' and the following is a brief account 

 of the results arrived at in the paper. 



In the case of the variation of short duration, India, 



Large Areas." By 



1 " Barometi 

 Dr. William J. 



. Lockyer. 



1906. 



1874 to 1877, &c., were determined. The curves here 

 shown are all composed of such means, and arc 

 formed by connecting the points plotted on squared 

 paper ; the same scales are employed througliout. 



Fig. I illustrates the series of curves, all drawn 

 on the same scale, for the Indian, East Indies, and 

 Australian areas. Bombay and Madras represent the 

 pressure changes of the first, Batavia the second, 

 and Adelaide, Melbourne, Sydney, and Perth are 

 types for Australia. 



An examination of these curves leads one to the 

 following conclusions : — 



First, the Indian curves are very alike, and suggest 

 a variation of an oscillatory nature, the maxima or 

 minima occurring about every ten or eleven years. 

 Second, the amplitude of these curves, that is, the 

 difference between the maximum and minimum 



NO. I919, VOL. 74] 



