458 



Sir NV Lockyer and Dr. W. J. S. Lockyer. [Apr. 13, 



which have a regular and pronounced annual pressure variation, such 

 as India, and where the yearly barometric range is of far greater 

 magnitude than any other aperiodic fluctuation. 



In those regions where the mean yearly curve is more misleading 

 than otherwise, the division, according to the two seasons included in 

 the two groups of months, April to September and October to March, 

 is best adapted.* 



The system adopted in the present investigation was to take the 

 pressure variations over India and Cordoba as the chief types of each 

 region, denoting those of the former by the symbol ( + ), and those of 

 the latter by ( - ). The pressure curve of any other place was then 

 taken and compared with each. If, for example, it was found that the 

 curve extending over several years exhibited an excess pressure at 

 those epochs when the Indian pressure curve was in excess, then it was 

 classified as being similar to the Indian type and represented by a ( + ). 

 If it was seen that although it was more like the Indian curve than 

 that of Cordoba, but yet not quite the exact counterpart of India, 

 then it was denoted hy (+1). In a similar way pressure curves like 

 Cordoba were classified as ( - ), and those more like Cordoba than 

 India as ( - T). 



In some regions the pressure variation curves were distinctly a 

 mixture of both the Indian and Cordoba types, and it was difficult to 

 classify them satisfactorily by the above method. The symbol adopted 

 for these cases was ( ± f). Again, there were further some curves in 

 which even this mixed type of symbol was not sufficient to exhibit the 

 relationship of their variations to the other curves, so a special symbol 

 (?) denoting ambiguity was used. 



In the present investigation of this similarity or dissimilarity of 

 atmospheric pressure changes over large areas, it was found that the 

 special types were apparent sometimes in the yearly curves, sometimes 

 in those for one or other of the high or low pressure groups of months, 

 or sometimes in both of these. It did not, however, appear to follow 

 that, because the type was distinguishable in the yearly curves, it was 

 necessarily apparent in both the curves of the high and low pressure 

 months. 



The accompanying table, although yet somewhat incomplete, gives 

 a tabulated statement of the data employed in the present survey. 

 The table explains itself, but it may be remarked that in Columns 6 



* To show the misleading nature of the mean annual pressure-variation curve 

 over, for example, the British Isles, it is only necessary to plot the actual monthly 

 values of pressure for any one jear on this mean curve and draw a curved line 

 through them, when it will be seen that there is practically no relationship what- 

 ever between the two curves. If, on the other hand, the actual monthly pressure 

 values during any one year be plotted on the mean annual pressure-variation curve 

 for India, the former follow very closely the swing and amplitude of the latter. 



