TRANSACTIONS OF SECTION A. 547 



the scientific process consists in identifying corresponding changes. This method 

 has given some remarkable results by the comparison of the sequence of changes 

 in the meteorological elements in the hands of Pettersen and Meinardus, and by 

 the comparison of the variation of pressure in different parts of the globe by Sir 

 Norman Lockyer and Dr. W. J. S. Lockyer ; as regards the earth and the sun 

 the subject has reached the stage of productive discussion. As a matter of fact, by 

 continn"ing this Address I am preventing Sir Norman Lockyer from telling you all 

 about it. 



For the purpose of dealing ■with periodicity in any form we substitute for 

 nature an ideal system obtained by using mean values instead of individual 

 values, and leaving out what, from this point of view, are called accidental 

 elements. The simplification is perfectly legitimate. Passing on to the con- 

 sideration of periodicity in the stricter sense the process which has been so 

 effective in deaUng with tides, the motions of the liquid layer, is very 

 attractive as a means of attacking the problems of the atmosphere, because, in 

 accordance with a principle in dynamics, to every periodic cause there must 

 correspond an effect of the same period, although the relation of the magnitude of 

 the efl'ect to the cause is governed by the approximation of the natural period of 

 the body to that of the cause. 



There are two forms of the strict periodic method. One is to examine the 

 generalised observations for periodicities of known length, whether it be that of the 

 lunar rotations or of sunspot frequency, or of somelonger or shorter period. In 

 this connexion let me acknowledge a further obligation to Professor Schuster for 

 tackmg on to his Address of last year a development of his work on the detection 

 of hidden periodicities by giving us a means of estimating numerically what I may 

 call the reality of the periodicity. The other method is by harmonic analysis of a 

 series of observations with the view of finding causes for the several harmonic 

 components. I may say that the Meteorological Office, supported by the strong 

 opinion of Lord Kelvin, has favoured that plan, and on that account has for many 

 years issued the hourly results for its observatories in the form of five-day means 

 as representing the smallest interval for which the harmonic analysis could be 

 satisfactorily employed. Sir Richard Strachey has given some examples of its 

 application, and the capabilities of the method are by no means exhausted, hut as 

 regards the general problem of dynamic meteorology harmonic analysis has not as 

 yet led to the disclosure of the required generalisation. 



I ought to mention here that Professor Karl Pearson, with the assistance of 

 Miss Cave, has been making a most vigorous attempt to estimate the numerical value 

 of the relationship, direct or inverse, between the barometric readings at different 

 places on the earth's surface. The attempt is a most interesting one as an entirely 

 new departure in the direction of reducing the complexity of atmospheric pheno- 

 mena. If it were possible to find coordinates which showed a satisfactory corre- 

 lation it might be possible to reduce the number of independent variables and 

 refer the atmospheric changes to the variations of definite centres of action in a 

 way that has already been approached by Teisserenc de Bort and Hildebrandsson 

 from the meteorological side. 



Years ago, when Buys Ballot laid down as a first law of atmospheric motion 

 that the direction of the wind was transverse to the barometric gradient and the 

 force largely dependent upon the gradient, and when the examination of syn- 

 chronous charts showed that the motion of air could be classified into cyclonic 

 and anticyclonic rotation, it appeared that the meteorological Kepler was at hand, 

 and the first step towards the identification of a working meteorological unit had 

 been taken — the phenomena of weather might be accounted for by the motion and 

 action of the cyclonic depression, the position of the ascending current, the baro- 

 metric minimum. The individual readings over the area of the depression could 

 be represented by a single symbol. By attributing certain weather conditions to 

 certain parts of the cyclonic area and supposing that the depression travelled with 

 more or less unchanged characteristics the vagaries of weather changes can be 

 accounted for. For thirty years or more the depression has been closely watched, 

 and thousands of successful forecasts have been based upon a knowledge of its 



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