80 Messrs. W. X. Shaw and 11. W. Cohen. On the, Seasonal 



connection between barometric pressure and atmospheric temperature, 

 it seems not unlikely that the one effect plays some part in the causa- 

 tion of the other. 



The striking correspondence between the period of occurrence of the 

 maximum of the second-order component of the gradient for westerly 

 winds (London Aberdeen) and the corresponding components for tem- 

 perature, leads naturally to the suggestion that the periods of maximum 

 and minimum of the second-order component of temperature are 

 related to special barometric conditions. They may, and indeed must, 

 be connected with some stage in the process of annual migration of 

 the centres of high and low barometric areas, which is the result of the 

 unequal distribution in longitude of the land areas in the northern 

 hemisphere. A cursory survey of the mean monthly distribution of 

 pressures in Dr. Buchan's volume of ' Bartholomew's Atlas,' shows 

 that April is an exceptional month of transition, but up to the present 

 we have not found any means of representing the successive stages of 

 the transition in such a way as to enable a comparison to be made 

 between them and the second-order curve of temperature. 



The data used in this paper have been compiled in the Meteorological 

 Office, or extracted from the various publications in the library of 

 the office. The harmonic analysis has also been carried out there 

 under the supervision of Mr. 11. H. Curtis. 



Summary. 



The harmonic analysis of the curve of mean atmospheric temperature 

 at stations in the British Isles shows that there is, compounded with 

 the primary solar variation, a secondary half-yearly meteorological 

 variation. 



The effect of this variation is to moderate and lengthen the winter, 

 and to intensify and shorten the summer. 



The second-order curve which represents it, has maxima which oscil- 

 late, in different years, over the first ten days of February and August 

 respectively, and minima which oscillate over the first ten days of May 

 and November. 



With some exceptions, the effect is generally apparent in a single 

 year's observations, and, owing to its varying position, its curve ap- 

 pears to have a larger amplitude in the analysis of the temperature 

 curve of a single year than in that of the mean curve of a number of 

 years. 



It is independent of the relative frequency of occurrence of cyclonic 

 and anticyclonic weather, and of the relative temperature of the air 

 during the prevalence of these different types of weather. 



It is partly due to a periodic variation in the relative frequency of 

 occurrence of the "cold," "warm," and "temperate" winds, but is 



