Variation of Temperature in the British Isles, &c. 81 



to some extent apparent during the prevalence of winds from all 

 quarters. 



It is not found, with maxima at the same epoch, in purely Continental 

 stations, such as Vienna and Agra. 



A similar effect is found in the temperature variation of the sea- 

 water at stations surrounding these islands, and the atmospheric effect 

 is probably connected with this. 



A similar second order periodic effect is also found in the variation 

 in the magnitude of the barometric gradient between London and 

 Valencia and London and Aberdeen. It is probable that this periodic 

 variation in pressure plays some part in causing the similar variation 

 in temperature. 



Note on the Effect of Sea Temperature upon the Seasonal Variation of Air 



Temperature. 



It has been pointed out in referring to Table II. that the harmonic 

 components of the first and second orders of the seasonal variations of 

 the temperature of the sea in the neighbourhood of a coast station 

 show a relative lag as compared with the corresponding components of 

 the air temperature of the stations. On comparing Table II with 

 Table I, it will readily be seen that the first-order curves for the coast 

 stations show a definite lag as compared with the inland and Continental 

 stations. Thus the earliest occurrence of a first-order maximum for 

 a coast station in Table II is July 30, and in Table I July 28, while 

 the relatively inland station, Kew, shows the first-order maximum on 

 July 23, and Oxford (for a single year, however) on July 12. Vienna 

 is earlier than Kew, July 18, and Agra still earlier, June 29. We 

 may thus regard the sea as causing a considerable lag in the occurrence 

 of the seasonal variations of temperature in the adjoining coast stations, 

 while in the sea itself the lag is still greater. 



Now we may regard the periodic variation of sea temperature as a 

 cause affecting the air temperature of the stations, and, without know- 

 ing precisely the process connecting the cause and its effect, we may 

 form some estimate of the relative magnitude in the following 

 manner. Dealing only with the first-order components of the two 

 related phenomena, we note that they are harmonic variations of the 

 same period. We may assume that the effect of the periodic cause is 

 itself simply harmonic and of the same period, and that the resultant 

 first-order component for the air temperature is made up of a part which 

 is independent of the sea temperature and a part which is dependent 

 upon and caused by the sea temperature. We may thus assume the 

 resultant first-order curve to be made up by compounding the two 

 separate component curves. 



The composition of harmonic curves of the same period is a geo- 



VOL. LXIX. G 



