70 Lieut.-General R. Strachey. Harmonic Analysis [Mar. 2, 



a curve of sines, the first component being more than ten times as 

 large as any of the others, which therefore influence the temperature, 

 relatively, very little. 



The relation of the epoch of the first maximum of the component 

 of the third order to the time of sunrise is decidedly marked, the 

 former occurring, on the average, about 12°, or 48 m. after sunrise ; 

 the mean deviation of the interval from that amount being only 7°, 

 or 28 m. 



The periodical variation in the position of the maximum leads, 

 during the winter months, to a positive maximum of this component 

 about 1 p.m., which is combined with negative maxima four hours 

 earlier and later, which correspond to the reduced temperature in the 

 mornings and afternoons of the shorter days. In like manner, in the 

 summer months, when this component has a negative maximum about 

 1 p.m., instead of a negative minimum, as in winter, there will be two 

 positive maxima, one four hours earlier, the other four hours later, 

 corresponding to the higher temperature in the mornings and after- 

 noons of the longer days. 



It will be seen that these positions of the midsummer and mid- 

 winter maximum phases correspond respectively to days of 16 hours 

 with nights of 8 hours, or days of 8 hours and nights of 16 hours, 

 and that at these seasons, when the variations of temperature, due to 

 these differences, are greatest, the amplitudes of this component are 

 also the greatest. At the equinoxes, with 12-hour days and nights, 

 the component becomes a minimum; and at this season the change 

 in the position of the maximum takes place as already noticed. 



It might be supposed that an analogous relation between the 

 fourth component and the occurrence of days of 18 hours, combined 

 with nights of 6 hours, and vice versa, is likely to arise. But the data 

 are not forthcoming to test this. 



Although the several components of the temperature curve cannot 

 be regarded as indications of specific physical efficient causes, the 

 examination of the graphical representations of the various curves 

 presents points to which attention may usefully be drawn. The chief 

 of these are the following : — 



In the summer months the time of mean temperature is nearly 

 where the first component becomes zero, the second and third com- 

 ponents then balancing one another. 



In the winter the time of morning mean temperature is later than 

 in summer, and occurs when a positive value dl the first component is 

 equal to a negative value of the second. 



The time of afternoon mean temperature throughout the year is 

 somewhat either before or after 7 p.m., and almost exactly coincides 

 with the time when the first and second components are equal, with 

 opposite signs. 



