G2G 
LIEUT.-GENERAL R. STRACHEY ON HARMONIC ANALYSIS OE 
components P and p, represent well marked truly periodical variations, even, as far as 
the fourth order, in which the maximum value of P hardly exceeds one-third of a 
degree Fahrenheit. 
The component of the first order, which in the winter months is more than double 
the magnitude of any other, and in the summer more than ten times as large as any 
of them, gives the dominant character to the diurnal curve of temperature. There is 
an obvious tendency for high or low values of P^ to be associated with high and low 
values, respectively, in the other components, though there are many deviations from 
such a rule. 
In the series of 20 years instances may be found in almost every month, of 
variations in the value of P^ for different years, of as much as 100 per cent., and the 
maximum values of the amplitude P in all components is frequently double the 
minimum. Nevertheless, these irregularities for the most part disappear even in 
the mean of a series of five years, and the monthly mean values for 20 years are as 
a whole remarkably consistent, the mean difference between the five-year and the 
20-year values being less than 5 per cent. 
The progression of the magnitude of Pj^ in the course of the year follows ap¬ 
proximately the Sun’s meridional zenith distance, and the empirical formula 
P^ = 10 cos z — '91 gives a close approximation to the mean monthly values shown 
in the Tables, if a “ lagging ” of about eight or ten days is allowed in reckoning the 
zenith distance, as the following comparison will show :— 
Month. 
Meridional 
zenith 
distance. 
Cos s. 
Value of P^. 
Error of 
formula. 
! 
1 
From formula. 
From tables. 
January 
0 
73 
37 
•282 
1-91 
1-90 
1 
-1“ '01 1 
Eebruary . 
66 
47 
•394 
3-03 
3-16 
- 13 
March 
55 
42 
•564 
4-73 
4-67 
+ ■06 1 
April . . . 
44 
31 
•713 
6-22 
6-50 
-•28 ! 
iMay .... 
36 
4 
•808 
7-17 
7-30 
-•13 1 
June .... 
29 
23 
•871 
7-80 
7-71 
-f09 i 
July . . . . 
29 
11 
•873 
7-82 
7^75 
-1--07 
Augxist . 
34 
29 
•814 
7-23 
717 
-b-06 : 
September . 
45 
39 
•699 
6-08 
6^46 
— •38 i 
October . 
57 
20 
•540 
4-49 
4-16 
+ •33 
November . 
67 
47 
•378 
2-87 
2-80 
+ •07 
December . 
73 
38 
•282 
1-91 
1-65 
+ •26 
The amplitude of the component of the second order, P 2 , has two clearly marked 
maxima, about the time of the equinoxes, with a principal maximum at midsummer. 
The component of the third order, Pg, varies in a converse manner, having two well 
marked minima at the time of the equinoxes, with a principal maximum at midsummer. 
In the fourth order, P^ appears to combine the characters of the two former, having 
two maxima near the equinoxes, and a principal minimum in the winter. 
