ON “QUIET” DAYS DUEING THE ELEVEN YEAES 1890 TO 1900, ETC. 
381 
where t represent G.M.T. counted from the first midnight of the day, an h.our l)eing 
taken as equivalent to 15°; whilst aj, 6^, c^, . . . are constants. Between the 
constants of the two expressions there exist the relations 
a,, = tan~^ — ^/a;^ + />/. 
The constants with suffix n occur in terms in which the period is '2i/n hours. The 
periods of the terms in the above expressions are respectively 24, 12, 8, G hours. 
In a previous paper dealing with Kew data I explained that terms with periods 
shorter than 6 hours were so small as to be of little account, and similar conclusions 
have been reached elsewhere. Tims attention will he confined to the first four 
pei'iods. The values of a,„ have been found by calculation from the well known 
formulse; the values so obtained are, it should he noticed, independent of whether 
tei'ms of the higher orders are neglected or not. 
The data employed in the calculations refer to G.M.T. If results are wanted 
relating to local solar time, all that is necessary is the application of a table of 
corrections to the values found for the angles a^, a^, &c. In the case of aj, the 
correction simply represents the angular equivalents of the equation of time and the 
longitude of the place of observation. The corrections to ao, ccg, and are obtained 
by multiplying those for by 2, 3, and 4 respectively. The corrections for the 
individual months and seasons of the year applicable to Kew are given in Table XIX. 
They represent the mean of the results given in the ‘ Greenwich Magnetical and 
Meteorological Observations’ for tlie four years 1892 to 1895, with the addition 
required to allow for the longitude of Kew, viz., 19' W. The corrections to — 
omitting of course the 19'—are in close agreement with cori'esponding results given 
by General Strachey,* tliere being sensiljle differences only in August and Septenil)er. 
In these months, however, they are in good agreement with results given more 
recently by Angot.I 
'fhere are probably for a mean year errors of 1' in the values of in the table, and 
these of course multiply up for a.,, a.^, and ; but tlie results are probably sufficiently 
exact for all practical purposes. 
For the seasons, tlie use of local solar time fortunately makes but little difference; 
but in several montlis, notably February and November, the distinction between 
local solar time and G.M.T. is very appreciable. 
* ‘Phil. Trans.’ A, for 1893, p. 646. 
t ‘ Ann. du Bureau Central Meteorologique de France,’ 1899, “Memoires,” p. B. 98. 
