ON “QUIET” DAYS DURING THE ELEVEN YEARS 1890 TO 1900, PHC. 
357 
than I had feared. Influenced by this investigation, I accepted the data for 18D0 
to 189G as published in the annual ‘ Reports,’ and all the tables of diurnal inequalities, 
and numerous others, were originally calculated on this basis. The results so obtained 
were utilised in the “ Preliminary Note on the Relationships between Sun-spots and 
Terrestrial Magnetism,” published in the ‘ Roy. Soc. Proc.,’ vol. 71, p. 221. Subse¬ 
quently I noticed that the ranges in the mean annual inequalities for V for 1898 
and 1899 appeared larger relative to the ranges in earlier years than one would have 
anticipated from the relationships I had observed between Sun-spots and Teri’estrial 
Magnetism, and I had little difficulty in tracing the cause to the neglect of the 
temperature correction in the earlier years. Owing to the smallness of the tempe¬ 
rature coefficient in the H magnetograph, the absence of a temperature correction in 
the data for years prior to 1897 is of little importance. It became obvious, however, 
that the earlier ^mai's’ data for Y must he corrected or dispensed with. The correction 
has entailed a great amount of labour, involving the complete recalculation of all the 
diurnal inequalities for Y and I and of the corresponding Fourier coefficients, &c. 
The thermograph records from 1897 to 1901 gave smooth diurnal inequalities of 
temperature for each month of the year. The range varied largely with the season, 
being much less at midwinter than at midsummer, but the hours of maximum and 
minimum and the general features were nearly independent of the time of year. The 
magnetograph room and its environment remained unchanged from 1890 to 1901, 
and the readings from the mercury thermometer throughout the period cojiflrmed 
the view that if the mean diurnal inequalities of temperature obtained for the 
several months of the year since 1897 were assumed to apply to years prior to 1897, 
satisfactory average corrections would he obtained. This accordingly has been done, 
and I do not think that the results now published are aflected by any serious source 
of uncertainty. 
§ 20. Diurnal inequalities for D, I, II, A", N, W, and T appear iii Tables XI. 
to XA"II. In the case of the first four elements inequalities are given for each 
month of the year. The values ascribed to any one month, say January, were 
obtained by adding together the hourly values from eacli January of the period 
included, applying a non-cyclic correction to the sums thus formed, and then taking 
the arithmetic means. The annual ine(|uality for the year was obtained by summing 
up the mean hourly values for the 12 months—as corrected for non-cyclic eflect—and 
taking their means. In tlie case of D and H mean ineipialities are given, not merelv 
for the whole 11 years, but for two combinations of years, viz., 1890, 1891), and 1900, 
years of few sun-spots, and 1892 to 1895, years of great sun-spot frequency. 
The inequalities for N, W, and T were derived by means of the formulae :— 
SN 3= cos D SH - H smD 8D, 
oW = sin D SH + H cosD SD, 
8T COS I 8H + sini 8Ah 
