4:13 
OX “QUIET” DAYS DUEIXG THE ELEVEN YEARS 1890 TO 1900, ETC. 
uot exclusive source of the luagnetic diurnal inequality to be external to the Earth 
I must confess, however, that whilst I consider the conclusion, which has been arrived 
at by other physicists from other points of view, to be pro1)ably in the main correct, I 
think that the investigation ought to he repeated when practicable for at least two 
different groups of years, one representing numerous, the other few sun-spots ; and 
trustworthy data from at least ten or twelve well distributed observatories 
would be highly desirable, if not absolutely necessary. Meantime, 1 suspect, the 
vertical force is likely to prove a stumbling block. 
Nature of the Bdatiomhf between Smi-Spots and Terrestrial Magnetism. 
§71. It was pointed out in the Preliminary Note” that Wolfer’s table of 
sun-spot frecjuencms is liased on all days for which solar oliservations exist, whereas 
the Kew magnetic data refer to five days a inontli only. If one divided the days 
of a month for which sun-spot data exist into two equal groups, the one containing 
the days of the lowest frequency, and if from this group one selected five days at 
random, the mean frequency for these days would often he hut a small fraction of 
Wolfer’s own mean for the month. Thus ifi as has frecpiently been suggested 
sun-spots were the immediate cause of magnetic disturbances, the disturbance beina 
synchronous with the spot, the presumption would lie that the sun-spot frequency for 
all and for quiet days would be altogether different, the latter being mucli the smaller. 
As explained in the “ Note,” data which Wolfer does publish for individual davs 
were not used for the following reasons; (1) Ijeing onlv provisional they a^e 
presumably less trustworthy than the finally accepted monthly values appearing in 
fable I; (2) not infrequently data were wanting for several of the selected quiet dtys. 
I liave, howeyer, looked careftilly into the matter, to see wl,ether the results here 
arrived at would hare been materially modified if sun-spot frequencies Iiad been taken 
rom qxmt days only. For this purpose, frequencies were calculated for each month 
using Wolfer’s provisional values for the selected quiet days. These are comiiared 
in Table XLIX. with the corresponding data derived by Wolfer from his provibral 
values for all days. In forming the means I have left out of account three months 
when tliere were less than'tv'o quiet days. 
Table XLIX. ^Wolfer’s Provisional Sun-8pot Frequencies. 
All 
clays. 
Quiet 
flays. 
All 
days. 
Quiet 
days. 
1 
All 
days. 
Quiet 
day.s. 
Jamiary . . 
Eebniaiy. 
March 
April . . . 
40-1 
40- 4 
35-6 
41- 2 
47-6 
42-0 
.30-0 
;l9-8 
May . . . 
June . . . 
July . . . 
August . . 
43-.5 
4.5-3 
45-4 
45-7 
42-2 
43 • 3 
42-3 
47-4 
1 
i 
Septemlier . 
October . 
Xoyember . 
Decendier . 
45-6 
41 -6 
32-4 
35 • G 
45 • 7 
41-1 
35-1 
37-3 , 
VOL. CCII.—A. o - 
