ON “QUIET” DAYS DUKING THE ELEVEN YEAKS 1890 TO 1900, ETC. 
347 
Agaiu, the annual inequality, when such exists, implies an increment or decrement 
in the course of 24 hours, varying with the time of year; and if Table IV. may be 
trusted, this increment at certain seasons sliould be consideral)ly larger than that due 
to the secular change. But even the most pi'ominent monthly change in tlie table, 
viz., from 2'Iy to 14Ay in the case of V, would represent an increment of only about 
0‘4y in a day. Any increment, of course, due to annual inequality would change in 
sign with the season, and would cut out for the year as a whole. 
There are other known causes tending to introduce a difference between the values of 
the magnetogram ordinates answering to the beginning and end of a day. There aie 
the irregular movements visible in most curves, especially at times of sun-spot 
maximum, and imperfections in the temperature corrections applied, at least in tlie 
case of V. These two last mentioned sources v-ould, liow^ever, only introduce iri'egu- 
larities into the monthly values of the non-cyclic effect, without appreciably affecting 
the mean for the year. 
A more troublesome source of non-cyclic effect is instrumental change. In the case 
of D there does not seem to he any appreciable nncertainty on this liead. This might 
he inferred from tlie nature of the declination magnetogi'aph, l)ut it is at least 
desirable to have observational confiimation. During the eleven years dealt with, 
the magnet mirror had been re-adjusted only once, viz., in 1808, when its azimuth 
was altered so as to increase the curve ordinate by the equivalent of about 36' of 
declination. Allowing for this, the curve ordinate had diminislied l)y the equivalent 
of about 53', between tlie taking of absolute observations in January, 1800, and 
December, f 000, which showed an actual deciease of 63 m I). 1 his leaves only 
about 10'of shift unaccounted for by true secular change. S})read over 11 years, 
this would represent an e.xceedingly small non-cyclic effect. In the case of the hifilar 
magnetograph also, only one slight artificial alteration had been made in the distance 
between the trace and the liase line. Allowing for this, the curve ordinate increased 
by about 65y, whilst H really increased by about 280-)/. The deficiency 215y repre¬ 
sents presumably in the main decrease in the moment of the hifilar magnet. Spread 
over 11 years, this represents a mean non-cyclic effect of aliont 0’05y per day. This, 
as we shall see, is only about a sixtieth of the average lujii-cyclic effect actually 
observed on (julet days. If we combine the effects due to true secular change and 
instrumental causes, we have a mean non-cyclic effect of only about 0'02y per day. 
This is in the same direction as the non-cyclic effect actually observed in hi, hut is 
wholly insignificant. 
Ilesnlts confirmatory of our conclusion as to the smallness of the non-cyclic effect 
in H, due to secular change and instrumental causes comllined, were obtained from 
measurements of the curve ordinates at the heginning’ and end of a series of individual 
o O 
years. 
In the case of V, the magnet mirror had been altered seven or eight times in the 
course of 11 years, usually early in January, at the time of the scale determinations. 
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