338 DR. C. CHREE; AXALYSIS OE RESULTS EROM THE KEW MAC4XETOGRAPHS 
D = declination, 
I = inclination, 
H = liorizontal force, 
V = vertical force. 
N = northerly component of H, 
W = Ave.sterly ., 
T = total force (resnltant of H and V, or of X, 
AY and Y). 
§ 4. Taking first the case of D, as simplest, we have the scale value—that is the 
equivalent of 1 centim. of curve ordinate—de23endent only on the distance between 
the photographic paper and the mirror carried by tlie declination magnet. There is 
no occasion to move this mirror except sliglitly in azimuth, as the magnet turns with 
secular change, and no appreciable change of scale value has occurred during the 
period dealt with. In this case no temperature compensation is necessary, so far as is 
known; and the scale value being determined once for all, there remains only to 
determine the value of the declination which answers to the base (or time) line, from 
which ordinates are measured. This is dojie by comparing the results of the absolute 
observations, taken usually three or four times a montii, with the corresponding curve 
ordinates; each month is dealt with by itself With the exception of a few months 
early in 1890, the same magnet has been used throughout for the absolute observations, 
and an elaborate intercomparison of it with the magnet previously accepted as the 
standard, before the latter was laid aside, gave mean results as nearly as possible 
identical. The magnetometer employed has been in use for about half a century. 
Certain additions were made to it in January, 1891, but were discarded after a few 
weeks’ trial, and the results oljtaiiied with it during these weeks were rejected. 
Coming next to H, the scale value here is affected by change either in the 
magnetograph magnet itself or in its bifilar susjDension. During the whole eleven years 
the aim has been to keep the scale value at 1 centim. = 50y (where ly = 1 X 10“^ 
C.G.S. unit). The magnet is an old one, whose moment alters extremely slowly, and 
when the instrument is left to itself, the change of scale value taking place in twelve 
months seldom amounts to 1 per cent. As in the case of D, the value of the base line 
is determined l^y comparing absolute values of H, determined three or four times 
a montli, wdth the coiTespondiiig curve ordinates, and each mouth is treated 
independently. A temperature correction is applied, which allows for the difference 
between the temperature of the magnetograph room at the hour considered and at the 
times of tlie absolute determinations of H in the month in question. The temperature 
coefficient of the H magnetograpli is very small, and the correction comparatively 
unimportant. 
The procedure in the case of Y and I is less direct. Here the absolute observation 
gives I, while the curve gives A’^. The value of 1 centim. of ordinate is found as in 
the case of H by direct experiment, and the instrument is adjusted—usually once a 
year—so as to keep the sensitiveness as close as possible to 1 centim. = 5Uy. Three 
or four absolute observations of I are made each month, usually on the days of the 
observations of H. Die H observation is corrected to the time of the corresponding 
