OF TERRESTRIAL MAGNETIC FORCE IN THE HORIZONTAL PLANE. 681 
curves traced on the photographic paper which is wrapped upon the barrel; and 
the inconvenience produced by this doubling was soon found to be so great that 
I thought it necessary to alter the clock-work so as to produce a revolution of the 
barrel in twenty-four hours. The records of the change of western declination 
from the north, and of the change of horizontal force, are made on the same barrel ; 
and by alterations, first suggested by myself about 1881 , and carried out by the 
present Astronomer Boyal (then Chief Assistant), the two curves are now so traced 
that the simultaneous records of the two instruments at all times are in close 
juxtaposition. 
While the observations were made by eye, at every two hours, the mean of the 
two-hourly readings was adopted as base for the day, and the excess of each 
two-hourly reading above the mean was adopted as “ magnetic inequality ” of that 
ordinate for that hour ; producing twelve measures of “ inequality ” for each day. 
When the photographic system was introduced, the elevation of a pencil curve drawm 
by eye so as to smooth down the irregularities of the photographic trace above a 
photographic base was measured for every hour, producing twenty-four measures 
of “inequality.” 
In the instances of excessive and rapid disturbances of the magnets during 
magnetic storms, no measures of ordinates were taken for the present purposes. 
Thus the daily measures at each hour or two hours were obtained. 
The next step was to collect for each month all the daily measures on corresponding 
hours through each month, and to take their mean. These are the measures for 
the hours which are actually treated in the present memoir. By combining (for 
each month) the inequality of magnetic horizontal force at every two hours or each 
hour, as abscissa, with the inequality of magnetic declination (on the same scale 
of measure) at the same two hours or hour, as ordinate, points were defined in 
every monthly curve representing completely the mean diurnal changes of magnetism 
for each month. On the recommendation of the Board of Visitors of the Boyal 
Observatory, reduced photographic copies of these curves were prepared by the 
Astronomer Boyal for publication with the volume of Greenwich Observations for 
1884. 
The number, and the character, of the curves produced uninterruptedly on this 
plan, and the circumstance that they are intended for publication in the Greenwich 
Observations, appear to render them unfit for dissemination in the Boyal Society’s 
