538 MR JOHN ALLAN BROUN ON THE HORIZONTAL FORCE 
Variations of the Daily Means. 

65. The daily means for four places in -24,, of the value of X at each place, will 
be found for the years 1844 and 1845, in Table XX XIII, p. 550.* In considering 
the daily means of horizontal force, we shall perhaps derive the most general in- 
formation from an examination of the projections in Plate XX VII. It should be 
remarked jirst, that the daily means, commencing each week at any place, are not 
strictly comparable with the means for the corresponding days at other places; 
since the means are for the Gottingen astronomical day (0" to 23"), one mean in 
each week is made up partly of observations on Saturday afternoon, and partly of 
observations on Monday morning; and as Saturday ends and Monday begins at 
different hours (Gottingen) on different meridians, the hourly observations forming 
the day, made up of these two parts, are not simultaneous at the different places. 
It should be remarked, second, that the means of 24 hourly observations, are not 
in all cases enough to give the true daily mean force; and this is especially true 
for days of disturbance in high latitudes. 
66. The conclusions deducible from Plate XX VII. are as follow :— 
1st, The movements from day to day resemble each other generally, at all the 
places; and this whether we consider the amount or direction of movement. 
2d, In certain seasons there is the appearance of a period of about 26 to 30 
days, equally well marked, or nearly so, at all the stations. I shall proceed to 
examine these conclusions more minutely. 
67. From the first it follows, that the daily mean horizontal intensity increases 
at the same time at all the stations, and diminishes at the same time at all the 
stations; so that, if this holds for all the points on the earth’s surface (as it does 
for all the points on which observatories have been placed), we may conclude 
that the intensity of the magnetism of the whole earth is variable, increasing or 
diminishing from day to day. 
68. The only marked cases in which the direction of movement in the curves, 
Plate XX VITI., is different at one place from that at any other, are those of the 
daily means for April 17 and June 28, 1844, which show an increase at Makerstoun, 
while there is a diminution at all the other places on the first date, and little or 
no movement on the second; all the other differences are of the smallest kind, 
and generally within the amount of errors of correction, or of means, when dis- 
turbances have existed. The increase at Makerstoun, April 17, 1844, which was 
not experienced in the places farther south, was due solely to an increase of hori- 
zontal force during the disturbance, from 17° 1° to 6" (Gottingen mean time) at 
* The value of the unit coefficient of the bifilar magnetometers at Makerstoun and Trevandrum 
was very nearly k=0-000140. As the results in this paper were first deduced from the observations 
at these two places, the same value of the unit was adopted for the other places in these tables. 

