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XII.— Results of the Makerstoun Observations, No. I. On the relation of the Varia- 
tions of the Horizontal Intensity of the Earth's Magnetism to the Solar and 
Lunar Periods. With Two Plates. By J. A. Broun, Hsq. Communicated 
by Sir T. M. BrisBane, Bart. 
(Read January 5. 1846.) 
1. Tue following communication is intended to be the first ofa series, in which 
I propose to consider the results of observations made at Makerstoun, near Kelso, 
Roxburghshire, in the Observatory of the President of this Society. These ob- 
servations, and the tabular results, will be found ultimately in the volumes of 
Makerstoun Observations, constituting volumes of the Society’s Transactions. 
2. It has been found convenient to separate the observations of the varying in- 
tensity of the earth’s magnetism into two parts, namely, its resolved components 
in the horizontal and vertical planes. I shall treat at present of the variations 
of the horizontal component. These variations are observed by means of the 
bifilar magnetometer, an instrument devised by M. Gauss, and modified by Dr 
Luoyp, described in the Introduction to the Makerstoun Observations for 1842. 
It consists simply of a magnetic bar, suspended by two silver wires, the latter 
being twisted out of a vertical plane, the magnet is forced from the magnetic me- | 
ridian; the variations of its position afterwards are due to two causes, namely, 
variations of the horizontal component of the earth’s magnetic force, or of the 
moment of free magnetism of the bar; the former are due to changes of the total 
force or of its dip, the ordinary variations of the latter are due to temperature; 
and it is, accordingly, a point of much importance to determine the correction for 
temperature with accuracy, in order that the simple effect of varying intensity 
may be obtained. I have pointed out, in a paper read before this Society last 
session, the imperfections of the method usually adopted for the determination of 
this correction, and the method which has been adopted for the correction of the 
Makerstoun Observations. I shall afterwards exhibit an example of the very dif- 
ferent results to be deduced, after correcting observations by the two methods (11). 
3. The horizontal force varies throughout the solar day, having, in that period, 
two maxima andtwo minima. The hours of the principal maximum and minimum 
were first pointed out by M. Hansrren, but I am not aware of the first determina- 
tions of the secondary points. Inthe years 1844 and 1845, observations were made 
at Makerstoun every hour excepting on Sundays. (See Curve, No. 1, Plate III.) 
From the means of the whole observations for the year 1844, the principal mini- 
mum occurs about 20” past 10 a.m. (Makerstoun mean solar time is used through- 
_ out), or exactly when the sun is on the magnetic meridian of Makerstoun; the 
force then increases rapidly till between 3 and 4 p.m., when there is a slight in- 
flexion; again it increases with its previous rapidity till about 5} p.m., when the 
VOL. XVI. PART II. 20 
