516 MR JOHN ALLAN BROUN ON THE HORIZONTAL FORCE 
this was chiefly instrumental, I have not thought it necessary to give the yearly 
means.* 
8. These results are affected by errors. The alternate increase and diminution 
at the Cape is doubtful, especially the sudden increase after the insertion of a 
new magnet October 1844. We may conclude, however, with considerable cer- 
tainty, that the law of change of the yearly mean force is different at different 
places on the earth’s surface. We may conclude also, especially when the results 
(§ 66) are known, that the yearly mean horizontal force for the whole surface of 
the earth is not a constant quantity. 
9. From these conclusions it follows that the yearly mean change cannot be 
explained wholly by a mere movement of the magnetic poles and of the lines of 
force. As this movement is known to exist, we must conclude that the yearly 
mean change of horizontal force is a complex result, due partly to a movement of 
the magnetic poles, partly to an absolute change of the earth’s magnetic force. 
Monthly Mean Variation of the Horizontal Intensity and Annual Period. 
10. No annual period of horizontal intensity has apparently been proposed as 
yet, carrying with it a sufficient weight to be adopted by physicists as a well- 
determined law. The great errors connected with instrumental changes have 
frequently rendered verification impossible. Erroneous temperature corrections 
also will give false results, even when the observations have been sufficient for 
the determination of the law. Nothing is more probable than to find an apparent 
connection between the maximum and minimum force and the maximum and 
minimum temperature; since, if the temperature coefficient be too great, we shall 
have the observed horizontal force increased in summer; and if it be too little, 
the force will be increased in winter. The error hitherto has been the employ- 
ment of too great a coefficient, and consequent apparent maximum of force in 
summer. 
11. In 1844, having computed the coefficient for the Makerstoun bifilar by the 
process described in the Introduction to the Makerstoun Observations, I obtained 
from the observations for the year 1842, corrected by this coefficient, the 
remarkable result that the horizontal intensity was a maximum near the sol- 
stices and a minimum near the equinoxes. This result, it should be remarked, 
was very unlikely to proceed from an error of coefficient, for the equinoxes (the 
* Copy of Abstracts of Observations made at Simla (lat. 31° 6’ N., long. 5h. 9m. E. of Green- 
wich) under the direction of Colonel Borzzav, is in the Library of the Trevandrum Observatory. I 
haye not been able to discuss these observations satisfactorily, as the temperature of the magnet is 
not given, excepting for 1842. I have, however, taken some pains to arrive at an approximate 
correction, and can state as results that at Simla the horizontal force increased throughout the whole 
period. The curve for 1842 is projected, Plate XXIII.; but the range of temperature in the bifilar 
box is too considerable to allow us to put much value in the conclusion that may be drawn that the 
variations of the monthly means are less than at other places. . 


