AND ANNUAL VARIATION OF THE TERRESTRIAL MAGNETIC FORCE. 205 
Lefroy, R.A., F.R.S., for the purpose of illustrating the objects which I have in view. 
I might also have availed myself of similar observations made at the observatory at 
Hobarton in Van Diemen Island, under the direction of Commander Kay, R.N., 
F.R.S., but for the fear of encumbering this communication with too much detail. 
The magnets employed for the monthly series of observations on the absolute hori- 
zontal force at Toronto are solid cylinders of 0'3 inch diameter; the suspension 
magnet being 3 inches and the deflecting magnet 3‘67 inches in length. The same 
magnets have been used throughout the series. The observations are made about the 
same period in every month, and are extended over three days, usually the 16th, 17th 
and 18th of the month. Three distances are employed, the least being 1 foot, and 
the greatest 1 foot and four-tenths from the centre of the suspended magnet. The de- 
flections are read on a circle of 6 inches diameter, having two verniers reading to 20". 
The deflections vary, according to the distance, from 6° to 10°. The reading telescope 
is attached to and moves with the azimuth circle ; the deflecting magnet is therefore 
always perpendicular to the suspended magnet when the deflections caused by the 
latter are read off. The deflecting magnet is suspended for vibration in a stirrup with 
a mirror, in a detached wooden box, by a silk thread, of which the line of detorsion is 
brought approximately into the magnetic meridian. The time of vibration is deter- 
mined by the mean of 300 vibrations in very small arcs, the commencing arc being 
always the same, i. e. 50', and a correction for the arc is applied. Actual changes in 
the horizontal force of the earth occurring between the two parts of the experiment, 
i. e. between the experiments of deflection and vibration, are eliminated by a correc- 
tion derived from the Observatory Bifilar, which is read off concurrently with the de- 
flections and vibrations, and both are reduced by this means to what they would have 
been, had the horizontal force, at the time of each observation, coincided with the 
mean horizontal force shown by the Bifilar on the same day. The reduction might 
obviously have been made with equal convenience to the mean reading of the Bifilar 
in the month-, but this reduction would have involved the question of the dependence 
to be placed on the Bifilar itself for longer periods than for a few hours ; and as the 
absolute determinations extend in every month over three days, I have preferred to 
keep their results independent of this reduction, except in two instances, viz. October 
and November 1848, when disturbances of unusual magnitude and continuance less- 
ened the force on the days of observation to a more than ordinary degree. In those 
two instances therefore the results of the absolute determinations have been reduced 
to the mean reading of the Bifilar in the respective months, instead of its reading on 
the respective days of observation. 
Differences of temperature occurring in the experiments of deflection and vibration 
have been eliminated by a temperature correction applied to the deflecting magnet, 
in which the coefficient has been very carefully determined by the usual process 
directed in the Revised Instructions of the Royal Society. 
The constant, depending upon the value of the moment of inertia of the deflecting 
