xxviii INTRODUCTION TO THE MAKERSTOUN OBSERVATIONS, 1845 AnD 1846. 
whence 
PEO TIMOAL TG) 
eS ree) i 
If the deflecting bar be now employed to deflect a freely-suspended unifilar magnet, 
in order to determine the value of = as in the ordinary observations for absolute 
horizontal intensity ; w being the angle of deflection for a distance 7, we have 
2M 
—— = a tan wu 
De ah 
142444 
aared 
Tey 
If the bifilar and unifilar bars are of the same dimensions p and q, which are quan- 
tities depending upon the distribution of the magnetism in the bars, may be consi- 
dered equal to p, and q, and if the deflections for both bars be made at the same 
distances, or r=7, then 
peice iy 
XS > 
and ASS (3.) 
If, however, the bifilar and unifilar magnets are of different dimensions, the value 
2M 
x 
p, and q being eliminated ; that value being substituted in equation (2.), and deflec- 
tions of the bifilar being obtained for different values of r, p and q also may be 
eliminated. 
of 
should be obtained from the deflections of the unifilar at different distances, 
33. Wooden beams having been placed in the prolongations of the bifilar mag- 
net, and at right angles to these, lines were drawn upon them, passing through the 
centre of the magnet,—one in the magnetic meridian, the other at right angles to 
it; several distances from the centre of the suspended magnet were marked off on 
each side with a beam compass; a similar structure was erected for the declinome- 
ter. The following observations were then made :— 
34. 1st, A cylindrical magnet, 3-65 inches long, was employed to deflect the bifilar 
and declinometer magnets; these two magnets are of the same dimensions, 15 inches 
long, and were obtained at the same time from the same maker. The short de- 
flecting bar was placed at different distances to the east, and at the same distances 
to the west, of the bifilar bar, and the deflections of the bifilar were observed in 
seale divisions. Observations of deflection of the declinometer magnet were then 
obtained with the same deflector—the deflector, however, being placed at the same 
distances, as in the other case, to the north and south of the declinometer magnet : 
in both cases, the prolongation of the suspended bar, in its normal position, passes 
through the centre of the deflector. The results are obtained in the Ist portion 
of Table 3. 
