AND ANNUAL VARIATION OF THE TERRESTRIAL MAGNETIC FORCE. 203 
been by no means found to be the case with the magnets employed in the different 
Bifilars of the Colonial Observatories, whose loss of magnetism appears to be subject 
to no general or systematic law, and even occasionally to intermit and to recommence 
without' any apparent or discernible cause. To eliminate the effect of the loss, the 
magnetic moment of the bar would require therefore to be examined from time to 
time, and at short intervals ; but the removal of the magnet for this purpose would 
break the connexion and thus interrupt the continuity of the series. 
The source of the other irregularity in the indications of the Bifilar, and which 
does not appear to have been anticipated, is still somewhat obscure : the effect is of 
the opposite character to that of the loss of magnetism in the magnet bar ; and the 
position of equilibrium of the bar has been in more than one instance so much 
affected by it, that the bar has in a few months progressively passed out of the field 
of view of the telescope. The position of equilibrium is determined on the one side 
by the two variables, the magnetism of the earth and that of the bar ; and on 
the other side by the supposed constants, the weight of the bar, the length and 
distance between the parallel suspension wires, and the angle of torsion. The weight 
of the bar cannot alter ; the angle of torsion and the distance between the wires, ob- 
served before and after the change in the instances referred to, were ascertained to 
have undergone no alteration. A lengthening of the suspension wire has been there- 
fore supposed to be the cause of the irregularity in question, the effect being pre- 
cisely that which would be produced by an elongation of the wire ; but no direct 
proof of this has yet been obtained, because when the Bifilars were first adjusted no 
such effect was anticipated, and the exact length of the suspension wire at the period 
of the first adjustment appears to have been in no case measured*. 
The Bifilar being thus found to be affected by two sources of instrumental irregu- 
larity opposite in character, neither of which admitted of being satisfactorily elimi- 
nated, the comparability of the differential results obtained with that instrument, 
constructed according to the specifications in the Instruetions of the Royal Society, 
could only be relied on for short intervals, and they would consequently admit of 
no certain inference being drawn from them in respect to secular changes. 
The observations made during the first years of the Colonial Observatories having 
thus failed in accomplishing a very important, if not the most important, part of the 
objects contemplated, I requested Captain Riddell, who was at that time my assist- 
ant in the superintendence of the British Colonial Magnetic Observatories, to make 
* The Toronto Bifilar was adjusted on the 25th February 1843, with the intention that it should remain un- 
disturbed. Between this date and the 11th of October of the same year the reading of the scale had altered 
470 scale divisions, equivalent (approximately) to '044 parts of the force. At Hobarton the Bifilar was ad- 
justed, with the intention of its being left undisturbed, on the 16th of July, 1843. On the 1st of March, 
1846, the scale reading had altered 78 scale divisions, equivalent (approximately) to '017 parts of the force. 
The alterations in both cases are in the opposite direction to what would be occasioned by a loss of force in 
the magnet bar, and must be regarded as the excess of one instrumental irregularity over a second, the real 
value of either being unknown. 
2 D 2 
