2 Captain Sabine’s experiments to determine the 
Many of these, indeed, are not difficult of detection, and 
admit of compensation by certain known methods of observ- 
ing; such are those which are caused by incorrect graduation, 
by the eccentricity of the needle with respect to the divided 
circle, by the agate planes which support the axis of the 
needle not being truly horizontal, or by their non-coincidence 
with the right line joining the zeros of the circle. The errors 
to which even the most careful determinations are subject, are 
to be referred chiefly to faults in the construction of the 
needle itself; ist, to imperfection in the axis, whereby the 
needle, on being made to oscillate, will not return with 
certainty in repeated trials to the same division of the limb ; 
and, 2ndly, to the difficulty which even the most skilful ar- 
tists experience, in the endeavour to make the axis of motion 
pass through the centre of gravity; a condition which is 
essential to accuracy in the usual mode of observation, but of 
which it may be safely said, that its accomplishment admits 
of no very certain proof, and that it is rarely or never suc- 
ceeded in. 
It is obvious that a needle, of which the balance is not 
thus correctly adjusted, will not assume, on being suspended 
freely in the plane of the meridian, the direction which mag- 
netism alone would have imparted to it, and that it will, in 
consequence, err sensibly from the true dip ; the remedy 
which has been recommended for this inconvenience, and 
which has become the usual practice, is to reverse the poles 
of the needle, and to take the arithmetical mean of the arcs 
indicated in four positions, as the true magnetic dip. The 
sanction of this method by persons who are regarded as au- 
thorities, is a sufficient indication, that observations of the dip 
