88 Lovering on Magnetic Observations at Cambridge. 
ponent of the intensity leave us nothing to desire, their determi- 
nation being now reduced to a degree of precision, hardly (if at 
all) inferior to that of astronomical measurements. ‘The same 
thing, however, cannot be said respecting the third element, as 
hitherto observed. In the Dublin Magnetical Observatory, and in 
the Observatories since established by order of the Government 
and of the East India Company upon the same plan, the third 
element chosen for observation has been the vertical component 
of the intensity, the instrument for the measurement of which 
has been already submitted to the notice of the Academy. The 
principle of this instrument, it will be remembered, is to balance 
the vertical component of the magnetic force by a fixed weight, 
and to observe the changes of the position of equilibrium, under 
the action of the changing force. Unexceptionable as this princi- 
ple is in theory, the accuracy of the results has not been com- 
mensurate with that of the other two instruments. This inferiority 
is to be traced to the large influence which the unavoidable 
errors of workmanship must necessarily have on the position of 
equilibrium of a magnet supported on a fixed axle. It has been 
shown, that the effect of magnetizing a bar, under the most ad- 
vantageous circumstances of form, and at the part of the globe 
where the vertical component of the magnetic force is greatest, is 
the same (as to its position of equilibrium) as if its centre of 
gravity had been transferred about the goth of an inch towards 
the north end; so that the moment of the force, exerted by the 
vertical component of the earth’s magnetism, can never exceed 
this small quantity multiplied by the weight of the bar. Now, in 
order to render the results of this instrument comparable to those of 
the Horizontal-Force Magnetometer, it should enable us to meas- 
ure changes of the vertical force, amounting to the roo,soodth 
