6 Mr Harris on Magnetic Intensity by the 
per card np isa narrow ring, about half an inch wide, and admits 
of the bar vibrating within it. It can be altogether removed if 
required. 
The bar has fine index wires, ii, Fig. 4, slightly held in shal- 
low notches at each end. These indicate the precise degree on the 
graduated cards, so that very great precision may be arrived at, 
if required, in noting the are of vibration. 
It may be readily perceived, that, in the above mechanical ar- 
rangements, there is no considerable mass of metal in the vicinity 
of the needle, which can disturb its state of oscillation, the parts 
being constructed, for the most part, of non-metallic bodies, 
whilst every possible facility is obtained, requisite to carry on the 
experiments, free from the disturbing influence of the atmo- 
sphere. 
11. An extensive series of experiments, with magnetic bars 
and needles, vibrating in air, and in an exhausted receiver, 
under various conditions, has led me to conclude, that we cannot 
really depend upon observations embracing extremely minute 
differences in the rate of vibration, as applicable to terrestrial in- 
tensity, until the usual instruments of research have been so per- 
fected, as to place such differences beyond the operation of other 
causes, foreign to the object of experiment, (a) (b). Indeed, it 
will be readily admitted, on due reflection, that, considering the 
vast object to be attained, no less a one than an accurate mea- 
surement of the magnetism of the Earth, the same kind of prac- 
tical perfection is requisite, and should be continually sought in 
the construction of the magnetic pendulum, as is obtained in that 
of the ordinary pendulum, oscillating by the force of gravity, 
without which we can never hope to arrive at results worthy of 
confidence. Do we require the rate of oscillation, such as to de- 
tect extremelysmall differences in terrestrial intensity ? A given 
number of oscillations, which admit of being considered isochro- 
nous, is absolutely requisite. Now, this can be readily effected 
with the instrument above described, Fig. 2, by observing the vi- 
