2 Mr Harris on Magnetic Intensity by the 
8. The identity in principle of a needle vibrating by the di- 
rective force of the earth’s magnetism, with an ordinary com- 
pound pendulum, oscillating by the force of gravity, would lead 
us to infer, that the mere friction and resistance of the air has 
little or no influence on the time in which a given number of vi- 
brations would be performed, supposing every other condition of 
the experiment to be the same, as may be seen in Professor 
Airy’s profound paper on the disturbances of pendulums, forming 
a distinguished portion of the Third Volume of the Cambridge 
Philosophical Transactions. But although the presence of a re- 
sisting medium, such as the atmosphere, may not be thus far pre- 
judicial to the rate of the magnetic oscillation ; yet, in consequence 
of the are of vibration becoming more or less rapidly diminished, 
by occasional changes in the density of the air, or otherwise, in 
consequence of the uniformity of the oscillation being impeded 
by invisible currents, known to take place in it from various causes, 
the presence of such a medium is extremely undesirable, more 
especially when it is considered necessary to vibrate a needle in 
large ares. 
4. The following experimental illustrations of the differences 
which may arise in the times and ares of vibration, in conse- 
quence of the presence of a resisting medium ; as also of the dif- 
ferences in the rate of vibration, arising solely from differences 
in the extent of the arc, may not be here out of place. 
(a.) A magnetic bar about 6 inches in length, jth of an inch 
wide, and 4th of an inch thick, being delicately suspended by a 
silk filament without torsion, and quite free from the disturbing 
influence of metallic substances, was deflected from its meridian, 
and again set free at an angle of 45°. The times and arcs, cor- 
responding to every tenth vibration, were carefully noted, until 
the are of vibration was reduced to 10°. This experiment, 
frequently repeated, both in open space, and when the bar 
was covered by a glass shade, led to the following result: In 
a perfectly open space without the glass shade, the terminal 
