Oscillations of the Horizontal Needle. 3 
are at 10° corresponded to the 130th vibration ; whereas, under 
the glass, the 180th vibration was completely distinguishable be- 
fore the are was reduced to 10°; a slight difference also was ob- 
servable in the time of 100 vibrations, which in the first case was 
somewhat less. 
The room in which the experiments were conducted, was se- 
lected expressly for the purpose, being about 12 feet square, and 
not liable to sensible currents of air from artificial heat ; it had 
a stone pavement, and was not furnished with a fire-place. 
(b.) A magnetic bar, about 5 inches long, and 4th of an inch 
square, being delicately suspended as before, was deflected and 
set free under an exhausted receiver, at an angle of 60° from its 
meridian. The following differences in the rate of vibration were 
observed as the ares of vibration became reduced: they were ac- 
curately ascertamed by means of a valuable chronometer, ex- 
pressly constructed for taking time, and which could measure 
the ;{;th part of a second *. 
When the arc was between 50° and 40°, the rate of vibration 
amounted to 9.5 per minute ; between 40° and 30° it was 9.7 per 
minute ; between 30° and 20°, it increased to 9.75 per minute. 
In an are of about 10° the rate of vibration had increased to 10 
in a minute. 
This experiment is not adverted to as comprising any very 
unexpected result, but merely in illustration of the action of a 
particular magnet, the state of magnetic inquiries being such as 
to render any instance of this kind worthy of notice. 
5. The great advantage likely to attend a method of experi- 
ment which admits of the magnetic oscillations being freed from 
the disturbing influence of thé surrounding air, has led me at va- 
rious times to the construction of instruments calculated to at- 
* I have to acknowledge the obligation I am under to my friend Colonel Hamil- 
ton Smith, for a long use of this valuable instrument. 
A (9) 
AA 
