Oscillations of the Horizontal Needle. 3 



arc at 1 0° corresponded to the 1 30th vibration ; whereas, under 

 the glass, the 180th vibration was completely distinguishable be- 

 fore the arc 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 1 9, 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 ^th 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 arcs of vibration became reduced : they were ac- 

 curately ascertained by means of a valuable chronometer, ex- 

 pressly constructed for taking time, and which could measure 

 the 7-V tn 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 arc 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 the 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- 

 se of 



A 2 



ton Smith, for a long use of this valuable instrument 



