39 



Janxuiry 6, 1834. 



Sir T. M. BKISBANE, President, in the Chair. 



The following Communications were read : — 



1. On the Investigation of Magnetic Intensity, by the Oscil- 

 lations of a Horizontal Xeedle. By "William Snow Harris, 

 F.E.S. 



The chief disturbing causes by -which the magnetic intensity, as 

 ascertained by the oscillations of the horizontal needle, are effected, are 

 1. Variations in the air in which they are performed j 2. The influ- 

 ence of changes in the mechanical conditions incidental to the mode 

 of suspending the needle ; 3. Changes in the disposition and intensity 

 of the magnetism of the needle from heat and other causes. 



These causes of disturbance the author proceeded to investigate. 



I. He compared the oscillations of the needle vibrating in air, 

 with those of the same needle oscillating in vacuo ; and he minutely 

 described the apparatus which he had contrived for allowing the 

 needle to vibrate freely in an exhausted receiver, and his mode of 

 determining the arcs of vibration. This apparatus enabled him to 

 appreciate the resistance of air to the oscillations of the needle, and 

 its effect in rendering iineqiuil the duration of vibrations performed 

 in long and in short arcs. Hence he inferred the impossibiUty of 

 ascertaining the alleged diurnal changes of magnetic intensity by the 

 common apparatus. 



II. The second source of disturbance he endeavoured to obviate by 

 a more accurate mode of suspending the needle ; by which its centre 

 of gravity and point of magnetic neutrality should be made to coin- 

 cide. This the author proposes to accomplish by greater care in 

 finding its true centre, and in adjusting its horizontality by means 

 of small sliding counterpoises of platinum on each arm. 



III. The influence of increase of temperature on the magnetic 

 needle has generally been considered as loicering the tension of its 

 magnetism ) and it has been represented as again restored by cold : 

 but the author's experiments seemed to prove the contrary, when the 

 comparative experiments were made in vacuo. He considers, how- 

 ever, that if the needle be prepared, by being previously exposed to 

 a variation of temperature from 212° to 0° of Fahrenheit, its tension 

 will not afterwards be affected by ranges of temperature within these 

 limits. 



One of the most interesting parts of Mr Harris's paper is his mode 

 of determining changes of magnetic tension in a particular magnet. 



