Royal Institution of Great Britain. 



557 



French philosopher, he may be said to have weighed the earth in 

 this delicate balance. 



But of all substances glass is the most perfectly elastic, and 

 from the facility with which it can be drawn into threads of 

 any degree of fineness, Mr. Ritchie prefers it to wires in all 

 kinds of torsion balances. He showed the application of this 

 property to a torsion electrometer, to a magnetometer, galva- 

 nometer, and torsion balance ; but as the most important of these 

 applications are already described in the Transactions of the Royal 

 Society, and at page 29 of the present volume, we shall refer the 

 reader to these works for more complete information. 



Towards the end of the lecture, Mr. Ritchie applied the force of 

 torsion to demonstrate experimentally the two following propositions : 

 1st. If a magnetic needle or pendulum be deflected from its 

 state of rest, the force with which it endeavours to return to its 

 former position is proportional to the sine of the arc or angle of 

 deflection. 



Let E B F be a vertical cir- 

 cle, and let C B be a small 

 wooden pendulum, turning free- 

 ly on an axis C. 



Let one end of a glass thread 

 six or eight feet long be at- 

 tached to the axes of the pen- 

 dulum, and the other end fixed 

 to a torsion key, as in the tor- 

 sion balance. Turn round the 

 key, and observe the degrees 

 of torsion which the thread has 

 undergone in raising the pen- 

 dulum to different heights, and it will be found that these degrees 

 are directly proportional to the smes of the arcs D K, G B, E B, 

 through which the pendulum has been raised. If, for example, it 

 requires 300 of torsion to raise the pendulum through an arc D B 

 of 30, it will require 600 of torsion to raise it to 90, or to bring it 

 to a horizontal position, the sine of 30, being half the sine of 90. 



The second proposition, to which the property was applied, is 

 the following: If a pendulum be made to oscillate, the forces which 

 cause it to oscillate are inversely as the squares of the number of 

 oscillations performed in the same time. This proposition was 

 experimentally demonstrated by ascertaining the relative strength 

 of two threads of glass, and then suspending them from a fixed 

 point, and attaching a small horizontal pendulum to the lower end, 

 which was then turned round, and allowed to vibrate by the elastic 

 force of the glass threads. The squares of the oscillations per- 

 formed in the same time being inversely as the elastic forces of the 

 threads employed. 



In the library, were a specimen of the platycercus unicula, or 



