Royal Institution of Great Britain. 551 



free, and without being directly opposed to any substance capable of 

 acting on it; the extremities of the bar are furnished with two 

 light indexes of gold wire, which indicate the arc of vibration on 

 the ring above, and the whole is enclosed in a good pneumatic void, 

 on a pump plate of fine slate. The entire instrument is made up of 

 non-metallic bodies, with the exception of the exhausting tubes, and 

 which are away from the influence of the vibrating magnet. The mag- 

 netic bar is drawn aside, and liberated at any given angle from its 

 meridian, by means of a double stop of light brass wire, attached at 

 right angles to a vertical rod, moveable in an air-tight collar, through 

 the middle of the pump plate ; and which, being inconsiderable as to 

 mass near the magnetic centre of the bar, and otherwise at a con- 

 siderable distance when the bar is set free, do not operate in dis- 

 turbing the result of the experiment. By means of this double stop, 

 the bar may be brought completely to a state of rest ; so that when 

 set free it vibrates steadily and without a swinging motion. 



In applying this instrument to determine the magnetic energies 

 of non-ferruginous bodies, Mr. Harris observed, that it was im- 

 possible to deduce the comparative values of these energies, from 

 the mere number of oscillations made in a given arc, under the 

 influence of these substances, since the result is compounded of 

 the retarding force of the given body under examination, and that 

 retarding force by which the bar would be brought to rest, when 

 oscillating in free space. To get the former force, he divides the 

 number of oscillations made in a given arc in free space, by the 

 number of oscillations performed in the same arc, whilst under the 

 influence of the given substance, and subtracts one from the 

 quotient, by which he considers that we shall obtain, in every case, 

 a fair value of the force we seek to determine ; for as the time of 

 performing a given number of vibrations is not caused to sensibly 

 vary in this species of action, we cannot resort to the common law 

 of pendulums, and take the square of the number of oscillations 

 performed in a given time as a measure of the force.* 



There was also on the library table, by Mr. Harris, a beating 



* Mr. Harris has deduced his formula in the following mannei : 



Let r =: the retarding force in free space, 



R = the retarding force of the given substance, 



Then r -f- R will be the whole force in action. 



Let a the number of vibrations in a given arc in free space, 



And b =1 the number of vibrations in the same arc whilst under the influ- 

 ence of the given substance, 



Then, since these oscillations may be supposed in the inverse ratio of the 

 retarding forces we have 



r : r -f R ; : 6 ; a , 



Hence ra 6 (r + R) r= 6 r + 6 R. That is, 

 ra b r fa \ 



K- T ^ r (-7 1 j but as r is constant in every case,' or 



may be taken as unity, we have 

 a 

 y 1 for the value of the force in action. 



202 



