Mr. R. H. M. Bosanquet on the Theory of Sound. 343 



is turned always in a direction contrary to that of the mag- 

 netizing current. 



The precise character of the mixed optical effect in the 

 case of oblique incidence can be determined only by exact 

 measurements. This much, however, appears to be clearly 

 proved by the preceding experiments, that the rotation due 

 to magnetization of the mirror is impressed upon the light 

 neither effectively before incidence, nor effectively after re- 

 flection. 



No effect was obtained in any case without the presence of 

 a submagnet. I think it certain that the only use of this 

 piece is to concentrate or intensify the magnetic force upon 

 the iron mirror by inductive action. 



The powers applied were barely adequate to produce all the 

 effects. Some of the phenomena were quite imperceptible 

 when the battery began to work, and afterwards, when it had 

 worked at intervals for three or four hours. Much better 

 effects may certainly be expected with higher electromagnetic 

 powers and finer optical appliances. 



Glasgow, 26th March, 1877. 



XLIV. Notes on the Theory of Sound. By R. H. M. 



Bosanquet, Fellow of St. John's College, Oxford. 



[Continued from p. 278.] 



2. On the Energy per second of a Pendulum-vibration in Air. 



THE flow of energy per second along a column of air 

 transmitting a pendulum-vibration is 



^ 2 ^)%. 2 xi.4m,(^) 2 , 



where p is the density of the air, 

 IT the atmospheric pressure, 

 v the velocity of sound, 

 A amplitude of vibration, 

 X wave-length. 



This was proved in a paper in the Philosophical Magazine, 

 March 1873. The kinetic and potential energy were esti- 

 mated separately ; each is equal to half the above quantity. 



This result may be obtained more conveniently by supposing 

 a disk to oscillate in an infinite cylinder. The changes of pres- 

 sure on the two sides of the disk are always equal and opposite ; 

 and the work done in any small movement is the product of 

 the displacement by the difference of the pressures. The total 

 work done by the disk is the energy supplied to keep up the 



