Measure of Intensity in the Theories of Light and Sound. 215 



the pressure on the studs due to rifling is but a small frac- 

 tion (about 2|- per cent.) of the pressure required to give trans- 

 lation to the shot. 



3rd. That the substitution of the parabolic for the uniform 

 rifling has reduced by about one half the maximum pressure- 

 on the studs. 



4th. That the increment of the gaseous pressure, or the pres- 

 sure tending to burst the gun, due to rifling is exceedingly 

 small*, both in the case of the uniform and parabolic rifling. 

 This result is entirely confirmed by the experiments of the Ex- 

 plosive Committee, who have found no sensible difference of 

 pressure in the 10-inch gun fired in the rifled and un rifled states. 



5th. That, small as the increment in gaseous pressure due to 

 rifling is, it is still less in the parabolic than in the uniform 

 system of rifling. 



Elswick Works, February 15, 1873. 



XXVII. Note on the Measure of Intensity in the Theories of Light 

 and Sound. By R, H. M. Bosanquet, Fellow of St, John's 

 College, Oxfordf. 



MR. MOON, and more recently Dr. Hudson, have, in the 

 Philosophical Magazine, controverted the position that 

 the energy of the forms of motion, which constitute light and 

 sound, with factors depending on the wave-length or periodic 

 time, is to be regarded as the measure of the intensity of these 

 impressions on our senses. The matter is one for experimental 

 evidence. Now direct evidence bearing on the point at issue is 

 not wanting, in the case of either sound or light. 



The question is whether the intensity is measured by the 

 square of the amplitude, or by the amplitude, for given periodic 

 times. 



Mr. Moon has not offered any answer to the remark made at 

 the end of my paper of last November, although, if he under- 

 derstood it, it is conclusive in the case of light. It may be 

 worth while to mention the point again. There exists an expe- 

 rimental law known as the law of Malus, which connects the 

 intensities of the two rays, polarized in planes at right angles to 

 each other, into which a plane-polarized ray is decomposed by a 

 doubly refracting crystal. The intensities of these two rays are 

 proportional to the squares of the sine and cosine of the angle 



* Although the increase of strain due to rifling is inconsiderable, yet the 

 decrease of the strength of the structure of a gun inseparable from rifling 

 may be, and in many systems is, considerable ; but the discussion of this 

 question is outside of the scope of my paper. 



t Communicated by the Author. 



