64 The Hon. J. W. Strutt on Gaseous Pressure, 



imperfect octave is due to the beats of the combination-tone of 

 the first order with the lower of the two fork-tones. The former 

 sound is much weaker than the latter ; the variations of inten- 

 sity cannot, therefore, be considerable, though the sourness of 

 effect is marked. A similar observation applies to the case of 

 two simple tones forming the interval of an imperfect fifth, where 

 the dissonance arises between combination-tones of the first and 

 second orders, the latter being extremely weak compared with the 

 former. Further, with composite sounds, such as those of most 

 musical instruments, the dissonance of all intervals wider than a 

 tone, or tone and a half, is due to the beats of over-tones of dif- 

 ferent orders. These are, in general, correspondingly different 

 in loudness ; and, accordingly, variations of pitch are developed at 

 the expense of those of intensity. I should, in particular, attri- 

 bute the slight imperfections by which the ordinary concords of 

 the scale (fifth, fourth, thirds, &c.) are rendered less smooth 

 than the unison or octave, mainly to the pitch-variations, since 

 the difference in order, and consequently in strength, between 

 the beating over-tones is here usually sufficient to render the in- 

 tensity-variations inconsiderable. 



V. On Mr. Moon^s views on Gaseous Pressure. By the Hon. J. 

 W. Strutt, M.A., late Fellow of Trinity College, Cambridge, 



To the Editors of the Philosophical Magazine and Journal, 

 Gentlemen, 



I WISH to make a few remarks on some views of Mr. R. 

 Moon promulgated in your June Number and (more at 

 length) in the Number for August 1868. It is a received opi- 

 nion among physicists that, in the case of such motions of air as 

 constitute sound, the differential pressure is proportional to the 

 differential density. Mr. Moon, on the contrary, holds that 

 density (and temperature) do not determine pressure, and brings 

 forward a mathematical argument (which I shall consider pre- 

 sently) to show that velocity also is a datum which it is necessary 

 to know before pressure can be calculated. The first question 

 that arises is. What is here meant by velocity ? If it be absolute 

 velocity which is intended, two repetitions of Boyle's [not Ma- 

 riotte's) experiment wdth an interval between of twelve hours 

 during which the earth's diurnal motion at the place of obser- 

 vation would be reversed, would suffice to settle the question. 

 Perhaps Mr. Moon means the velocity relatively to the contain- 

 ing vessel ; but this does not alter the matter, because in the 

 case chosen for illustration the cylinder may be moved in the 

 direction of its length with any velocity, and leave the air behind 



