320 Intelligence and Miscellaneous Articles. 



SiO., 138-40, A10 a 2-80, FeO 15-52, (MgO 39-88,) H 2 3-40=100. 

 — Silliman's American Journal, September 187(>. 



ON THE INFLUENCE OF SONOROUS VIBRATIONS UPON THE 

 RADIOMETER. BY J. JEANNEL. 



I have ascertained that the rotatory movements of the radio- 

 meter can be determined by certain sonorous vibrations. In re- 

 gard to this I have, in conjunction with M. Coulier and M. Alver- 

 gniat, made various experiments, of which I think I ought to render 

 an account to the Academy. 



In semiobscurity three radiometers A, B, C, of unequal sensi- 

 tiveness and at perfect rest, were placed on the inner shelf of a 

 saloon- organ. The bass notes (those of the first three octaves) 

 determined the rotation-movements. The deepest bass notes have 

 the greatest action ; nevertheless the fa and the fa sharp of the 

 lower octave, especially with the bourdon stop, produce the most 

 rapid rotation. Ut, re, and mi, though deeper, have much less action. 



The radiometers do not all behave in the same way as regards 

 the speed and the direction of their rotations. Thus, through the 

 effect of fa or fa sharp of the lower octave, radiometer A (the 

 least sensitive to light) rotates with the velocity of about one turn 

 per second, with the black faces of the vanes in front — that is to 

 say, the inverse of the motion produced by light ; B and C (more 

 sensitive ta light) turn more slowly, and in the direction of the 

 light-produced motion, viz. with the bright faces in front. 



These facts I propose to explain thus. As certain notes pro- 

 duce no effect, it is evident that the needle which forms the internal 

 support of the revolving arms must be able to vibrate in unison 

 with the notes of the organ in order that the rotatory movement 

 may be produced. Certain vibrations of the organ-shelf, trans- 

 mitted to the needle, communicate to it circular or angular vibra- 

 tions, from which results the rotation of the arms which it supports. 

 What appears to demonstrate that this explanation is w T ell founded 

 is, that on pressing the soft part of the finger upon the top of the 

 radiometer we prevent it from vibrating, and, at the same time, 

 from turning. 



The shelf of a piano produces similar effects, but in a less degree. 



When the above experiments are tried in a medium where the 

 diffused light is nearly sufficient to start the radiometer, even the 

 feeblest deep sounds determine the rotation in the usual direction : 

 the roll of the vehicles is sufficient. 



Here the explanation presents itself to the mind spontaneously. 

 The arms undergo the impulsion of the force which is to put them 

 in motion ; but this force is counterbalanced by the friction of the 

 top piece upon the point of the needle ; no motion can take place. 

 But if the vibrations interpose, the top piece, raised above the 

 needle-point and thus removed from friction during a certain space 

 of time, can obey the impulse. Hence it results that under the 

 influence of the vibrations the radiometer appears much more 

 sensitive to light than when it is at rest. — Comjrtes Benches de 

 V Academic des Sciences, vol. lxxxiii. p. 445. 



