1874] WRITINGS OF JOSEPH HENRY. 399 



From this series of experiments it appears that a diminu- 

 tion of pressure is attended with a comparatively small 

 diminution in the penetrating power of the siren. 



In regard to this unexpected result, — of great practical im- 

 portance, the following appears to be the explanation. It is 

 a well-known principle in aerial mechanics that the velocity 

 of the efflux of air from an orifice in a reservoir does not 

 increase with an increase of condensation, when the spouting 

 is into a vacuum. This is evident when we reflect that the 

 weight or density of the air moving out is increased in pro- 

 portion to the elasticity or pressure ; that is, the increase in 

 the propelling force is proportional to the increase in the 

 weight to be moved, hence the velocity must remain the 

 same. 



In the foregoing experiments with high pressures — large in 

 proportion to the resistance of the air, the velocity of efflux 

 should therefore be but little increased with the increase of 

 pressure, and inasmuch as the velocity is the most impor- 

 tant factor in the expression mv^, which indicates the effect 

 on the tympanum, the penetrating power of the sound should 

 be in accordance with the above experimental results. 



A similar result cannot be expected with the use of the 

 whistle, or the trumpet, since in the former, the stiffness of the 

 aerial reed depends upon its density, which will be in pro- 

 portion to the pressure in the boiler; and in the case of the 

 latter — on the one hand, no sound can be produced unless the 

 pressure be sufficient to overcome the resistance of the reed, 

 and on the other, the sound must cease when the pressure is 

 so great as to prevent the recoil of the reed. 



5. An experiment was made to determine the effect of a 

 small whistle inserted into the side of a trumpet near the 

 small end. The whistle being sounded before and after it was 

 placed in the trumpet, the result was as follows : The pene- 

 trating powers were in the ratio of 40:51, while the tone was 

 considerably modified. From this experiment it appears 

 that a whistle may be used to actuate a trumpet or to 

 exercise the functions of a reed. In order however to get 

 the best results, it would be necessary that the trumpet 



