362 Tufts — Transmission of Sound 



a single vibrating system, and any vibration of the membrane 

 must be accompanied by a to and fro motion of the air in the 

 tray. The block containing the membrane, microscope and 

 tray was mounted on a suitable support, about fifty centimeters 

 above an organ pipe, O, provided with a sliding piston, P, 

 so that the effective length of the pipe could be varied, and 

 the pipe thus brought in unison with the fundamental note of 

 the apparatus above it. The organ-pipe was connected to a 

 source of compressed air, and by means of a suitably placed 

 stop-cock and manometer, the pressure at which the pipe was 

 blown was kept constant. When the note emitted by the pipe 

 was of about the same pitch with the fundamental note of the 

 membrane, this would be set in quite violent vibration, and the 

 amplitude of the vibration could be determined by measuring 

 with the microscope the width of the band of light produced 

 by the vibration of the slit, A. 



A few preliminary experiments indicated that when the 

 fundamental note of the membrane was as low as two hundred 

 vibrations per second, no appreciable change in the fundamental 

 note of the system, composed of the membrane and air in the 

 tray above it, was produced by the gradual addition of shot to 

 the tray. Under this condition, the diminution in the ampli- 

 tude of vibration of the membrane, when shot is poured into 

 the tray, is due chiefly to the increased resistance offered by 

 the shot to the to and fro motion of the air particles, and only 

 a very small amount is due to the slight change produced in 

 the fundamental note of the system composed of the mem- 

 brane and air in the tray. 



In the experiments for the purpose of comparing the resist- 

 ance of different thicknesses of the granular materials, the 

 organ pipe was tuned to the vibrating system, K and T, and 

 blown at a constant pressure. The amplitude of vibrations of 

 the membrane was measured after each addition of a half cen- 

 timeter thickness of shot. The differences between the recip- 

 rocals of these amplitudes should theoretically be proportional 

 to the resistances offered by the various thicknesses of shot 

 introduced. The data given in Table II are representative of 

 the results obtained. 











Table II. 











T 



A 



B 



C 



A' 



B' 



C 



A'/T 



B'/T 



C'/T 







60-0 



60*0 



60-0 















1 



24-0 



18*0 



10-5 



•0250 



•0389 



•0786 



•0250 



•0389 



•0786 



2 



15*0 



11-0 



5*5 



•0500 



•0743 



•1652 



•0250 



•0376 



•0826 



3 



10-5 



7-5 



4-0 



•0786 



•1167 



•2334 



•0262 



•0389 



•0778 



4 



8*5 



6-5 



3-0 



•1010 



•1500 



•3167 



•0252 



•0375 



•0792 



5 



6'5 



5-0 



2-0 



•1272 



•1834 



•4834 



•0254 



•0367 



•0967 



6 



5-5 



4'0 



2"0 



•1652 



•2334 



•4834 



•0275 



•0389 



•0806 



