through Porous Materials. 363 



The note used in this particular case was about 150 vibra- 

 tions per second. The numbers in the first column of the 

 table headed T represent the thickness, in half centimeters, of 

 the material in the tray. The next three columns, A, B and 

 C respectively, contain the amplitudes of vibration of the 

 membrane, in eightieths of a millimeter, after the addition to 

 the tray of the corresponding thicknesses of the different mate- 

 rials A, B, and C respectively. The numbers given in the 

 columns headed A', B', and C, are obtained by subtracting the 

 reciprocal of the amplitude of vibration with the empty tray 

 in place, from the reciprocals of the amplitudes of vibrations 

 when 1, 2, 3, etc. half centimeters of one of the three sizes of 

 shot A, B, or C, had been placed in the tray. These numbers 

 should be, theoretically, proportional to the resistances offered 

 by 1, 2, 3, etc. half centimeters of the different materials used. 

 In the last three columns of the table, headed A'/T, B'/T and 

 OyT, are given the quotients obtained by dividing these num- 

 bers by the corresponding thickness of the material. An 

 examination of these quotients shows them to be practically 

 constant for a given size of shot. The variations are no greater 

 than can be accounted for by experimental errors, since the 

 amplitudes of vibration could not be read to a greater accuracy 

 than two or three units in the first place of decimals, thus mak- 

 ing the percentage error greater for small than for large 

 amplitudes. Experiments with much modified forms of the 

 apparatus indicated the same relation between resistance and 

 thickness. This relation may be stated as follows : The 

 resistance offered by granular materials to the to and fro 

 motion of the air particles in a sound-wave is proportional to 

 the thickness of the material, other things being equal. Hence 

 the relation between resistance and thickness is the same as was 

 found to hold for direct currents of air. 



Observations were also made upon the transmission of sound 

 and of direct currents of air through porous materials of a woven 

 texture, such as cheese cloth, cambric and cotton flannel, and 

 also through materials like felting and cotton batting. With 

 such materials it was much more difficult to obtain consistent 

 results than with shot, owing to variations in packing. By 

 subjecting the materials always to the same pressure, the 

 variation due to packing was almost eliminated, and the results 

 then showed that the resistance which such materials offered 

 to the transmission of sound and of direct currents of air, was 

 directly proportional to the thickness or number of layers of 

 the material used, as was the case with granular materials. 



