through Porous Materials. 



361 



Flow of sound through granular materials. 

 These experiments were undertaken for the purpose of ascer- 

 taining if there were any variations in the resistances offered 

 by the materials A, B and C, to the transmission of sound 

 through them, corresponding to the variations already detected 

 in the case of air currents. A number of different forms of 

 apparatus were used in studying the relation between the 

 resistance offered by such materials to the transmission of 

 sound and the thickness of the material used. The form of 

 apparatus which seemed least subject to disturbing influences is 

 shown schematically in fig. 2. 



Fig. 2 



It consists essentially of a thin rubber membrane K, stretched 

 over a brass ring, which fits snugly in the opening through the 

 block of wood, B. To the center of the membrane is attached 

 a rectangular index of tinfoil, D, having a very narrow 

 slit cut in it near its outer end. A microscope, M, provided 

 with a micrometer ocular, is so mounted that it can be 

 focused on the slit in the index, the slit being illuminated by a 

 lamp, L. A tray, T, three centimeters deep, and about three 

 centimeters in diameter, provided with a bottom of wire 

 gauze, sits snugly on the block, directly over the membrane, 

 and in it is placed the granular material under investigation. 

 The application of a little cement around the edges of the tray 

 makes an air-tight joint between it and the block, B. Thus 

 the membrane together with the air in the tray above it forms 



