Tufts — Transmission of Sound through Solid Walls. 451 



but that which proved the most satisfactory was obtained 

 by dropping a metal ball upon a pine board, the height 

 through which the ball dropped being adjusted until the sound 

 was sufficiently intense to be heard through the discs under 

 investigation. An observer in the adjoining room, with the 

 ear-piece, h, in his ear, could, by alternately closing and open- 

 ing the rubber tubes, c and d, easily ascertain which of the 

 two discs transmitted the loudest sound. 



The rigidities of the discs were measured in the following 

 way ; an upright index was cemented to the disc under inves- 

 tigation, and a microscope, provided with a micrometer ocular, 

 was focused on a mark on the index. The disc was then sub- 

 jected to a pressure of air through the tube, c or d, and the 

 displacement of the index read off in the microscope. The 

 excess of pressure of the air on the inner surface of the disc 

 over the atmospheric pressure was measured by a suitable 

 manometer. From the data thus obtained the displacement of 

 the center of the disc, for a pressure of one gram per square 

 centimeter of surface, was calculated. The value of this dis- 

 placement is of course a measure of the rigidity of the disc. 

 The following are some of the results obtained with the appa- 

 ratus just described. 



I. A lead disc 10-5 cm in diameter and -012 cm thick, and a 

 glass disc of the same dimensions, were clamped in the two 

 unions respectively, and the intensities of the sounds trans- 

 mitted through the two discs compared. It was found that 

 the lead disc transmitted sound better than the glass one. 

 The displacement of the center of the lead disc, for a pressure 

 of one gram per square centimeter of surface, was '000106 cm 

 and of the glass disc -000053 cm . 



II. A disc of white pine -65 cm thick was compared with a 

 disc of leather of the same thickness. Both discs had been 

 treated with paraffine to render them impervious to air. The 

 displacement of the center of the pine disc was '000013 cm and 

 of the leather disc '0(3021 2 cm for a pressure of one gram per 

 square centimeter of surface. It was found that the leather 

 disc transmitted sound very much better than the pine disc. 



In both of the above cases the more rigid disc was found to 

 be the poorer conductor of sound, although, in both cases it 

 was composed of a material much better suited to the 

 transmission of an elastic wave than the less rigid disc. 



III. A brass disc *015 cm thick was braced by soldering to it 

 a few cross strips of brass. This disc was compared with one 

 formed of two thicknesses of cardboard treated with paraffine. 

 The total thickness of the cardboard disc was -44: cm . The 

 displacements of the two discs for a pressure of one gram per 

 square centimeter of surface were found to be the same, 



