C. GENERAL PHYSICS. 177 



to 230 feet may be regarded as too small to be noticed. 

 From a discussion of the acoustic conditions of churches, in 

 this way, he concludes, first, that the ceilings, which in thea- 

 tres help the efiect in the upper tiers, in churches are too 

 high for this, and may produce resonance or echo, and that it 

 is therefore necessary to diffuse the sound reflected from 

 them ; and he gives the results of a comparison of different 

 forms of ceilings obtained by graphic construction ; second, 

 tliat the walls require to be not less carefully investigated, 

 since defects are often attributable to walls rather than ceil- 

 ings ; and since they can not always be conformed to acous- 

 tical demands, the most practical question is frequently how 

 to render the reflected rays without effect by suitable manage- 

 ment of the surface ; third, that surface and material, partially 

 touched upon before, need scientific investigation to complete 

 our knowledge in regard to the part they play ; fourth, that 

 sounding-boards, generally constructed empirically, ought to 

 be regulated in size and form by suitable construction in each 

 case, and that the material should be carefully selected. 

 Technische Blatter, III., 1872, 187. 



VIBRATION OF CORDS. 



M. Gripon has investigated the vibration of cords and rods 

 in resisting media, and his results, as communicated to the 

 society at Lille, are as follows: He noticed at first that a 

 cord attached at one end to a vibrating diapason not only 

 vibrated in unison with it, but in certain cases the division 

 of the cord into nodes presented a peculiar character in that 

 the number of nodes became less than it should be, and the 

 cord, divided into equal sections, seemed to vibrate as if its 

 extremities were fixed, and gave forth sounds of lower pitch 

 than those of the diapason. Duhamel had shown that the 

 latter mode of vibration is due to the initial state of the cord, 

 and is not permanent, but is soon destroyed by extraneous 

 resistances. Gripon finds that the second movement is as 

 necessary as the first, and under proper conditions quite as 

 permanent; and he concludes, from numerous experiments on 

 cords, ribbons, etc., by varying their size, length, density, 

 shape, and tension, that a diapason may generate in any at- 

 tached cord vibrations of any other pitch whatever lower 

 than its own, a condition Avhich had been thought impossible 



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