L. MECHANICS AND ENGINEERING. 529 



difference between the lengths of the paths of the direct and 

 reflected rays from the origin of sound, and the question of 

 jDractical acoustics rests essentially on a knowledge of the 

 limits within Avhich this difference exerts a disturbing influ- 

 ence. The observ' ations of Orth, which differ from those of 

 Langhaus, show that a difference of from sixteen and a half 

 to twenty-three feet not only causes no disturbance, but un- 

 der some circumstances may produce favorable re-enforcement 

 of the direct by the reflected sound, and that, with dimin- 

 ished intensity, the difference may be somewhat greater, but 

 under no circumstances should it exceed thirty- three feet. 

 On the other hand, the intensity at a difference of 196 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 theatres, help 

 the effect in the upper tiers, in churches are too high for this, 

 and may produce resonance or echo, and that it is therefore 

 necessary to dift\ise the sound reflected from them ; and he 

 gives the results of a comparison of difterent forms of ceil- 

 ings obtained by graphic construction ; second, that the walls 

 require to be not less carefully investigated, since defects are 

 often attributable to walls rather than ceilings ; and, since 

 they can not always be conformed to acoustical demands, the 

 most practical question is frequently how to render the re- 

 flected rays without effect by suitable management of the 

 surface ; third, that surface and material, j^artially touched 

 upon before, need scientific investigation to complete our 

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

 ing-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, 1872, in., 187. 



IXJURY TO ATLANTIC CABLES BY ANIMALS. 



When submarine cables were first proposed, much trouble 

 was anticipated from the boring of marine animals through 

 their external envelopes, and thus allowing the salt-water to 

 penetrate to the wires. A cable would also in time become 

 covered with adherent mollusks and similar objects, which 

 would be likely to attract fishes and other animals, the teeth 

 of which might produce serious mechanical injuries. Al- 



Z 



