36 Dr. E. C. 0. Neumann on an Apparatus for directly 



Let k' be the average external resistance between two given 

 highest limits, k" the average external resistance between two 

 given lowest limits ; then we have 



x-\-y — k', 



x + y 



x and y being the resistances of the two equal parts of the space ; 

 thus 



For example, in" the bridges usually used in cable-testing we 

 have, according to formula (2), 



k' =1009 Siemens's units, 

 and 



k"= 109 

 therefore 



x = 124-41 



y = 884-6 J » 



which values may be each corrected, as regards insulating cover- 

 ing, either by equation (3) or by formula (4) . 



VI. On an Apparatus for directly measuring the Velocity of 

 Sound in the Atmosphere. By Dr. E. C. 0. Neumann*. 

 [With a Plate.] 



EVERY sound-wave which starts from any point in the atmo- 

 sphere exerts an impact on any body opposed to it ; so that 

 if the body is a stretched membrane, it is agitated in such a man- 

 ner that the first and strongest displacement must ensue in the 

 direction of the motion of the wave of sound. A second mem- 

 brane stretched in the same manner, but at a different distance, 

 will be agitated in the same manner, but after an interval of 

 time whose magnitude depends on the difference in the distances 

 of the two membranes from the source of sound. By the aid of 

 the apparatus I am about to describe, it is possible to fix the in- 

 stants of the first displacements of those membranes, and thus 

 determine the velocity of sound in the air. 



The velocity of sound is the same in tubes as in the open air. 

 On this principle a box was constructed about 82 centims. in 

 length, 66 centims. in breadth, and 7 centims. in height, of whose 



* Translated from Poggendorff 's Annalen, vol. cxxviii. p. 307. 



