38 On direct Measurement of the Velocity of Sound. 



the rods/and #, it was found that/ (answering to d) struck out ex- 

 actly 10 milliins., d, on the contrary, 8 millims. After the termi- 

 nation of these preliminary trials, the first object was to determine 

 the distance of the stroke of the two rods / and g. With this 

 view the disk P, whose anterior surface was covered with white 

 paper, was brought within 6 millims. of the blackened rods, and 

 in such a manner that they must strike the periphery of a circle 

 30 centims. in diameter drawn on the disk P. As long 

 as the disk was not rotated, after each discharge two black points 

 were formed at the same distance on the same periphery, which 

 was to be accepted as the normal difference of stroke. But 

 when the disk was rotated, the distance was greater or less, ac- 

 cording to the direction of the rotation. When the rotation had 

 a definite velocity of one turn in a second, the difference between 

 the distance of the marks in that case and in the normal case was 

 constant and amounted to 16 J- millims. It follows thence that 

 with the given velocity of rotation of the disk a point of the 

 periphery of the above circle of 30 centims. diameter passes 

 through 16^ millims. in the time which sound requires to tra- 

 verse 6 metres. If we put the velocity of a point of the peri- 

 phery = a, and call b the length of the path of a point of the 

 periphery in the time which sound requires for its path in the 

 apparatus, the length of which shall be /, the velocity c of sound 

 is obviously determined from the equation 



al 



c =r 



As al was previously known, it is only necessary in each expe- 

 riment to divide by b to find c at once. 



The above values substituted gave c = 346-217 metres at 

 22° C. 



Of course in such apparatus it is necessary that there be — 



(1) Perfect unalterability of the position of the plane P 

 during the rotation of the disk. 



(2) Perfectly uniform rotation. 



(3) An accurate determination of the path of the sound in the 

 tube. 



In the present apparatus the rotation of the disk is effected 

 by hand, the time being taken by a seconds-pendulum, which is 

 sufficient for instruction in schools. But considering that this 

 rotation might be made more certain and more rapid by a good 

 clockwork, that the dimensions of the tube might then be consi- 

 derably smaller, and the source of sound feebler, it will be seen 

 that this apparatus is capable of an amount of improvement 

 sufficient for obtaining the most accurate results. 

 Dresden, April 1866. 



