46 M. A. Kundt's Acoustic Experiments. 



there is not too much lycopodium in the tube, the heaps are very 

 sharp and well defined, so that their distances can be measured 

 with great accuracy. If by pushing the cork the length of the 

 column of air be altered by half a wave-length, a totally different 

 dust-figure is obtained — that is, the dust-heaps which form in 

 sounding tubes at the nodes each being separated from the next 

 by an empty space surrounded by a ring. One of the nodes is 

 always at the moveable cork. 



The shape of the dust-figures seems always to depend on the 

 intensity of the vibrations of air; for when the external tube 

 is considerably wider than the sounding one, and thus the end 

 which produces the motion small as compared with the section 

 of the column of air, the heaps of dust are not formed, but the 

 figure with the holes. When the tone is feeble, the figure with 

 the holes appears first, and it is only gradually on continued 

 sounding that the dust accumulates in heaps. It would seem to 

 follow from this that the intensity of the motion of the air is con- 

 siderably greater if the column of air is an entire multiple of haif 

 a wave of air than if this is not the case. And in fact the os- 

 cillation in the loops must be greater the nearer the tone- 

 exciting terminal surface, with its given amplitude of oscillation, 

 is to a node. In agreement with this, sand or another heavy 

 powder would not move at all in the tube if the dust-figure with 

 the holes is formed. But if the apparatus is so arranged that 

 the dust collects in heaps at the nodes, if pure quartz sand be also 

 present, this is violently moved and forms accumulations be- 

 tween the heaps consisting of extremely regular ripples of sand, 

 while it rests at the nodes or sometimes forms holes similarly to 

 the lycopodium. It would thus appear that the intensity of the 

 motion of the air which is sufficient to collect the lycopodium dust 

 in heaps, can only arrange sand in such a form as lycopo- 

 dium assumes when the motion of the air is less intense. In 

 all modifications and transitions of the dust-figures, the dis- 

 tance of two successive similar points is equal to half a wave- 

 length. 



M. Kundt has executed a series of measurements for deter- 

 mining the velocity of sound in various bodies. In the case of 

 solid bodies, the results show that the method is capable of great 

 accuracy. 



The bar in which the velocity of sound was to be determined 

 was mostly fastened at two nodes ; and over one of its free ends 

 the glass tube for the dust-figures was pushed. The ratio of the 

 length of one dust-heap to half the length of the bar was then 

 the ratio of the velocity of sound in the air to that in the bar. 

 The length of the bar could be measured with great accuracy. 

 The lengths of a great number of beautifully formed dust-waves 



