PROFESSOR DVORAK'S SOUND-MILLS. 259 



nators vibrates in response, and the apparatus begins to rotate. Rota- 

 tion will take place even if there is only one resonator, properly bal- 

 anced ; but the phenomenon is more marked and certain if there are 

 four. 



A second style of apparatus — ^the " rotating resonator " — is repre- 

 sented in plan and elevation in Fig. 2. It consists of a short cylin- 

 drical box of stiff paper, having four projections, each of which bears 

 at its side a short open tube of paper. It is hung on a silk fiber, and 

 is supplied with a small needle, projecting below to steady the motion 

 during its rotation. 



The operation of these instruments depends on the principle which 

 has been pointed out by Lord Rayleigh and Professor Mayer as well 

 as by Professor Dvorak, that " when sounds of great intensity are pro- 

 duced, the calculations, which are usually carried only to the first order 

 of approximation, cease to be adequate, because now the amplitude of 

 motion of the particles in the sound-wave is not infinitely small as com- 

 pared with the lengths of the sound-waves themselves. Mathematical 

 analysis shows that under these circumstances the wave of the pressures 

 in the condensed part, and in the rarefied part of the sound-wave, is no 

 longer equal to the undisturbed atmospheric pressure, but is always 

 greater. Consequently, at all nodal poifits in the vibrations of the 

 air in tubes or resonant boxes, the pressure of the air is greater than 

 elsewhere ; and therefore any resonator closed at one side and open 

 at the other is urged along bodily by the slight internal excess of 

 pressure on the closed end." The apparatuses therefore rotate by 

 reaction. 



To produce vibrations of sufficient intensity. Professor Dvorak 

 uses heavy tuning-forks mounted on resonant cases, and excited elec- 

 trically. For this purpose he places between the prongs of the fork 

 an electro-magnet, in which the core is composed of two plates of iron, 

 separated by a sheet of paper, and cut of such a breadth as to lie be- 

 tween the prongs without touching them. The core is overwound 

 with insulated copper wire, as shown at E, Fig. 3, and the electro- 

 magnet is mounted by a bent piece of wood, ab c, upon the sound- 

 ing-box, K, of the fork. The wires are connected in a circuit with the 

 battery, and with the electro-magnet of a self -exciting tuning-fork of 

 the same note. The sounding-boxes of the forks must not touch the 

 table, but the arm a 5 c is clipped at about the point J in a firm sup- 

 port ; and particular care must be taken to have the wood of the reso- 

 nant boxes tuned into exact accord with the tone of the fork and of 

 the air within the cavity of the box. 



The third apparatus is the " sound-radiometer," and was described 

 by Professor Dvorak before the Imperial Viennese Academy in 1881. 

 It is more simple than the two instruments previously described, but 

 its cause of action is less easily explained. It is shown in Fig. 4. It 

 consists of a light cross of wood pivoted by a glass cap upon a vertical 



