258 



THE POPULAR SCIENCE MONTHLY, 



sucli analysis plainly can not be claimed by a department of inquiry 

 whicli deals with phenomena physically demonstrable alone ; be those 

 results sound or unsound, conclusive or tentative, final or provisional, 

 such as they are, they are the property of introspective psychology 

 alone. Furthermore, there is a large class of psychological concepts 

 framed on a combination of both kinds of evidence, subjective and 

 objective. 



PEOFESSOE DYOEAK'S SOUND-MILLS. 



PEOFESSOE SILYANUS P. THOMPSON has made known, 

 through the columns of " Nature," an interesting series of ex- 

 periments by Professor V. Dvorak, of the University of Agram, in 

 the production of an apparatus which should rotate under the influ- 

 ence of sound-waves in the same way as the radiometer introduced by 

 Professor Crookes rotates under the influence of rays of light and 

 heat. The same idea was suggested independently to several men, 

 among whom were our countrymen, Professor A. M. Mayer, of Ho- 

 boken, and Mr. Edison, all of whom have made in the matter re- 

 searches of great scientific interest. Professor Dvorak has devised 

 four kinds of " sound-mills," as they may be called, two of which de- 

 pend on the repulsion of resonant boxes, and two on different prin- 

 ciples. 



One of the instruments is represented in Fig. 1. It consists of a 

 light wooden cross, balanced on a needle-point, and carrying four light 

 resonators — hollow balls of glass, forty-four millimetres in diameter, 



Fia. 1. 



Fig. 2. 



with an (Opening of four millimetres at one side, and responding to the 

 note g', or the middle G, of the piano-forte (= 392 vibrations). When 

 this note is forcibly sounded by the tuning-fork, the air in the reso- 



