O. N. Rood— Reflexion of Sound- Waves. 133 
Art. XVIII.—On a Method of Studying the Reflexion of Sound- 
Waves; by O. N. Roop, Professor of Physics in Columbia 
College. 
It has been the custom for several years to introduce in cer- 
tain forms of the melodeon a revolving fan for the purpose of 
obtaining rapid alternations in the intensity of the notes. This 
arrangement is called a “tremolo,” and its action was consid- 
ered by its inventor and by those interested in it to depend on 
the currents of air produced by the motion of the fan. An 
examination of the apparatus soon convinced me that this idea 
was erroneous, and that the alternations in the loudness of the 
sound was due to reflexion or non-reflexion from the face 
of the revolving fan, for I found that the same effects could be 
produced by the aid of a circular dise consisting of open and 
closed sectors and revolving in its own plane. A disc of this 
kind constitutes a useful piece of apparatus for studying the 
reflexion of sound-waves, and some results obtained with it 
were communicated by me to the National Academy of Sci- 
ences, as long ago as October, 1876. 
no account of these experiments has ever been published, 
a short description of them may not be without interest to those 
engaged in experimental researches on sound, as with their aid 
the following facts may be easily demonstrated : 
st. At a perpendicular incidence the short sound-waves are more 
coprously reflected than those that are longer, and the regular reflex- 
ton 1s more copious from large than from small surfaces. 
The diameter of the zine disc was in the first set of experi- 
ments 21 inches =53°3 centimeters; alternate quadrants were re- 
moved, and the rate of rotation varied from two to four turns in 
asecond. The tuning-forks were mounted on their resonance 
boxes and gradually removed away from the revolving dise till 
the alternations could no longer be distinguished by the ear 
placed near the fork. The results are given in the table below, 
in which “ distance” indicates that of the open end of the tun- 
ing-fork from the disc : 
Diameter of disc 21 inches. 
Ut, fork; alternations heard at 13. “ distance. 
tes “ “ 20 66 “ 
Ut. “ “ é< 96 ‘<< 6 
5 
_ When the same experiments were made with a disc having a 
diameter of only 84 inches or 21°5 centimeters, it was found 
hecessary to bring the forks much nearer to the dise before the 
alternations could be perceived. 
