134 0. N. Rood—Reflexion of Sound- Waves. 
Diameter of disc 84 inches. 
t, fork; alternations heard at 2 “ distance. 
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2d. When the sound-waves fall upon small flat surfaces at an 
acute angle, the reflexion is most copious in the same direction as 
with light, but the reflected and inflected waves can be traced all 
around the semicircle. 
Experiments on this point were made in the open air, the 
larger disc being used with angles of 60° and 70° (from the per- 
pendicular); the Ut, and Ut, forks were employed. 
e regularly reflected waves could be heard at a distance of 
ten or twenty feet from the disc, the fork being held a foot or 
two from it; inflected waves were easily distinguishable all 
around the disc and even a few feet behind the fork. 
hen the forks were placed in the plane of the disc the alter- 
nations of loudness were reduced to a minimum, but in the 
open air and in a room never wholly disappeared. This I sup- 
se to be owing to the fact that the source of sound is not a 
point but a surface. Even under these circumstances, feeble 
alternations were heard all around the disc, the inflected waves 
actually returning to their source. With a plain disc alterna- 
tions were not perceived. 
8d. Qualitative comparisons between the power of different sub- 
stances to reflect sound can easily be made. 
or example, a disc of card-board in which filter paper is 
fastened over the open sectors gives alternations, owing to the 
difference of the reflective powers of the two substances. 
+ a composite sound-wave falls on the rotating dise the 
shorter waves will undergo regular reflecion more copiously than 
the other components. 
This experiment is most easily made with a reed without ts 
pipe. Ut,, Ut, Ut, reeds give alternations but mainly in their 
high overtones; the alternations consequently have a ringing 
metallic sound. : 
5th. The reflewion of sound from very small surfaces is easily 
demonstrated. 
If an Ut, or Ut, reed without its pipe be employed, alterna- 
tions are easily obtained by moving a visiting card properly 
near the reed: By substituting a gas-flame for the card the 
reflexion from the flame can be demonstrated. The gas-burner 
should be attached to along slender rod. 
Almost all of these experiments are so easily performed as to 
be suitable for lecture-room purposes. 
