to determine the Direction of Sound. 331 
direction, when using ear-tubes and facing the source, 
remains almost the same when the sound becomes louder, 
the angle of displacement becoming, i£ anything, somewhat 
smaller. In the case of the notes of greater frequency, 
however, the sound when dying away will, in my case, 
move through a considerable angle to the left, if its original 
position is near the source to start with — this is probably 
due to my left ear being more sensitive for feeble shrill sounds 
than the right ear. 
In connexion with these methods of examining sound- 
images, an interesting experiment is to take two tubes of 
2 inches or more in diameter, one say 12 inches long and 
the other 4 inches, and listen to a band of three or four 
instruments played in the open — the notes will be found to 
be scattered over a wide range, most being to the side of the 
short tube, some being in front and some being to the side 
of the long tube. 
In listening with such a pair of tubes to two dogs furiously 
barking, the effect is at first quite alarming — one seems to 
be in the middle of a pack of dogs some of which are rushing 
viciously at one's throat, Preferably the tubes should be of 
metal sheet, and it is best, though not necessary, to surround 
them with a sheet of some rubber cloth or composition. 
It remains to say something about the interference of 
trains of waves after reflexion in producing a change in the 
position of the image. 
In one experiment, an organ-pipe was placed 5 feet from 
a flat wall and the observer stationed on a line, making an 
angle of 30° with the wall, through the foot of the perpen- 
dicular from the organ-pipe on the wall. There was more 
difficulty in settling the direction than when there was no 
wall, and the apparent position seemed to be at the foot of 
the perpendicular on the wall : with other positions of the 
observer, the apparent image of the source varied between 
the source and this point. 
When the positions of the observer and the source were 
interchanged, I obtained these results when working with a 
note of wave-length 2 6' 8 inches. 
Source of sound 10 feet distant. 
Ear 30 inches from the wall — the only image lay on a 
line from the ear parallel to the wall. 
(The image of the first harmonic of the stopped pipe was 
displaced 10°.) 
As the head was moved nearer to the wall, the image 
