156 DES. GUY BARLOW AND H. B. KEENE ON THE ANALYSIS OF SOUND. 
harmonic series for low and high speeds are represented by continuous and broken lines 
respectively. All three experiments were carried out under identical conditions as 
regards the position of the receiver, which was placed on the bottom at a depth of 
15 feet. The only difference made in the conditions was in the natural frequency of 
the receiver : in the first experiment this was made as low as possible, about 40/sec., 
by using the full volume of the air-cavity ; in the second it was raised to about 80/sec., 
and in the last to about 100/sec. The last two natural frequencies are only known 
approximately. 
It will be seen that in all three cases the responses n, 2 n, 3 n, are increased with increased 
speed. In the first experiment 4 n is very prominent, and showed a decrease with speed, 
evidently due to its being put out of resonance with the diaphragm. The effect of 
diaphragm resonance is also shown in the second and third experiments, which give 
strong responses near 80 and 100 respectively, frequencies for which no responses were 
detected in the first experiment. On the other hand, there seem to be certain perma¬ 
nent features in the spectrum—for example, the third harmonic for the lower speed 
always being very weak, and the harmonic near 68 being always prominent. These 
characteristics may be associated with either the boat or the motor. 
Variation of Spectrum with Direction. —The receiver was placed in various directions 
with respect to the axis of the boat, the distance from the propeller being constant. 
Variations in the spectrum were observed, but as consistent results were not obtained 
on repeating the measurements for any one position, no definite conclusions can be 
drawn. In general the experiments showed that direction has no marked influence on 
either the intensity or distribution in the spectrum. 
Variation of Spectrum with Depth. —It Avas not possible to moor the motor-boat in the 
deeper parts of the reservoir. Some experiments were therefore made with the motor 
circling round a cork marker indicating the position of the receiver, which Avas— (a) on 
bottom, (6) on tripod 6 feet above bottom. The depth of water Avas 26 feet, and the 
radius of the circle either 20 feet or 60 feet. The spectrum of the boat in motion did 
not differ essentially from results obtained Avith the boat moored in shalloAV water. 
The motor frequency Avas 14-2/sec. instead of 10-11/sec., so that no exact comparison 
is justifiable. The intensities observed were less than those for equal horizontal dis¬ 
tances in shallow Avater, but not in greater proportion than is accounted for by the 
greater actual (oblique) distance betAveen receiver and boat. 
Variation of Amplitude with Distance. —Experiments Avere made to determine the 
variation of amplitude Avith distance in the case of the most prominent vibrations up 
to the fifth harmonic. The most useful results would be obtained at distances great 
compared Avith the dimensions of the boat, but on account of the rapid decrease in 
intensity it Avas quite impossible to make observations at distances greater than 70 feet. 
At these distances disturbances Avere comparable in magnitude with the effects to be 
observed. 
Since the source is on the surface and receiver on or near the bottom, any change in 
