422 
DR. W. S. TUCKER AND MR. E. T. PARIS ON 
be of such a nature that the outward movement of the air takes place principally along 
the central axis of the neck, while the inward movement takes place close to the walls 
of the neck. Since the platinum grid is placed centrally in the neck and does not 
occupv any space near the walls, the effect of the jet could be represented by adding 
a term V' to V 0 , so that (approximately) 
Y 0 — Y cos (9 + a) + V'. 
The fundamental would then be suppressed when 
Y cos (6+a) +Y ; = 0. 
If a = —11° (as in the case of the grid used in the experiment described above), then 
. V' 
9 will be 125° if ===== 0 -4. 
A further point in favour of this hypothesis is. that the angle at which the microphone 
must be tilted for the octave to be heard best depends to some extent on the intensity 
of the sound. 
On the other hand, if a jet were formed, then since the amplitude of the fundamental 
is approximately proportional to V 0 , the amplitude when 9 = 0 should be to the 
amplitude when 0 = tt in the approximate ratio 
Y + V' 
V - V'* 
And since we must have 
V' 
V 
about equal to 0 -4 in order to make 9 — 125 degrees when the octave is heard best, 
it follows that the microphone should be more than twice as sensitive to the fundamental 
vibration when it is held upright than when it is held upside clown. This is not borne 
out by experiment, which shows that the sensitivity is only slightly reduced by turning 
the microphone upside down (see fig. 15). 
Rayleigh* points to the near agreement between observed and calculated pitch in 
support of his view that jets are not formed “ to any appreciable extent at the mouths of 
resonators as ordinarily used.” The further argument, however, that “ the persistence 
of the free vibration . . . seems to exclude any important cause of dissipation beyond 
the communication of motion to the surrounding air,” does not apply to the resonators 
used in the present experiments, for it is shown in § 4 that the dissipation caused by the 
communication of motion to the surrounding air is negligible compared with the total 
amount of dissipation which occurs. 
It appears, therefore, that the jet hypothesis, while offering a plausible explanation 
of the suppression of the fundamental at such large values of 9 as 125 degrees, is open to 
objection on account of 
(i) The nearly equal sensitivity shown by the microphone in the erect and inverted 
positions ; and 
(ii) The near agreement between observed and calculated pitch (see § 4). 
* Loc. cit., p. 217. 
