Meperîments on the highest audible tone.
107
mean variation for a set of results seldom exceeded 5 per cent, it may
well be supposed that the constant of precision of the measurements
was in this case determined by technical errors and not by psycho¬
logical variations. In future experiments it will be necessary to main¬
tain the room at an even temperature during each set of experiments ;
and to calculate the pitch with the appropriate value of v for each
change, possibly also to be on guard against sudden barometric
changes.
It may seem remarkable that we should have neglected to record
the temperature of the room. We give the following as reasons : 1.
we did not expect after the elimination of the error of air-pressure
to find the psychological sources of error smaller than the technical
ones ; 2. the temperature of the air has not been regarded in pre¬
vious experiments ; 3. it is not the custom of psychological labora¬
tories to pay attention to the psychological and instrumental errors
due to changes in temperature.
Several means of blowing the whistle have been employed.
Galton1 used a single rubber bulb. Zwaardemaker2 used a fun¬
nel with a rubber membrane stretched across the large opening, the
smaller end being connected with the whistle by a rubber tube ; the
funnel was depressed through a constant distance. These methods
are none of them strictly reliable. The pressure is intermittent and
cannot be accurately regulated. The only possible method of obtain¬
ing a current of air of constant intensity, seems to be by use of a
rotary-fan blower. This method has been previously described and
tested.3 The source of power used by us was an electric motor run
by the city-current ; there were no perceptible fluctuations in
speed. The fan-wheel of the blower made from 13 000 to 15 000
revolutions a minute. The blast was carried by a rubber hose into
a room in another part of the laboratory ; thus all noise from the
machinery was avoided. The hose led to a rubber tube, in which
was a stop-cock. The rubber tube ended in a glass T-coupling with
a rubber tube on each end of the cross-arm. One of these led to the
whistle, the other to a water-manometer. The manometer scale was
graduated to millimeters ; the height of the column of water gave the
1 G Alton, Whistles for audibility of shrill notes; Inquiries into Human Faculty,
38, New York 1883.
2 Zwaardemaker, Der Umfang des Gehörs in den verschiedenen Lebensjahren,
Zt. f. Psych, u. Phys. Sinn., 1894 VII10.
8 Scripture, A constant blast for acoustical purposes, Am. Jour. Psych., 1892
IV 582.