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to the experimenter. The latter sits at ease with his hand in an unconstrained position, and
lifts the weights in turn between his -finger and thumb, the finger pressing against the top,
the thumb against the bottom of the cartridge. Guided by the touch alone, he arranges them
in the tray in what he conceives to be their proper sequence; he then returns the tray to the
operator, who notes the result, the operator then reshuffles the weights and repeats the
trial. It is necessary to begin with coarse preparatory tests, to accustom the operatee to the
character of the work. After a minute or two the operator may begin to record results, and
the testing may go for several minutes, until the hand begins to tire, the judgment to be
confused, and blunders to arise. Practice does not seem to increase the delicacy of
perception after the first few trials, so much as might be expected.
The base of the inner tube of the whistle is the foremost end of a plug, that admits of
being advanced or withdrawn by screwing it out or in; thus the depth of the inner tube of
the whistle can be varied at pleasure. The more nearly the plug is screwed home, the less is
the depth of the whistle and the more shrill does its note become, until a point is reached at
which, although the air that proceeds from it vibrates as violently as before, as shown by
its effect on a sensitive flame, the note ceases to be audible.
The number of vibrations per second in the note of a whistle or other “closed pipe”
depends on its depth. The theory of acoustics shows that the length of each complete
vibration is four times that of the depth of the closed pipe, and since experience proves
that all sound, whatever may be its pitch, is propagated at the same rate, which under
ordinary conditions of temperature and barometric pressure may be taken at 1120 feet, or
13,440 inches per second,—it follows that the number of vibrations in the note of a whistle
may be found by dividing 13,440 by four times the depth, measured in inches, of the inner
tube of the whistle. This rule, however, supposes the vibrations of the air in the tube to be
strictly longitudinal, and ceases to apply when the depth of the tube is less than about one
and a half times its diameter. When the tube is reduced to a shallow pan, a note may still
be produced by it, but that note has reference rather to the diameter of the whistle than to
its depth, being sometimes apparently unaltered by a further decrease of depth. The
necessity of preserving a fair proportion between the diameter and the depth of a whistle is
the reason why these Previous page Top Next page