es 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.

D.--WHISTLES FOR TESTING THE UPPER LIMITS OF AUDIBLE SOUND IN
DIFFERENT INDIVIDUALS.

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plug, that admits of being advanced or withdrawn by screwing it out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 instruments, having necessarily little depth, require to be
made with very small bores.

The depth of the inner tube of the whistle at any moment is shown by
the graduations on the outside of the instr