ome rather elderly and self-satisfied personages. They
are indignant at being thought deficient in the power of hearing, yet
the experiment quickly shows that they are absolutely deaf to shrill
notes which the younger persons hear acutely, and they commonly
betray much dislike to the discovery. Every one has his limit, and
the limit at which sounds become too shrill to be audible to any
particular person can be rapidly determined by this little instrument.
Lord Rayleigh and others have found that sensitive flames are
powerfully affected by the vibrations of whistles that are too rapid
to be audible to ordinary ears.

I have tried experiments with all kinds of animals on their
powers of hearing shrill notes. I have gone through the whole
of the Zoological Gardens, using an apparatus arranged for the
purpose. It consists of one of my little whistles at the end of a
walking-stick--that is, in reality, a long tube; it has a bit of
india-rubber pipe under the handle, a sudden squeeze upon which
forces a little air into the whistle and causes it to sound. I hold
it as near as is safe to the ears of the animals, and when they are
quite accustomed to its presence and heedless of it, I make it sound;
then if they prick their ears it shows that they hear the whistle; if
they do not, it is probably inaudible to them. Still, it is very
possible that in some cases they hear but do not heed the sound. Of
all creatures, I have found none superior to cats in the power of
hearing shrill sounds; it is perfectly remarkable what a faculty
they have in this way. Cats, of course, have to deal in the dark
with mice, and to find them out by their squealing. Many people
cannot hear the shrill squeal of a mouse. Some time ago, singing
mice were exhibited in London, and of the people who went to hear
them, some could hear nothing, whilst others could hear a little, and
others again could hear much. Cats are differentiated by natural
selection until they have a power of hearing all the high notes made
by mice and other little creatures that they have to catch. A cat
that is at a very considerable distance, can be made to turn its ear
round by sounding a note that is too shrill to be audible by almost
any human ear. Small dogs also hear very shrill notes, but large
ones do not. I have walked through the streets of a town with an
instrument like that which I used in the Zoological Gardens, and
made nearly all the little dogs turn round, but not the