strings may be released so as to be
free to vibrate.

Other interesting details are:

1. The strings are stretched over a thin sheet of wood called
the sound-board. This aids greatly in intensifying the tone.

2. The soft pedal (the one at the left) in an _upright piano_
causes the hammers to move up nearer the strings, and the
shorter swing thus afforded causes a less violent blow and
consequently a softer tone. In the _grand piano_ this same
pedal shifts the mechanism to one side so that the hammers
strike only one or two of the strings, this resulting in a
softer tone of somewhat modified quality.

These details regarding the mechanism of the piano can easily be
verified by removing the front of any ordinary upright piano and
observing what takes place when the keys are struck or the pedals
depressed.

3. There are two familiar types of _organ_ in use at the present time,
(1) the reed organ, (2) the pipe-organ.

The _reed organ_ is very simple in construction, the tone being produced
by the vibration of metal reeds (fixed in little cells), through which
air is forced (or sucked) from the bellows, the latter being usually
worked by the feet of the player. More power may be secured either by
drawing additional stops, thus throwing on more sets of reeds, or by
opening the knee swells which either throw on more reeds (sometimes
octave couplers) or else open a _swell box_ in which some of the reeds
are enclosed, the tone being louder when the box is open than when
closed. More tone may also be secured by pumping harder.

4. The essential characteristic of the _pipe-organ_ is a number of sets
or registers of pipes called _stops_, each set being capable (usually)
of sounding the entire chromatic scale through a range of five or six
octaves. Thus for example when the stop _melodia_ is drawn (by pulling
out a stop-knob or tilting a tablet), one set of pipes only, sounds when
the keyboard is played on: but if the stop _flute_ is drawn with
_melodia_, two pipes speak every time a key is depressed. Thus if an
organ has forty _speaking stops_, all running through the entire
keyboard, then each time one key is depressed forty pipes will speak,
and if a chord of five tones is played, two hundred pipes will speak.
The object of having so many pipes is not merely to make possible a very
powerful tone, but, rather, to give greater variety of tone-color.

The pipe-organ usual