until the jack falls out of place; then by lifting the key off the
balance pin it can be drawn out. The back check will sometimes rub so
hard against the regulating button that it will be bent somewhat, and
must be adjusted after the key is replaced.

The bottom is often found to have shrunken; it rattles at every stroke
of the key. This can generally be stopped by simply turning the back
screw down until tight, which can be done without taking the key out.
This will rarely be found to alter the jack enough to cause it to fail
to return to the notch in the butt. After doing this, however, it is
well to examine for such a condition.

A sluggish motion of the jack is often found in old square pianos
caused by the swelling of the wood, at the point where the jack is
hinged to the bottom, or by the center pin's becoming foul from oxide.
This will cause the jack to fail at times to operate on the hammer,
especially in quick repetition. The key is struck with no response.
Take out the bottom entirely, and with the fingers press the sides of
the bottom inward; at the same time, work the jack back and forth.
This will generally free it if the jack-spring is all right.

3. _Jack_.--As in the upright, the behavior of the jack depends
entirely upon the surrounding members. A very common occurrence in the
square piano is a broken jack-spring. This spring is concealed in a
groove on the under side of the bottom, with a linen thread leading
around the end of the jack and held fast by a wooden plug. If the
spring is found to be long enough, drive out the plug, attach a new
thread to the spring, and fasten as before. If a new spring is needed,
one may be made by wrapping some small wire round a piece of music
wire of the right size.