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.

4. The back check, hammer stem and regulating button are subject to the same faults as their counterparts in the upright, which may be remedied in the same way.  Bridles and hammer springs are not needed in the square, as the weight of the hammer, moving in a vertical direction, is sufficient to bring it to its rest position.

5. Hammers, when made of felt, will of course require the same treatment as those in the upright.  In many old squares the hammers are built up of buckskin.  If this becomes beaten down hard, it is well to cap the hammer with a new soft piece of buckskin, gluing only at the back ends.

6. Butts and Flanges.—­A click just as the key comes up, indicates that the felt cushion, against which the jack rests, is worn out and must be replaced.

In all square actions the center pin, in the butt, is held by friction alone, but rarely gets loose; if it should be found loose, put in a larger pin.

The flange, shown in the cut, is what is called a split flange.  By the set screw T, the jaws can be regulated so that they neither clamp the center pin so tightly as to make the action sluggish nor so loosely as to let the hammer wabble.

If the bushing cloth is found to be badly worn, it is better to put in new, which must be done neatly, or the result will not be satisfactory.

Hammer flanges, like all other wooden parts, shrink away from the screw heads and allow the hammer to drift to one side or rattle.  While the action is in the piano, strike the keys to see if there are any that strike improperly.  Mark the keys so as to indicate just what the trouble is, so that you will know how to remedy it when the action is out.  If the hammers are set so close that they rub against each other, you may have to cut off a slight shaving of felt, but this is rarely necessary; for if properly placed, there is generally room for all; yet sometimes the expansion of the felt or warping of the shank makes cutting necessary.

7. Dampers.—­The dampers in the square action depend entirely upon their weight for their efficiency in damping the strings and returning after being raised by the key.  Often, after the key is struck, the damper will not return to its place and the string is undamped.  This is generally found to be caused by the wire sticking in the hole through which it passes, the wire being rusty or bent