re end of the steel shaft into the square hole
in the block, and, through a hole carefully drilled for the purpose, put a
screw down through the hole in the end of the steel shaft (Fig. 224); this
will firmly fix the block on the end of the knob. Of course, the knob must
be inserted through the door before the block is permanently fastened
upon the end of the shaft. Fig. 225 shows the edge of the door with the
three knobs in place. If these knobs are so turned (Fig. 226) that their
flat edges are parallel with the crack of the door, there is nothing to
prevent you from opening the door; but if the knobs are so turned (Fig.
227) that the blocks overlap the crack of the door, the door cannot be
opened without breaking the lock.

It is evident that we must have some sort of a mark to tell us how to make
the proper combination so that the door may be opened. To do this, take
the metal washer of the door-knob (the upper figure in Fig. 228) or a
circular piece or disk of tin and divide it up like a compass (Fig. 228).
Fasten these disks securely on to the door with nails or screws; place all
of the disks with the north point pointing to the top of the door and in
line with each other. File in the circular base of each door-knob (Fig.
224) a little notch at the black mark where the finger is pointing, then
put the door-knobs in place and fasten them there (Fig. 225) by screwing
the block on their ends (Fig. 224) and securing the screws in the blocks
by running them through the shaft. Carefully turn the knobs so that the
block on the inside fits like those shown in Fig. 226. Jot down in your
notebook the position of the index on each knob (finger point, 224); one
may read northeast, another may read southwest, and another may read
south. When one wants to open the door one must turn the knobs so that
they will read according to the notes and the door may be opened; but
unless the indexes read as noted some of them will be turned as in Fig.
227, locking the door, and it may not be opened.

When the door is closed, twist the knobs around and it will lock them so
that no one else can open the door unless they know the combination. The
fact that there _is_ a combination will not be suggested to a stranger by
the compasses, although it might be suggested if there were figures in
place of compass points. But even supposing they did suspect a combination
it would take a long time for them to work it out, and no one would do it
but a t