-1/2 ins. long, with a half-cut centre slot
similar to that of Fig. 370.

Fig. 372.--Six required, these being 1-1/2 ins. in length, and with slots
in the middle as before.

Fig. 373.--One of these last six requires special treatment, as it forms
the key block of the puzzle. After its slot has been cut, one half of the
narrow part must be sawn away, as shown in Fig. 373. The inner edge must
also be gently rounded. The special use of this vital piece, which we
will call the "key," will be fully explained presently.

Fig. 374.--Then, in addition to these, there are three central bars to
make. Like the other parts they are 1/2 in. by 1/2 in., but are each
4-1/2 ins. long, and are cut as shown in Fig. 374. The end projections
_a_ are 1/2 in. long, and the cut-away part is exactly half the depth of
the wood. Two of the three pieces (X and Y in Fig. 374) are similar, but
the slot _b_ of the third one (Z) is only 1/4 in. wide instead of 1/2 in.
As will be noticed, this 1/4-in. slot is _not_ in the centre, but
corresponds with the right-hand half of the larger slots of X and Y.

[Illustration: Fig. 374.--The Central Bars.]

In making these twenty-one pieces, what should be borne in mind is that
the different parts fit closely into each other. Consequently the slots,
in width, must be cut so as to grip the thickness of the wood; in depth
they must be exactly half this thickness.

FITTING THE PUZZLE.--The three central bars must first be joined, as
those form the skeleton framework of the structure. Fig. 375 shows them
in position, but as it is a puzzle in itself as to how they can be got
thus some explanation is necessary.

[Illustration: Fig. 375.--The Three Central Bars in Position.]

[Illustration: Fig. 376.--How to Adjust the Bars. (Note position of
projecting ends, _a_.)]

FIRST STAGE.--First take the bars X and Y (see Fig. 374) and arrange them
as shown in Fig. 376. It is most important that the projections _a_ of X
face upwards, and that the projections _a_ of Y face towards the centre.
Then take the bar Z and bring it flat into the slot of X. The little slot
of Z, however, must remain _above_ the slot of X. Then slide the bar Y
along to the centre, so that the part lettered _c_ slips into the little
slot of bar Z.

This may seem confusing to read, but it is easy to follow when the pieces
are in one's hand. The result of this rather clever arrangement is that
the six arms of Fig. 375 are all exactly the same le