[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 41⁄2 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.
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.
| Fig. 375.—The Three Central Bars in Position. | 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 length, width, and thickness. They are also arranged so that in each arm may be clasped one piece each of [Figs. 370, 371, and 372]. The three central arms may, of course, be set up in a different order, and here we have merely chosen the way that is the most simple to describe and illustrate.