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.

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 projectionsaof X face upwards, and that the projectionsaof 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 remainabovethe slot of X. Then slide the bar Y along to the centre, so that the part letteredcslips into the little slot of bar Z.

Fig. 376. - How to Adjust the Bars. (Note position of projecting ends,a.)

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.

Second Stage

In the remaining part of the work the chief difficulty is to keep the puzzle from falling to pieces before the key finally locks it. Take the longer cross parts, Fig. 370 , and clasp one to each arm. The six need not all be put on meanwhile, but only those which are most easily handled. The next size ( Fig. 371 ) may then be put on.

In the ordinary course each arm could be completed with its three cross pieces till the sixth was attempted, and here the reader would find that, at the last moment,his attempt was frustrated. He could not get the last small piece in, as other bars lock the puzzle. Here it is that the "key" comes in.