Each alternate spoke is now driven in by the blows of a mallet to a perfectly close bearing of the shoulder of the tenon, the workman guiding it as best he can. But it is evident that the position that the spokes will take is by no means certain. Owing to the wedge-like form given to the tenon, the spokes are driven home very tight, and wood not being of a homogeneous texture will yield more in one part than in another; and the mortise, cut in the way that it is, must be to some extent uncertain. Every alternate spoke being driven, or those in the same plane, the remainder are driven in between them in the same manner. [Illustration: Fig. 14.] In Fig. 13, _b_, _b_, _b_, are the mortises for the face spoke, and _c_, _c_, _c_, the mortises for the back spoke. Fig. 14 shows a section of an ordinary spoke, the hatched part showing the form of the greater part of its length, and the plain lines completing the rectangle show the extent of the swelling at the shoulder of the tenon at the nave. [Illustration: Fig. 15.] Fig. 15 shows a very handy adjunct to the wheelwright’s shop; it is called the “centring square.” It is found extremely useful in marking and setting out the mortises for the spokes. Its construction is very simple, being but a T square, whose stock is the segment of a circle. A is the blade, B the circular stock, the extremities of which should be protected by steel or brass, or, better still, have a steel edge round the whole of the inner surface of the stock, so that it will always keep true; for if it wears at all, of course its true circular form will be destroyed, and it will be rendered useless. And another important thing should be borne in mind, and that is to make the upper edge, C, of the blade in a line with the centre of the curve from which the circular head is struck; the reason of this will be apparent to the most obtuse, for unless the lines radiate from the given centre it is useless for the wheelwright’s purpose.