In shedding, the tappets may sometimes be placed on a third shaft, driven from the tappet shaft, and termed a twill shaft, because by altering the tappets and the speed the loom may be made available for weaving twills. When the tappets are fixed to the twill shaft they are smaller than those fixed to the tappet shaft. SETTING OUT TAPPET. The setting out of a tappet is an important problem. Two circles must be described round the same centre, the difference in the radii being the stroke of the tappet--thus, say A B or C D in Fig. 28 is 3-1/2 inches, then that is the stroke of the tappet. This is also the radius of the smaller circle. The large circle must now be apportioned according to the number of picks to the round in plain cloth, say two. The circle is then divided into two parts by E F. Suppose we wish the healds to be still during two-thirds of a revolution of the crank-shaft, then as E A F represents a whole revolution, divide it into six parts where marked, place four of these parts from G to H for the dwell, and leave E G to form part of the lifting, and F H part of the depression of the healds. [Illustration: FIG. 28.] The movement of the rising healds commences, however, before the falling heald has come to a pause; therefore we must trespass into the lower half to an extent equal to E G and H F, thus obtaining by drawing a diameter through G and the centre, also through H and the centre, the parts G J and H K for the rise and fall respectively. Divide C J now into six parts by radii to the centre, also divide by arcs of circles transversely, to these radii, the space between the small and large circles into six parts, then by drawing a line diagonally through the figures thus formed, we get a line G M giving an easy fall from the large circle to the small one, and by similar treatment a rise on the opposite side from N to H. This is the theoretical construction of a plain tappet, from M to N being the dwell from the heald when up. N to H the depression, and H to G the dwell for the heald when down; and it will be noted, that although G H is apparently larger than N M, the time of dwell is the same in each case, in consequence of the arc N M being so much nearer the centre. In practice, the hollows N and M may be found rather fuller than shown here. This method of construction applies to tappets with an increased number of picks to the round, a point which will be found described in Chapter VI.