The speed of tappets is an important subject for calculation in connection with the loom. When the tappets are on the tappet shaft they are always plain, and are driven at half the speed of the crank-shaft, in consequence of the latter shaft only representing one pick, while the tappet shaft,[3] carrying two picking plates, represents two picks. Should the tappets be on the twill shaft, and driven from the tappet shaft, then the calculation is simple--_e.g._, say four picks to the round are required, then the crank-shaft must revolve four times for the twill shaft once. The tappet shaft must revolve half as many times as the crank-shaft--that is, twice--and the ratio between the speeds is as 2 to 1, which is also the ratio of the wheels, say 16 and 32, or 20 and 40, the larger wheel being on the twill shaft. In this case the rule is to divide the number of picks to the round by 2, which will give the ratio of the wheels gearing the tappet and twill shafts. Occasionally, the shaft carrying the tappets is driven from the crank-shaft direct, and the ratio of the gearing will be as one is to the number of picks to the round--say 6 picks to the round--then such a pair as 12 and 72, or 15 and 90, will be required. Obviously, for a large number of picks, an intermediate pair would have to be inserted--say 13 picks to the round must be woven, the wheel on the crank-shaft being 25 and the last wheel on the same shaft as the tappets is 65. Then, to get the size of the pair of intermediate wheels on the stud, multiply 13 by 25 and divide by 65, which will give the ratio of the size of the two wheels:-- (13 × 25)/65 = 325/65 = 65/13 These, or a multiple of these, are the wheels required, which may be proved:-- (65 × 65)/(25 × 13) = 13 picks to a round. [3] The term tappet shaft is used here in the sense understood in North Lancashire--_i.e._, the shaft driven from the crank-shaft and carrying the picking plates and tappets if plain. SPLIT CLOTHS AND MOTIONS. Split cloths are sometimes woven in the ordinary loom when narrow widths are required and a perfect selvage is not a necessity. Thus, a 40-inch loom may weave a cloth with a few empty dents at the centre, whilst on each side of the space thus caused the warp threads are crossed (on the gauze principle) between each pick to make a firmer selvage than could be got by ordinary lifting. The weft threads are then slit along this space by a sharp knife, giving two cloths, each 20 inches wide. One system of forming the selvage is by having an extra loop attached to the top stave of one heald, and carrying a thread which is drawn through the other heald. By correct drafting this thread can be made to lift at every pick first on one side of its neighbouring warp thread and then on the other.