The length of colour to be printed on the warp thread must, of course, bear an accurate relation to the height of the wire used and the number of wires per inch. This is a matter of calculation. The length of yarn consumed in 1 inch is determined. This length, divided by the number of wires, or loops per inch, gives the length of warp yarn to be coloured for each wire. Thus, for a carpet of nine wires to the inch, 3 inches of warp yarn are required for each inch of the carpet. The length of each unit of colour to be printed is, therefore, 1/3 in. The mechanism of the printing drum is adjusted accordingly, and can be varied for different qualities. The normal yarn used in Tapestry varies in count from 10s 2 × 2 to 16s 2 × 3 worsted. It is scoured and dried, and then wound from hanks on to large bobbins. A set of six of these bobbins, or more or less, filled with the white yarn, is placed vertically on a stand in front of the printing drum; and the six ends of worsted are attached at equal intervals to a triple thread of worsted stretched across the width of the drum. The threads, properly tensioned, pass through guides, and are wound on to the face of the drum, the guides being mounted on a rod which is slowly moved laterally in such a manner that the threads are laid side by side on the surface of the drum. The revolution of the drum continues until the whole face is covered, or until the required length of yarn has been wound. In the latter case, fewer bobbins are put on, and the drum is left partly uncovered. To fill a drum of ordinary size, then, there will be required 1,176 threads, which will be obtained from 196 revolutions. Each of the six divisions is called a hank, and is tied separately. Whatever the number of threads and length of the hank, the whole length of the yarn on the drum represents one warp thread only. The printing drum is a large roller constructed with a wooden or tin face supported upon an iron frame and spokes, the central shaft and external mechanism being carried upon iron framework. Drums are of various sizes, varying in circumference from 12 ft. 6 in. to 40 ft., and in width of face from 18 in. to 72 in., capable of carrying from 700 to 1,200 threads side by side. The most usual width is 30 in. The size of drums is reckoned by the number of “scrolls” or “types,” that is, lines of colour, they will carry. These are commonly 216, 324, 432, 648, 864, or 1,072 scrolls; but an average size is 648.