Working on the centre S the eccentricity obtained is employed in depressing the healds, but by an intermittent movement. The shed must be opened quickly to economise time, but must be left open sufficiently long to enable the shuttle being picked through. A reference to the shape of the plate in Fig. 28 will show this. From N to H is the portion for depressing the heald, from G to H being an arc of a circle and the part by which the heald is held stationary, while G M corresponds inversely with F N, and allows the heald to rise. It is necessary for the eccentricity to be equal at each shed, a point which may be tested by noticing if the sway of the beam is the same at each shedding. To get this, the treadles must be level when the healds are, and the centre of the bowl directly under the centre of the shaft. The bowl should be in contact with the tappet during the whole revolution, and thus give an easy tread--jerky motion in either treading or picking being very detrimental to good weaving. A good shed should be almost as large as the shuttle at the point where it passes through, quite clear from obstructions of any kind, and the lower half of it not too low, or, as it is termed, not “bottoming” too much, for when too low the warp is frayed by the movement of the slay. A point regarding which weaving technologists often enter into profuse arguments and diagrammatic illustration, may here be summed up in a few words. The line of warp when the healds are level should be below a line drawn from the temple to the back rest, and the lower the healds are in this respect the better is the cover on the cloth, although attention must be given to the remarks in a previous paragraph regarding bottoming. Whether the lowness of the warp line is produced by depressing the healds or raising the back rest is immaterial. When plain cloth is woven in a twill loom, the tappets act above the centre of the treadle, and of course are of smaller size, the healds being connected to the end of the lever opposite to the fulcrum.