(234) It has been pointed out that the action of the rollers in attenuating the sliver is identical with that of those in the drawing frame, so that no special description need be given of them. But these machines are the first in which the process of twisting is carried out, and the rollers form an important part of the mechanism for this purpose. In introducing twist into any strand or sliver it is necessary that one end of it should be held, while the other is also held and turned at a higher or lower speed. If this is done the strand will be twisted, and the amount of twist is strictly defined by the number of times it is turned. In actual working it is not practicable to continue so to twist the strand without at the same time submitting it to the action of the spindles continuously. Unless it was so delivered it would be broken because of the shortening which takes place during twisting, and it is therefore necessary to furnish a fresh portion of the sliver to the action of the twisting mechanism. For this reason the sliver, while being firmly held by the nip of the front rollers, is also delivered by them at a definite rate, which depends on their size and rate of revolution. Now, assuming that no such delivery takes place, and that a length of sliver of 10 inches is turned 100 times, there would be in each inch of it 10 turns or twists. Suppose, now, that another 10 inches was delivered and the same number of turns made, a similar result would be obtained. It does not matter whether the delivery is constant or intermittent, provided only that the ratio of the length delivered and the number of revolutions of the twisting mechanism remain the same. Intermittent delivery would, however, be very inconvenient in practice in producing rovings, and thus it is requisite to provide for a steady and regular delivery of yarn, as well as a uniform speed of the spindle and flyer. Granting the attainment of these conditions, it is easy to define the amount of twist put into any thread, it being in the same ratio as the number of revolutions made by the twisting mechanism during the delivery of one inch of sliver or roving. Twists are always defined as being so many “turns” per inch, and are arrived at in the way just indicated. As a matter of fact there is a little slip in the rollers, which does not, however, to any large extent modify the rule enunciated. The constants in a machine of this kind are therefore the rate of revolution of the spindles and front rollers; and, generally speaking, the amount of twist increases as the roving becomes finer. This can be attained by an increase of spindle speed or a decrease of that of the rollers, as will be readily understood, but considerations of a mechanical nature generally lead to the latter course being pursued. The spindle speeds in a slubbing intermediate, and roving frame, dealing with the same class of roving, would be approximately 700, 800, and 1,100 revolutions per minute. A table of productions, speeds, etc., is given on page 174, which will throw some light on this point.