There are a considerable number of combing machines in use at the present time, but their differences are mechanical rather than in the function they perform. The Heilmann principle is the most commonly used in this country. Eight rolls from the ribbon-lapper are placed in separate rests, or heads, end to end, and each lap is fed through rollers between teeth of a very fine and rapidly oscillating steel comb. Every back and forth motion, known as a nip, delivers about half an inch of filmy sheet from which the short fibres have been combed out. The eight combed sheets are then once more condensed into a single sliver and coiled into a cylindrical can. [Sidenote: _Drawing_] [Sidenote: _Doublings_] Following the comb there are usually two drawing frames, each combining six slivers into one, and these are followed by the three or four roving frames as in the other process. In the ordinary process the last roving as it leaves the jack frame has been doubled 27,648 times; in the combed yarn there are 2,959,120 doublings before spinning begins. Spinning proper is done either on the mule or the ring spindle. Very little cotton is spun on mules in this country, although mules are extensively used in Europe. We shall concern ourselves here only with the ring spindle, and that in bare outline. (See also Part Two, Page 83). [Sidenote: _The Ring Frame_] The principle of the ring frame is very similar to that of the roving operations which immediately precede it. The thread is again drawn through two or three sets of rollers running at successively higher rates of speed, and then passes as shown on the accompanying sketch through a guide to a small metal loop, called the traveller, which runs around on a metal track or ring within which the spindle with its bobbin is revolving. Since the spindle pulls the traveller around after it, the yarn is twisted or spun as it is wound on the bobbin. Sometimes two spools of roving are spun into a single thread, but more frequently there is no combination. All the rings on one frame, usually about 256, are moved up and down together on their spindles, so that yarn will be wound evenly on the bobbin. [Illustration: _Ring Twister_] [Illustration: THE RING FRAME PRINCIPLE] [Illustration: _Yarn of Filling Bobbin, Warp Bobbin, and Spool_] [Sidenote: _Warp and Filling Bobbins_] Not only is a different bobbin used for spinning warp and filling yarns, but they are also wound differently on the bobbin. Warp yarn is wound evenly up and down the whole length of the bobbin, while the filling bobbins, which go straight from the spindle into the shuttle of the loom, are wound on in sections to facilitate rapid unwinding. [Sidenote: _Twisting_]