Since every complete layer of rove is wound on the bobbin in virtue of the joint action of the spindle and flyer, the rotating bobbin, and the builder, each complete traverse of the latter increases the combined diameter of the rove and bobbin shaft by two diameters of the rove. It is therefore necessary to impart an intermittent and variable speed to the bobbin. The mechanism by means of which this desirable and necessary speed is given to the bobbin constitutes one of the most elegant groups of mechanical parts which obtains in textile machinery. Some idea of the intricacy of the mechanism, as well as its value and importance to the industry, may be gathered from the fact that a considerable number of textile and mechanical experts struggled with the problem for years; indeed 50 years elapsed before an efficient and suitable group of mechanical parts was evolved for performing the function. The above group of mechanical parts is known as "the differential motion," and the difficulties in constructing its suitable gearing arose from the fact that the speed of the rove passing on to the various diameters must be maintained throughout, and must coincide with the delivery of yarn from the rollers, so that the attenuated but slightly twisted sliver can be wound on to the bobbin without strain or stretch. The varying motion is regulated and obtained by a drive, either from friction plates or from cones, and the whole gear is interesting, instructive--and sometimes bewildering--two distinct motions, a constant one and a variable one, are conveyed to the bobbins from the driving shaft of the machine. The machine illustrated in Fig. 18 is of special design, and the whole train of gear, with the exception of a small train of wheels to the retaining roller, is placed at the pulley end--that nearest the observer. The gear wheels are, as shown, efficiently guarded, and provision is made to start or stop the machine from any position on both sides. The machine is adapted for building 10 in. X 5 in. bobbins, i.e. 10 in. between the flanges and 5 in. outside diameter, and provided with either 56 or 64 spindles, the illustration showing part of a machine and approximately 48 spindles. The machines for rove (roving frames) are designated by the size of the bobbin upon which the rove is wound, e.g. 10 in. x 5 in. frame, and so on; this means that the flanges of the bobbin are 10 in. apart and 5 in. in diameter, and hence the traverse of the builder would be 10 in. The 10 in. x 5 in. bobbin is the standard size for the ordinary run of yarns, but 9 in. x 4-1/2 in. bobbins are used for the roves from which finer yarns are spun. When the finished yarn appears in the form of rove (often termed spinning direct), as is the case for heavier sizes or thick yarns, 8 in. x 4 in. bobbins are largely used.