Assuming that a "Cop" of yarn (see Fig. 27), showing the cops on the spindles, has been partly made upon each spindle, the roving or thread from the rollers would extend down to the cop and be coiled round the spindle upwards up to the apex. The spindle would probably twist the thread for 40's counts twenty-three or twenty-four times for each inch that issued from the rollers, there being a well-recognised scale of "twists per inch" for various sorts and degrees of fineness of yarn. Unlike the bobbin and fly frames, the roving or yarn is not wound on its cop or spindle as it is delivered, but a certain definite and regulated length of cotton is given out to each spindle, and fully twisted and attenuated before it is wound into a suitable shape for transit and for subsequent treatment. To keep each thread in tension, therefore, as it is delivered from the rollers, the carriage containing the twisting spindles is made to recede quickly away from the rollers, a common distance for such movement being 64 inches. All the time the spindles are quickly revolving and putting twist into the rovings, thus imparting strength to them to a far greater degree than at any previous stage. Often the carriage is made to recede from the rollers a little quicker than the latter, the difference in the surface speeds between the two being technically known as "_gain_." The object of this carriage "gain" is to improve the "evenness" of the yarn by drawing out any thick soft places there may be in the length of thread between each spindle and the roller, a distance of 64 inches. It is a property of the twist that it will run much more readily into the thinner portions of thread than the thicker, thus leaving the latter capable of stretching out without breaking. Arrived at the limit of 64 inches stretch (see Fig. 27), certain rods, levers, wheels and springs are so actuated that the parts which draw out the carriage and cause the rollers to revolve are disconnected, so that both are brought to a standstill for the moment. In many cases the spindles at this stage are kept on revolving in order to put in any twist that may be lacking in any portion of the stretch. Twisting being finished, the important operation of "backing off" commences. It maybe at once explained that "backing off" means the reversing of the spindles; the uncoiling of a portion of the yarn from the spindles; and generally putting all the requisite apparatus into position ready for winding or coiling the attenuated and twisted rovings upon the spindles. Here come now into action those most beautiful and ingenious applications of mechanical principles, the working out of which entailed so many years of arduous effort, and which rendered the mule practically self-acting and automatic.