The next important step in spinning was the introduction of drawing rolls, which were a series of rolls running at different speeds for drawing out or elongating the roving as it was spun into a thread. This was mainly due to Arkwright, a contemporary of Hargreaves. The principle of the drawing rolls had been foreshadowed in the British patents of Louis Paul, No. 562, of 1738, and No. 724, of 1758, but Arkwright made the first embodiment of it in practically useful machines, which were covered by him in British patents No. 931, of 1769, and No. 1,111, of 1775. Arkwright’s spinning machine is shown in Fig. 286, the drawing rolls being shown at the top of the figure. [Illustration: FIG. 286.--ARKWRIGHT’S ROLL-DRAWING SPINNING MACHINE.] Following these important inventions came the mule spinner. This was invented by Crompton between 1774 and 1779, but was never patented. It combined the leading features of Hargreaves and Arkwright. The spindles were mounted on a wheeled carriage that traveled back and forth a considerable distance from the drawing rolls, which were mounted in bearings in a stationary frame. The long travel of the carriage back and forth, and the simultaneous twisting and drawing of the yarns, produced threads of great fineness and regularity. The value of the long travel of the carriage may be briefly noted as follows: When the threads or slivers emerge from the drawing rolls they are not absolutely of uniform size, and the thick portions do not twist as tightly as the thinner portions. The stretching and drawing of these thicker parts down to a uniform size by the receding of the carriage is the distinctive feature of its action. As the thread has greater tensile strength at the thinner hard-twisted parts than it has at the thicker untwisted parts, it will be seen that the stretching action is localized on the thicker untwisted parts of the thread, which are thus brought down to uniform size by elongation. The drawing and twisting of the thread is effected as the carriage runs out, and when the carriage runs in these twisted lengths are wound around the spindles. The rendering of the action of the mule automatic or self-acting in its travel back and forth was the invention of Richard Roberts, of England, and was covered by him in British patents No. 5,138 of 1825, and No. 5,649 of 1830. The mule spinner shown in Fig. 287 is a good modern example of this machine. [Illustration: FIG. 287.--MULE SPINNING MACHINE.]