The first advance over the old-fashioned spinning wheel, which dates back to the fifteenth century, was in 1770, when the first spinning jenny was invented by Hargreaves. This consisted practically of a multiple spinning wheel by which one man could spin a large numbers of bobbins of yarn at the same time. It was at about the same time that Arkwright invented the drawing rolls which have played a most important part in the preparation of yarn, and this invention is worthy of our attention because it contains an interesting mechanical principle. As shown in Figure 66, a number of pairs of rolls are provided through which the roving passes, but successive pairs operate at higher velocities. Thus, the second pair of rollers through which the roving passes run at a little higher speed than the first pair, the third a little higher than the second pair, and so on. As a result, the roving is drawn out by the operation and issues from the last pair of rollers at a higher speed than it entered the first pair of rollers. The only way in which it can accommodate itself to this accelerated motion is to be attenuated or drawn out. Weights are used, as shown in the drawing, to press the upper rollers against the lower ones. THE SPINNING MULE Shortly after Arkwright’s invention came the mule spinner, invented by Crompton between 1774 and 1779. Machines operated on the same general principle as this are in general use to-day. In the old-fashioned method of spinning by hand the worker took a small quantity of cotton, pulled it out into a long sliver, attached one end to a bobbin and gave the bobbin a twirl between his hands in order to spin the fiber into yarn; then the yarn was wound up on the bobbin and the process was repeated. The spinning mule does practically the same thing, but infinitely faster and on a much larger scale. As shown in Figure 67, the bobbins of roving (_A_) are mounted on a stand and passed through a set of drawing rolls (_B_) which are regulated to pull the rovings out to the desired thickness of yarn. The roving then passes to the nose of a “cop,” or spindle (_C_), which is revolved at very high speed. The cop is carried by a carriage (_D_) which moves away from the bobbins of roving to the position indicated by dotted lines, while the cop is revolving. The cop has a travel of about five feet during the time the yarn is drawn out and spun. Then the carriage moves back toward the stand upon which the bobbins of roving are mounted and the spun yarn is wound up on the cop. The reason the yarn does not wind up on the cop while it is spinning is because it runs to the nose of the cop and, at each turn of the cop, the coil twists on the nose and slips off. On the return of the carriage, however, a set of wires (_E_), called “fallers,” press the yarn down so that the revolving cop will wind up the slack. [Illustration: FIG. 67.--DETAILS OF A SPINNING MULE]