The water-frame spun twist for warps, but it could not be advantageously used for the finer qualities, as thread of great tenuity has not strength to bear the pull of the rollers when winding itself on the bobbins. This defect in the spinning machinery was remedied by the invention of another machine, called the Mule, from its combining the principles of Arkwright's water-frame and Hargreaves's jenny. Like the former, it has a system of rollers, to reduce the roving; and, like the latter, it has spindles without bobbins to give the twist, and the thread is stretched and spun at the same time by the spindles, after the rollers have ceased to give out the rove. The distinguishing feature of the mule is, that the spindles, instead of being stationary, as in both the other machines, are placed on a moveable carriage, which is wheeled out to the distance of fifty-four or fifty-six inches from the roller-beam, in order to stretch and twist the thread, and wheeled it again to wind it on the spindles.

The Mule is a compound of the Jenny and the Water Frame, from which circumstances it derives its name, and was invented in 1775, by Samuel Crompton, of Bolton-le-Moors. In this machine, the roving passes from the back part through rollers to the spindles, which are placed in front on a moveable frame. As the spindles revolve, this frame recedes from the rollers, somewhat faster than they give out the roving. The first pair of rollers draw the roving from the bobbin, the second pair draw it out and lengthen it, as in the Water Frame, and the pull of the spindles as the frame recedes, stretches it still finer. When a certain quantity of roving is giving out, the rollers stop and shut fast the roving, as the clove does in the Jenny, the spindles still continuing to revolve and the frame to recede, drawing out the roving to the fineness required and giving it the necessary twist, the yarn is then wound upon the spindles by returning the frame to the first position.