The _throstle machine_ is usually made double, a row of bobbins, spindles, &c. occupying each side of the frame. The bobbins filled with rovings from the bobbin-and-fly frame, are mounted at the upper part of the frame in two ranges. The roving from each bobbin passes through three pairs of drawing rollers, where it is stretched out to the requisite fineness. On quitting the last pair of rollers, each thread is guided by a little ring or a notch of smooth glass, let into the frame, towards the spindles, which revolve with great rapidity, producing by the motion of their flyers through the air a low musical hum, which is supposed to have given the name of throstle to this machine. The roving, which may now be called yarn, passing through an eyelet formed at the end of one of the arms of the flyer, proceeds at once to other bobbins. The yarn is wound upon the bobbins by a curious contrivance. The bobbin fits very loosely upon the spindle, and rests on its end upon a kind of platform. The bobbin is not connected with the spindle, except by the thread of yarn, which has to be wound; therefore, as soon as the flyer is set spinning, the thread drags the bobbin after it, and makes it follow the motion of the spindle and fly; but the weight of the bobbin, and its friction on the platform, which is promoted by covering the end with coarse cloth, causes it to hang back, and thus the double purpose is served, of keeping the thread stretched and winding it on the bobbin much more slowly than the flyer revolves. The yarn is equally distributed on the bobbin by a slow up-and-down movement of the platform. [Illustration: Mule.] These effects are the same as were produced by the bobbin and fly-frame, but in the throstle they are attained by simpler means. In the former machine a distinct movement caused the bobbin to revolve quicker than the spindle. In the throstle the bobbin is made to revolve by the pull of the yarn, which is now sufficiently strong for the purpose; but the roving in the bobbin-and-fly frame would not bear the strain. The throstle is not often employed for very fine spinning, because fine yarn would not bear the drag of the bobbin; but in mule-spinning the yarn is wound at once upon the spindles without any strain. In the _mule_ the rolling is first drawn by the usual system of rollers, and then stretched by a moveable carriage. The effect of first drawing and then stretching is to make the yarn finer and more uniform, as will be explained presently. The spinning mule is the most interesting and impressive spectacle in a large cotton mill—on account of its vast extent, the great quantity of work performed by it, and the wonderful complication and ingenuity of its parts.