Two or more rovings are generally united in this operation. Thus, suppose that two rovings are introduced together between the first pair of rollers, and that the second pair of rollers moves with twice the velocity of the first. The new roving thus formed by the union of the two will then be of exactly twice the length of either of the original ones. It will therefore contain exactly the same quantity of cotton per yard. But its parts will be very differently arranged, and its fibres will be drawn out longitudinally, and will be thus much better fitted for forming a thread. This operation of doubling and drawing is repeated as often as is found necessary, and the requisite degree of twist is given by a machine similar to the spindle and fly of the common flax-wheel.
The spinning-mule, invented by Samuel Crompton, carried the mechanism of the cotton-factory many steps in advance. Long after Crompton, came the self-acting mule. It is a carriage some twenty or thirty feet long, travelling to and fro, and drawing out the most delicate threads through hundreds of spindles, whirling at a rate which scarcely permits the eye to trace their motion. Mr. Whitworth says,--"So great are the improvements effected in spinning machinery, that one man can attend to a mule containing 1088 spindles, each spinning 3 hanks, or 3264 hanks in the aggregate per day. In Hindustan, where they still spin by hand, it would be extravagant to expect a spinner to accomplish one hank per day; so that in the United States [and in Great Britain also] we find the same amount of manual labour, by improved machinery, doing more than 3000 times the work."
[Illustration: Samuel Crompton, inventor of the spinning-mule.]
Of the rapidity with which some portions of the machinery operate, we may form an idea from the fact that the very finest thread which is used in making lace is passed through the strong flame of a lamp, which burns off the fibres without burning the thread itself. The velocity with which the thread moves is so great, that we cannot perceive any motion at all. The line of thread, passing off a wheel through the flame, looks as if were perfectly at rest; and it appears a miracle that it is not burnt.