The object of dyeing machines, of course, is to effect the same treatment of the yarn as is done by hand as just described, but in a more expeditious way. A machine can, in fact, do nearly twice as much as a man, and in a more regular manner. That is to say, one man can mind a machine of the same capacity as a hand vat which requires two men; but it is desirable, if not, indeed, necessary, that he should have assistance in loading and unloading. Given the necessity for keeping the yarn in motion while immersed in the dye liquor, the problem of the designer of dye machines has been to find the most simple and effective way of doing this, keeping in view the need for convenience in preparing the yarn for the bath, dropping it in, and removing it. Dyeing machines are of two main types: those which move the yarn through the liquor, and those which move the liquor through the yarn. Each type has in common a frame of some kind to carry the dye-sticks on which the yarn is hung; and practically every type of frame is provided with sticks to support the skeins at one or both ends. Of the former and earlier type the movement of the frame carrying the yarn in the liquor may be effected in various ways. The frame in one machine is revolved in the vat upon a circular axis; in another the revolution is combined with a rocking motion; in a third a vertical movement is obtained by means of a hydraulic pump fitted with an automatic reversing mechanism. In each case the problem of moving the yarn in the liquor is complicated by the minor problems of moving the yarn on the sticks so as to avoid stick marks, and of raising and lowering the frame, or of stripping it in position, if irremovable. The former is solved by some worm or ratchet contrivance for automatically turning the dye-sticks, and the latter by cranes or tackle, or, as in one of the makes just mentioned, by a hydraulic pump.