But although the factory as a whole be driven by its steam engine, yet either some of the individual machines may require the aid of the workman for some of their movements (such aid was necessary for the running in of the mule carriage, before the invention of the self acting mule, and is still necessary in fine spinning mills); or, to enable a machine to do its work, certain parts of it may require to be handled by the workman like a manual tool; this was the case in machine makers' workshops, before the conversion of the slide rest into a self actor.