n, h, S m

When the cross-iron is opposite m — that is, S with m — then the screw will turn together with its nut, and the screw will pull no further. And were it not done so, everything would go to pieces. And when you wish to send this screw forward again, pull out the iron S and turn the handle h.

plane iron for planing iron.

You must further take care to arrange things so that, when the blade has passed entirely out of the drawplate, the screw still remains with as many teeth in its nut as are exactly the teeth of the nut; and that the screw then, without further effort at the shoulders, goes on consuming those teeth so that they all pass out of the nut, and there remains in that nut an equal length of screw without teeth — that is, smooth — so that the wheel of that nut may turn by itself alone, without turning or drawing the screw to it any further. And in this way the operator of it will be able to draw out the blade while the screw stays still, and do his work without danger to the instrument. And then he can pull the screw straight out, and quickly, and put it back on the other side, and return it to its first office. And if you wished the screw to pull right up to its last tooth, which borders on its smooth part, that tooth, being left alone to so much effort, would break at once.

c, n a b

Here it is shown how, for a certain amount of travel made by one motor, the beginnings of two moving bodies will be drawn equally along with it, while the travel made by their ends will be varied. At c everything joined to it is unfastened. This c moves and drags the ends of whatever is joined to it.