h, r, g, K, m, n, f

Easy release of a crossbow, which is done at f by pulling it downward. n goes forward, and so does m, and h returns backward. And g rises upward, and r comes down and leaves the nut free and shoots the bolt. Then the cord K is pulled downward, which draws back the nose g and strikes upon the nose h, and there it locks anew. The tooth f below is made to fold to one side, so that it can be put in by bending it upward, within the thickness of the crossbow's stock. A false key is then made behind the stock, in the manner of the others, but only one finger plays in the said tooth.

The one below is similar to this one above, except that it is released with the ordinary key of crossbows. The spring M is made so that the blow given by the member n does not break n itself.

n, m, a b c f d e f, h

On the measure of the lever and counter-lever of the screw. c d is the core of the screw; a c d f is the width of its thread. b e is the centre of the width of that thread, which must be taken into account in measuring the force of screws, for it is the part that must be set as the limit of the counter-lever, by the 5th proposition of the 7th book, on motion and force. And when you wish to measure the power of the lever against that of the counter-lever, let the lever be h f and the counter-lever f m; then calculate by the rule of the 3rd of the first book, and note the figures here at the left, which are to the same purpose.

M, n

On the screw and its lever. Note well what is said here at the left: in the counter-lever you will reckon only the dragging of the weight, which carries that part of the weight this friction drags along. And it is also fitting to reckon another potential counter-lever, which lies in the motion gained by the weight drawn in one whole turn of the screw against the motion made by the lever. And the lever is spoken of only where the centre of the hand is, or of the force that moves the bar.