The double reverse screw, a balancing spring, and a crossbow
Two nuts moving in opposite directions; a spring holding two weights; a marginal crossbow
The sheet studies mechanisms together with the mechanics of forces. Several notes and figures define the 'double' or 'reverse' screw, cut so its threads run from the middle out to both ends, so that its two nuts always travel in opposite directions—or, when one nut is fixed, the other doubles its travel. A separate note analyses a spring set between two weights (b 100, a 100): with force for 200 it holds a 200 load, but relieved of a little weight it straightens until it matches the remaining resistance. A crossbow is drawn in the left margin and marked Q.
On this page
The double reverse screw and its two nuts
The double screw of contrary turns has its threads beginning at the middle of its length and ending at the ends. It is given two nuts, which always make contrary motions to each other: turning the screw one way sends the nuts toward its ends, turning it the other way brings them back toward the centre.
Definition of the reverse screw; doubling the travel
The reverse screw is defined as one whose turns begin at the middle of its length and depart equally from that middle to end at the ends. If, while turning it, one of the two nuts is fixed, the moving nut is found to double its travel.
A spring balancing two weights (b 100, a 100)
A spring with force for 200 stays still under a weight of 200. Remove a minimum weight from each side and it straightens, moving the weights until it equalises with their resistance and then stops; the motion is proportional to the weight removed relative to the remainder.
Crossbow in the left margin
A crossbow (balestro) is drawn along the left margin and labelled with the letter Q.