Here is shown the nature of the screw and of its lever, and how it ought rather to be used in pulling than in pushing; and how it exerts more force being single than double, and slender than thick, when moved by an equal length of lever and equal force. And so a little discourse will be made on in how many ways it can be used, and of how many kinds of endless screws can be made; and how many motions are made without a screw that perform the very office of a screw; and in what way the endless screw is paired with toothed wheels, and how many screws ought to be used together. And it will speak of the nature of their 'mothers' [nuts], and whether they are more useful with many teeth or not. And it will speak of retrograde screws, and of screws that by one and the same pull both push and pull the weight, and of screws that, by a single turn given to them, will make their mother [nut] flee many of its circular turns. And so very many of their effects, and their various labours, strengths, slownesses and quicknesses; and the reason of all their offices and natures, and materials, and levers, and utilities will be proved. And it will tell in what way they ought to be made, and of the manner of putting them to work, and of who has been deceived through not knowing their nature. And such instruments will for the most part be figured without their armatures, or any other thing that might hinder the eye of him who studies them. Then those armatures will be told of, by means of lines; then of the levers by themselves; then of the strength of the supports and of their eternity [durability] and maintenance. And of the differences found between the lever of continuous force in its operation, that is the wheel, and the lever of variable power, that is the straight one; and why the one is better than the other, and why the other is more brief and convenient than the first. And it will treat of the ratchet wheel and of its servant; and of the wheel of augmentation, and of the impetus of motions, and the nature of poles and their wearing-away; and of ropes and pulleys and windlasses and rollers. And of driving air under water, and drawing out of it the very greatest weights, that is, of filling bags with wind after they are tied under water together with weights; and of filling boats with sand, and tying the weights to such boats that are under water, and then emptying such boats of their sand.