We shall discuss here the nature of the screw and of its lever, and how it [the screw] shall be used for lifting rather than for thrusting; and how it has more power when it is simple and not double, and thin rather than quick, when moved by a lever of the same length and force. We shall briefly discuss the many ways there are to use screws, the various types of endless [i.e. worm] screws, ... how the endless screw shall be combined with toothed wheels ... We shall examine the nature of their nuts ... We shall also speak of inverted screws and screws that by a single motion thrust and pull a weight, and screws which, by turning them only once, will rotate the nuts more than once. ... Their method of construction will also be discussed, how to put them to work ...
Leonardo da Vinci's introduction to the "Book of the elements of machines" (Trattato degli elementi macchinali, Madrid Ms. I, fol. 82r).
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