The fusee: equalising a spring's diminishing force
A tapered spring-barrel and pinion arranged so resistance falls as the spring's power falls.
A large three-dimensional drawing on folio 16r shows a tapered, spirally grooved barrel (a fusee) set inside a cylindrical toothed case, with a big wheel m at the left and a small pinion or spring n at the right; the parts are lettered h, n, m, X, b. The notes explain that a spring's power is 'pyramidal', diminishing toward its end, so its resistance must diminish too, being strong at the start together with the driving power and weak at the finish, the motion of the toothed case running from h to X (text 4). Text 3 adds that the pinion n m stays fixed in place though it turns upon itself while the spring b revolves about the pole n, and that to rewind, the case is held while its pole is turned the contrary way. A motto heads the page: 'Every art demands its own instrument' (text 1).
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Tapered spring-case whose widening screw equalises the drive
The spring-case turns about pole n, enclosed within an outer case b whose parts are very unequal from the spring's centre. This inequality is deliberate: the first motion of the toothed case comes from the part h and ends at X, and to rewind the spring the case is stopped while its pole is turned the contrary way to its working motion.
A 'pyramidal' spring force needs matching resistance
The power of the driving spring is pyramidal, diminishing toward its end. Because it begins great and grows less, its resistance must do the same, beginning strong together with the moving power and finishing weak.
Pinion n m fixed while spring b turns about pole n
The pinion n m keeps its place yet turns upon itself, while the spring b goes turning around the pole n.