Descent and Soaring of Birds, the Echo, and Rivers Moving Gravel

Why the bird descends along any line

The bird descends along whichever part of its weight falls from the position of equality, and along whatever obliquity it disposes itself. If it inclines its forward part and a wing equally, the motion runs midway between the two inclined parts, leaning more toward whichever inclines more. The tighter the circle, the less oblique the line the bird takes, and the wing's obliquity should always equal the body's.

Reckoning a spiral ascent above the mountain

Fitting the motor's force so that space c e resists the descent, the remaining wing gives lift along the obliquity b g, which lies below line r b and so diminishes 15/16 of the bird's weight for its motor. Because b g contains line b r three times, for every 300 braccia of obliquity the bird rises 100; so in thirty spirals of 300 braccia each it finds itself a mile above the mountain-top it rose from. The trial is to be made of chancery paper and gauze.

The voice of the echo

The echo is classed as continuous or discontinuous, single or accompanied, brief or long, sudden or distant: continuous where the vault is of uniform concavity, discontinuous where the place is broken. It fades by degrees of space and time in a bell, cistern or cloud, like a circular wave in a pool m, and is often heard in the reflection rather than at the true source, as when fire in the air gave twelve thunder-images in twelve clouds at Ghiera d'Adda.

Rivers sort and deposit gravel by weight

Currents move materials of various weights: the lighter travel furthest and the heavier keep near the bottom, moving most where the water is strongest. When the current can no longer overcome the gravel's resistance it halts, the water leaps crosswise to scour fresh ground, and the old channel is abandoned and silts up with new earth from the turbid water.

Gravel rolling in waves behind the current

Water striking the lowest stone lifts it by its bending and carries it up to a, whence it falls back down to f, and so successively, so that the mounds of gravel all migrate downstream in the wave-form a d, b e, c f.

The leading muscle a b as rudder of the wing

The feathers lie overlapped, and the rudder of the wing c a d is the muscle a b. When the bird is about to be overturned by the wedge of the wind, this muscle enters under the wind more quickly than the whole wing, makes itself a wedge, and helps to lower the whole wing. It is thus the wing's rudder from top to bottom, as the tail is a rudder from there to here.

Riquadro a:

Tre figure di uccello con varia inclinazione:

Figura di linea a zig-zag:

Riquadro b:

Quadrante di cerchio colle lettere:

Figura di uccello colle lettere e i numeri:

Riquadro c:

Figura colle lettere:

Capovolgendo il foglio figure connesse con quella del riquadro d:

Figura colle lettere:

Riquadro d:

Figura di corso d'acqua:

Figura di corso d'acqua colle lettere:

Figura d'ala colle lettere:

Box a:

Three figures of a bird with various inclination:

Figure of a zig-zag line:

Box b:

Quadrant of a circle with the letters:

Figure of a bird with letters and numbers:

Box c:

Figure with the letters:

Turning the sheet over, figures connected with that of box d:

Figure with the letters:

Box d:

Figure of a water-course:

Figure of a water-course with the letters:

Figure of a wing with the letters: