Of the various velocities of currents from the surface of the water to the bottom.

Of the various transverse obliquities between the surface and the bottom.

Of the various currents at the surface of the waters.

Of the various currents over the bottom of rivers.

Of the various depths of rivers.

Of the various shapes of hills covered by the water.

Of the various shapes of hills uncovered by the waters.

Where the water is swift at the bottom and not at the surface.

Where the water is slow at the bottom and swift at the surface.

Where it is slow below and above, and swift in the middle.

Where it is slow in the middle, and swift below and above.

Where the water in rivers widens and where it narrows, where it bends and where it straightens, where it deepens equally across the widths of the rivers and where it is unequal, where it is low in the middle and high at the sides, where it is high in the middle and low at the sides, where the current runs straight down the middle of the river, where the current meanders, throwing itself to various sides.

Of the various obliquities in the descents of the water.

This ball should be a half or a third of a braccio in diameter, and should be of clear glass and full of clear water, with a lamp in the middle, its light about the center of this ball; and, hung in the middle of a hall, it will give great light.