A quicksilver timer for measuring a ship's speed at sea
191v–190r: mercury clock and the descent of a weight versus a ship's run
The sheet develops an instrument for timing motion at sea. On folio 191v a ball is to keep a little quicksilver up to the mouth of the spout that pours it, and the mercury is to be distilled over one hour divided into degrees, minutes and seconds, with a 'rule of the course' kept in a register. A lettered procedure adjusts a wheel n by a screw until a descent is completed in one harmonic time-unit, then compares, within the right angle K a h, the distance a ship travels with the fall of powder, so that when the two sides are equal the motions are equal and everything can be measured. Folio 190r adds that the device serves only with the wind, because it jolts less, and that the horizontal motion a b (the ship's true motion) exceeds the central descent a c (the true fall of the weight).
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Quicksilver retained to the spout's mouth
In the ball a little quicksilver must always remain, up to the mouth of the spout ('cannella') that pours it. The detail governs how the mercury feeds evenly through the pouring device.
Mercury distilled over one graduated hour
Quicksilver is set to run for one hour, the hour divided into degrees, minutes and seconds, the mercury distilled drop by drop; and the 'rule of the course' is to be made in the register. This turns the flow of mercury into a graduated measure of time.
The right angle K a h: ship's run against falling powder
After opening the vessel of time at a, one notes how long the poured matter takes to strike at n and lowers the wheel n by a screw until the descent ends in one harmonic time-unit. Drawing the small line K h in the angle, when the sides of the right angle K a h are equal the motions are equal — the ship travels as far as the powder descends — and all can be measured.
Horizontal motion a b versus central descent a c
The horizontal motion exceeds the central motion by as much as the line a b is longer than a c, since a b is the true motion of the ship and a c the true descent of the weight; neither deceives experience, being hindered neither by wind nor by the ship's motion. The instrument serves only with the wind, because it jolts less.