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Some say that, making the transverse arm of the 'tempo' in a circular figure, it has the same contact with the air, and does the same office in resisting with the air where it strikes, as if it were straight or of other figures. But I believe that so it has seemed to them because, in making it round, there is added a certain crosspiece that supports that circle, whence weight comes to be added; and this weight makes it seem to be one and the same thing as the straight one.

With this wheel alone one can make a clock that shows the hours, which will be simple and good. But it needs to have many teeth and considerable size, and its vanes distant from the pole, and a small barrel where the cord of the counterweight winds. And the counterweight should be very heavy. And when the air thickens, it draws the vanes toward the center of its rod; or you increase the weight of its counterweight; and so, with another similar wheel, you could then make it strike the hours.

Some use, against the impetus of the motions that often can happen to the counterweights that move the wheels of clocks, certain instruments which they call 'time' [tempo], because it keeps the motion of the wheels that move it in time, and it goes slowing according as is required by the slowness and length of the hours. This instrument is, in this office of prolonging the time, a most useful thing. And it is made in several different ways, but all are of one same nature and value; but such variety is born of the necessity of the place where they sometimes must be placed.

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Rule of the value of such 'time' [tempo].

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The rule given for placing the two opposite teeth of the above-said 'tempo' is this. That is, let the space found between the centers of the thickness (or width) of the teeth be divided into 3 parts of equal width, of which the first must be the width of the upper tooth, the second will be that of the thickness of the rod where such tooth joins, and the third will be the width of the other similar tooth, placed on the opposite side of the rod. And the 2 lines that pass through the middle of the length of said teeth must be directed to the center of the thickness of their rod, in such a way that whoever looked at the rod in foreshortening, and could see said 2 lines, would see them lead together to the center of that rod, at a right angle. And if you made the angle of such lines smaller than a right angle, the turn of such 'tempo' would be a greater circuit of motion. And know that the rod of such teeth must be suspended from the upper part by a thread, and situated in such a way that when its linear center is found above, in the middle of the two teeth, below it meets the center of one of the opposite teeth. And for this the number of such teeth must be odd, and the rod must pass with its central line over the center of the toothed wheel. But if necessity constrained you, for want of place, or of some pole that impeded you from making the rod of the 'tempo' so that it did not pass over the pole of said wheel, you could pull it somewhat to one side. But make the number of teeth odd and of much greater number, or increase the wheel.