Summery on 10 Books of Architecture Book 9

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THE ANALEMMA AND ITS APPLICATIONS

Wherever a sundial is to be constructed, we must take the equinoctial shadow of the place. If it is found to be, as in Rome, equal to eight ninths of the gnomon, let a line be drawn on a plane surface, and in the middle thereof erect a perpendicular, plumb to the line, which perpendicular is called the gnomon. Then, from the line in the plane, let the line of the gnomon be divided off by the compasses into nine parts, and take the point designating the ninth part as a center, to be marked by the letter A. Then, opening the compasses from that center to the line in the plane at the point B, describe a circle. This circle is known as the meridian then, of the nine parts between the plan and the center on the gnomon, take eight, and mark them off on the line in the plane to the point C. This will be the equinoctial shadow of the gnomon. From that point, marked by C, let a line be drawn through the center at the point A, and this will represent a ray of the sun at the equinox. Then, extending the compasses from the center to the line in the plane, mark off the equidistant points E on the left and I on the right, on the two sides of the circumference, and let a line be drawn through the center, dividing the circle into two equal semicircles. Mathematicians call this line the horizon. By continuing this process we found the circle of the months,which is called Menaeus. This is the figure of analemma.

The scheme of hours is next to be drawn on the baseplates from the analemma, according

To the winter, lines or those of summer, or the equinoxes, or the months, and thus many different kinds of dials may be laid down and drawn by this ingenious method.

In general, the main use of this analemma is to know more about seasons.

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SUNDIALS AND WATER CLOCKS

Water clock first investigated by ctesibius the Alexandrian, which also discovered the natural pressure of the air and pneumatic principle. He began by making an orifice in a piece of gold, or by perforating a gem, because these substances are not worn by the action of water, and do not collect dirt to be stopped up.A regular flow of water through the orifice raises an inverted bowl, called by mechanicals the ”cork” or ”drum.” To this are attached a rack and a revolving drum, both fitted with teeth at regular intervals. These teeth, acting upon one another, induce a measured revolution and movement. Other racks and other drums, similarly toothed and subject to the same motion, give rise by their revolution to various kinds of motions, by which figures are moved, cones revolve, pebbles or eggs fall, trumpets sound, and other incidental effects take place. The hours are marked in these clocks on a column or a pilaster, and a figure emerging from the bottom points to them with a rod throughout the whole day. Inserting or withdrawing wedges must adjust their decrease or increase in length with the different days and months. By building systems, we can control the flow of water through it.

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