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PORTABLE SUN sundials - Page 8

The Roman sundial consists of a circular disk which can be moved around. inside a ring, for setting to the proper latitude. The declination of the sun is cut on the outer edge of the disk, upon which a movable bar is mounted. The bar is set to the correct day, then brought into the sun so that the vertical gnomon will cast a shadow on the curved hour circle. The construction is readily seen in the illustration. The front and back of a disk sundial appear in the upper left corner. The vertical lines represent the days as in the pillar sundial; the curved lines, the hours of the day. The principle of its construction is based on the altitude of the sun for each hour of the day. On the back is cut the correction to be applied for standard time. Mr. Bolster has made a variant of the analemmatic sundial, upper right corner. The analemmatic and horizontal sundials are usually laid out on one plate, the perpendicular gnomon being the only movable part; but here the vertical gnomon of the analemmatic is attached to a movable horizontal sundial plate. This is a simple arrangement different from the general run of such sundials. Conical sundials need not be relegated to the "obsolete shelf." Note the chalice or goblet sundial, lower left center. It is a form of conical sundial, with a vertical pin in the center. It is used in the same manner as the pillar sundial. To be sure it is unlike its ancestor, but nevertheless conical. At the left of the goblet is a ship sundial, so called because of its resemblance to the old galleons. Its principle of construction and use is similar to that of the quadrant sundial on the right which consists of a plate in the form of a quadrant. A string fitted with a weight and bead is attached to the corner of the plate.

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