[Illustration: FIG. 2.--SUN DIAL IN THE FORM OF A MANDOLIN,
CONSTRUCTED IN 1612.]
No. 2 of Fig. 3 represents the instrument open. It rests upon the tail
piece and neck of the mandolin. The cover is exactly vertical. The
bottom of the mandolin is closed by a horizontal silver plate,
beneath which is soldered the box of a compass designed to put the
instrument in the meridian, and carrying upon its face an arrow and
the indications S. OR. M. OC., that is to say, "Septentrion" (north),
"Orient" (east), "Midi" (south), "Occident" (west). One of the ends of
the needle of the compass is straight, while the other is forked. It
is placed in a position in which it completes the arrow, thus
permitting of making a very accurate observation (Fig. 2, No. 3).
Around the compass, the silver plate carries the lines of hours. It is
perfectly adjusted, and held in place by a screw that traverses the
bottom of the instrument. In front of the compass it contains a small
aperture designed to permit of the passage of the indicating thread,
which, at the other end, is fastened to the cover. The silver plate is
not soldered, in order that the thread may be replaced when it chances
to break. On the inner part of the cover are marked in the first place
the horary lines, traversed by curves that are symmetrical with
respect to the vertical and having the aspect of arcs of hyperbolas.
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