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.
At the extremity of these lines are marked the signs of the zodiac. At
the top, a pretty banderole, which appears at first sight to form a
part of the _ensemble_ of the curves, completes the design. Such is
this wonderful little instrument, in which everything is arranged in
harmonious lines that delight the eye and easily detract one's
attention from a scientific examination of it. Let us enter upon this
drier part of our subject; we shall still have room to wonder, and let
us take up first the higher question.

[Illustration: FIG. 3.--DIAGRAM EXPLANATORY OF THE MANDOLIN SUN DIAL.]

Let us consider a horizontal plane (Fig. 3, No. 2)--a plane
perpendicular to the meridian, and a right line parallel with the axis
of the world. Let P be a point upon this line. As we have seen, such
point is the summit of a very wide cone described in one day by the
solar rays. At the equinox this cone is converted into a plane, which,