SUN DIALS.


An article on the subject, recently published by us, has gained for us
the communication of two very interesting sun dials, which we shall
describe. The first, which we owe to the kindness of General Jancigny,
is of the type of the circular instrument, of which we explained the
method of using in our preceding article. The hour here is likewise
deduced from the height of the sun converted into a horary angle by
the instrument itself; but the method by which such conversion
operates is a little different. Fig. 1 shows the instrument open for
observation. We find here the meridian circle, M, and the equator E,
of the diagram shown in Fig. 3 (No. 4); but the circle with alidade is
here replaced by a small aperture movable in a slide that is placed in
a position parallel with the axis of the world. Upon this slide are
marked, on one side, the initials of the names of the months and on
the other side the corresponding signs of the zodiac. The sun
apparently describing a circle around the axis, PP¹, the rays passing
through a point of the axis (small aperture of the slide) will travel
over a circular cone around such axis. If, then, the apparatus be so
suspended that the circle, M, shall be in the meridian, the slide
parallel with the earth's axis, and the circle, E, at right angles
with the slide, the pencil of solar light passing through the aperture
will describe, in one day, a cone having the slide for an axis; that
is to say, concentric with the equator circle. If, moreover, the
aperture is properly placed, the luminous pencil will pass through the
equator circle itself; to this effect, the aperture should be in a
position such that the angle, a (Fig. 3, No. 4), may be equal to the
declination of the sun on the day of observation. It is precisely to
this end that the names of the months are inscribed upon the slide....