he corresponding hour-points,
will serve to find the time for a day intermediate to those set down,
the lid being turned till the style occupy a proper position between
the two divisions. The horizontality of the surface on which the
instrument rests is a very necessary condition, especially in summer,
when, the shadow of the style being long, the extreme end will shift
rapidly for a small deviation from the vertical, and render the
reading uncertain. The dial can also be used by holding it up by a
small ring in the top of the lid, and probably the vertically is
better ensured in that way.

_Portable Dial on a Card._--This neat and very ingenious dial is
attributed by Ozanam to a Jesuit Father, De Saint Rigaud, and probably
dates from the early part of the 17th century. Ozanam says that it was
sometimes called the _capuchin_, from some fancied resemblance to a
cowl thrown back.

_Construction._--Draw a straight line ACB parallel to the top of the
card (fig. 8) and another DCE at right angles to it; with C as
centre, and any convenient radius CA, describe the semicircle AEB
below the horizontal. Divide the whole arc AEB into 12 equal parts at
the points r, s, t, &c., and through these points draw perpendiculars
to the diameter ACB; these lines will be the hour-lines, viz. the line
through r will be the XI ... I line, the line through s the X ... II
line, and so on; the hour-line of noon will be the point A itself; by
subdivision of the small arcs Ar, rs, st, &c., we may draw the
hour-lines corresponding to halves and quarters, but this only where
it can be done without confusion.

Draw ASD making with AC an angle equal to the latitude of the place,
and let it meet EC in D, through which point draw FDG at right angles
to AD.

With centre A, and any convenient radius AS, describe an arc of circle
RST, and graduate this arc by marking degree divisions on it,
extending from 0 deg. at S to 23 1/2 deg. on each side at R and T. Next
determine the points on the straight line FDG where radii drawn from A
to the degree divisions on the arc would cross it, and carefully mark
these crossings.

[Illustration: FIG. 8.]

The divisions of RST are to correspond to the sun's declination, south
declinations on RS and north declinations on ST. In the other
hemisphere of the earth this would be reversed; the north declinations
would be on the upper half.