ain,
that when the shadow reaches the next line of reference, it is 1
o'clock by solar time, or, which comes to the same thing, that the
above plane through the style and through the sun has just turned
through the twenty-fourth part of a complete revolution; and so on for
the subsequent hours,--the hours before noon being indicated in a
similar manner. The style and the surface on which these lines are
traced together constitute the dial.

The position of an intended sun-dial having been selected--whether on
church tower, south front of farmstead or garden wall--the surface
must be prepared, if necessary, to receive the hour-lines.

The chief, and in fact the only practical difficulty will be the
accurate fixing of the style, for on its accuracy the value of the
instrument depends. It must be in the meridian plane, and must make an
angle with the horizon equal to the latitude of the place. The latter
condition will offer no difficulty, but the exact determination of the
meridian plane which passes through the point where the style is fixed
to the surface is not so simple. At present we shall assume that the
style has been fixed in its true position. The style itself will be
usually a stout metal wire, and when we speak of the shadow cast by
the style it must always be understood that the middle line of the
thin band of shade is meant.

The point where the style meets the dial is called the centre of the
dial. It is the centre from which all the hour-lines radiate.

The position of the XII o'clock line is the most important to
determine accurately, since all the others are usually made to depend
on this one. We cannot trace it correctly on the dial until the style
has been itself accurately fixed in its proper place. When that is
done the XII o'clock line will be found by the intersection of the
dial surface with the vertical plane which contains the style; and the
most simple way of drawing it on the dial will be by suspending a
plummet from some point of the style whence it may hang freely, and
waiting until the shadows of both style and plumb-line coincide on the
dial. This single shadow will be the XII o'clock line.

In one class of dials, namely, all the vertical ones, the XII o'clock
line is simply the vertical line from the centre; it can, therefore,
at once be traced on the dial face by using a fine plumb-line.

The XII o'clock line