being equal to the latitude of the place.

The diurnal motion of the stars is strictly uniform, and so would that
of the sun be if the daily retardation of about four minutes, spoken
of above, were always the same. But this is constantly altering, so
that the time, as measured by the sun's motion, and also consequently
as measured by a sun-dial, does not move on at a strictly uniform
pace. This irregularity, which is slight, would be of little
consequence in the ordinary affairs of life, but clocks and watches
being mechanical measures of time could not, except by extreme
complication, be made to follow this irregularity, even if desirable.

The clock is constructed to mark uniform time in such wise that the
length of the clock day shall be the average of all the solar days in
the year. Four times a year the clock and the sun-dial agree exactly;
but the sun-dial, now going a little slower, now a little faster, will
be sometimes behind, sometimes before the clock-the greatest
accumulated difference being about sixteen minutes for a few days in
November, but on the average much less. The four days on which the two
agree are April 15, June 15, September 1 and December 24.

Clock-time is called _mean time_, that marked by the sun-dial is
called _apparent time_, and the difference between them is the
_equation of time_. It is given in most calendars and almanacs,
frequently under the heading "clock slow," "clock fast." When the time
by the sun-dial is known, the equation of time will at once enable us
to obtain the corresponding clock-time, or vice versa.

Atmospheric refraction introduces another error by altering the
apparent position of the sun; but the effect is too small to need
consideration in the construction of an instrument which, with the
best workmanship, does not after all admit of very great accuracy.

The general principles of dialling will now be readily understood. The
problem before us is the following:--A rod, or _style_, as it is
called, being firmly fixed in a direction parallel to the earth's
axis, we have to find how and where points or lines of reference must
be traced on some fixed surface behind the style, so that when the
shadow of the style falls on a certain one of these lines, we may know
that at that moment it is solar noon,--that is, that the plane through
the style and through the sun then coincides with the meridian; ag