a little consideration will show that, owing to
the thickness of the plate, these styles will only one at a time cast
a shadow. Thus the eastern edge will give the shadow for all hours
before 6 o'clock in the morning. From 6 o'clock until noon the western
edge will be used. At noon it will change again to the eastern edge
until 6 o'clock in the evening, and finally the western edge for the
remaining hours of daylight.

The centres of the two dials will be at the points where the styles
meet the dial face; but, in drawing the hour-lines, we must be careful
to draw only those lines for which the corresponding style is able to
give a shadow as explained above. The dial will thus have the
appearance of a single dial plate, and there will be no confusion (see
fig. 3).

[Illustration: FIG. 3.]

The line of demarcation between the shadow and the light will be
better defined than when a wire style is used; but the indications by
this double dial will always be one minute too fast in the morning and
one minute too slow in the afternoon. This is owing to the magnitude
of the sun, whose angular breadth is half a degree. The well-defined
shadows are given, not by the centre of the sun, as we should require
them, but by the forward limb in the morning and by the backward one
in the afternoon; and the sun takes just about a minute to advance
through a space equal to its half-breadth.

Dials of this description are frequently met with. The dial plate is
of metal as well as the vertical piece upon it, and they may be
purchased ready for placing on the pedestal,--the dial with all the
hour-lines traced on it and the style plate firmly fastened in its
proper position, if not even cast in the same piece with the dial
plate.

When placing it on the pedestal care must be taken that the dial be
perfectly horizontal and accurately oriented. The levelling will be
done with a spirit-level, and the orientation will be best effected
either in the forenoon or in the afternoon, by turning the dial plate
till the time given by the shadow (making the _one_ minute correction
mentioned above) agrees with a good watch whose error on solar time is
known. It is, however, important to bear in mind that a dial, so built
up beforehand, will have the angle at the base equal to the latitude
of some selected place, such as London, and the hour-lines will be
drawn in directions cal