Fig. 2
Fig. 3
This dial does not show the time before seven o’clock in the morning or after five o’clock in the evening. The reason of this is, the days and nights at the equator being equal—viz., twelve hours each—the sun rises and sets at six o’clock. At six o’clock, the sun being exactly on the horizon, any object placed in the middle of a perfectly horizontal plane would cast an indefinite or unlimited shadow, as the shadow of the upper part would be parallel to the plane, and of course could not meet it. The dial can be made to show any time after six in the morning or before six in the evening by lengthening the dial-plate. [Fig. 4] will show how inconvenient it would be to have a plate to show the time before seven or after five o’clock. In [Fig. 4] the hour from five to six is divided into quarters, and shows that for 5.30 the plate must be about double, and for 5.45 about four times, the length required to show the time between seven and five o’clock. So that a plate about six feet long would be required for a dial having a style two inches high.
Fig. 4
The style is to be soldered to the dial-plate between the two lines A B and C D, and must be equal in thickness to the distance between them. The dial must be set up in a horizontal position with the gnomon directed due north and south.
Both these dials are horizontal. I will now explain the construction of vertical dials, or dials that are fixed in an upright position against a wall or house. The dialing scale, as already described, will be required for the construction of a vertical dial to be fixed on a wall facing the south.
Cut the zinc plate twelve inches square, and mark it with the inner square, the twelve-o’clock lines, and the six-o’clock line, as in the horizontal dial. From the line of latitudes, in the scale, take the length equal to the difference between 90 deg. and the latitude of the place (that is, not as in the horizontal dial, the latitude, but the complement of it).