To make the scale of chords, divide the arc D C into nine equal parts, and then with the compasses, with one leg placed on the point C, protract each division on the line C D. Mark the points on this line from C 10, 20, 30, etc.; the point D will mark 90 degrees.
To make the scale of latitudes, draw lines from the points of division in the arc D C parallel to the line D 0, and cutting the line 0 C in points, counting from 0, 10, 20, 30, 40, etc. Now draw straight lines from D through these points, cutting the arc B C in the points 10, 20, 30, etc., and with your compasses, with one leg on B, protract these distances on to the line B C, which will be the scale of latitudes. Now our dialing scale is finished.
To make the dial, which will be a horizontal one, you must get a piece of zinc plate about one foot square. On this mark all round it, and one inch from the edges, lines making a smaller square of ten inches a side. Plate 1⁄8 inch thick.
Bisect one line of this square, and draw a line from this point to a point bisecting the opposite side. Now draw two other lines, one on each side of, and one-sixteenth of an inch from, this line, and parallel to it. These lines will then be one-eighth of an inch apart. They are made this distance apart as the style, or gnomon, will be that thickness, and has to stand between them. Now divide the other sides into five equal parts, and join the two second points of division, counting from the bottom. This line, which is called the six-o’clock line, will cut the two parallel lines in the points A C ([Fig. 2]). Mark the other or top ends of these lines B and D.
Fig. 2.
Now with your compasses take from the scale of latitudes the latitude of the place where you wish to erect your dial. Suppose you are in London, put one leg of the compasses on the point B, and the other leg on the point in the scale of latitudes marking 511⁄2 degrees, which is the latitude of London. Now mark this distance off on the six-o’clock line from C to E and from A to F ([Fig. 2]). Now take the length of the line of hours from 12—6 in the compasses, and, putting one leg on the point E, intersect the line C D in the point G. Do the same on the other side, putting one leg on point F, and intersecting the line A B in the point H. Draw the lines G E and H E ([Fig. 2]).
These lines are the same length as in the line of hours; mark them as that line is marked, using your compasses to get the distances, marking the line from G to E and from H to E; now from the point C draw lines through the divisions on G E to the lines of the inner square; do the same from point A through the line H F. The fourth and fifth lines on the right side must be continued back through the point C to opposite side of square, and the seventh and eighth on the left be continued back through A to right side. Now mark the hours. The double line is the twelve-o’clock line, and must be marked twelve. The line to the right is the one-o’clock line, the next two, and so on to eight on the right side. On the left the line next the twelve-o’clock line is eleven, the next ten, and so on back to four. All the lines can be marked on the zinc with a pointed bradawl.
The dial plate is now finished.
