Now I must tell you how to draw the shape of the three-cornered board abc. Fig. 2 shows how it is done. The side ac should always be just five inches long. The side bc is drawn at right angles to ac, which you can do with an ordinary carpenter's square. The length of bc depends on the place for which the dial is made. The following table gives the length of bc for various places in the United States, and, after you have marked out the length of bc, it is only necessary to complete the three-cornered piece by drawing the side ab from a to b.

Table Showing the Length of the Side bc.

Place.b cPlace.b c
Inches.Inches.
Albany411-16New York43-8
Baltimore41-16Omaha43-8
Boston41-2Philadelphia43-16
Buffalo411-16Pittsburg43-8
Charleston31-4Portland, Me413-16
Chicago41-2Richmond315-16
Cincinnati41-16Rochester411-16
Cleveland41-2San Diego31-4
Denver43-16San Francisco315-16
Detroit41-2Savannah31-8
Indianapolis41-16St. Louis315-16
Kansas City315-16St. Paul5
Louisville315-16Seattle59-16
Milwaukee311-16Washington, D. C.41-16
New Orleans27-8

If you wish to make a dial for a place not given in the table, it will be near enough to use the distance bc as given for the place nearest to you. But in selecting the nearest place from the table, please remember to take that one of the cities mentioned which is nearest to you in a north-and-south direction. It does not matter how far away the place is in an east-and-west direction. So, instead of taking the place that is nearest to you on the map in a straight line, take the place to which you could travel by going principally east or west, and very little north or south. The figure drawn is about the right shape for New York. The board used for the three-cornered piece should be about one-half inch thick. But if you are making a window-sill dial, you may prefer to have it smaller than I have described. You can easily have it half as big by making all the sizes and lines in half-inches where the table calls for inches.

After you have marked out the dimensions for the three-cornered piece that is to throw the shadow, you can prepare the dial itself, with the lines that mark the place of the shadow for every hour of the day. This you can do in the manner shown in Fig. 3. Just as in the case of the three-cornered piece, you can draw the dial with a pencil directly on a smooth piece of white board, about three-quarters of an inch thick, or you can mark it out on a paper pattern and transfer it afterward to the board. Perhaps it will be as well to begin by drawing on paper, as any mistakes can then be corrected before you commence to mark your wood.

Fig. 3.

In the first place you must draw a couple of lines MN and M′N′, eight inches long, and just far enough apart to fit the edge of your three-cornered shadow-piece. You will remember I told you to make that one-half inch thick, so your two lines will also be one-half inch apart. Now draw the two lines NO and N′O′ square with MN and M′N′, and make the distances NO and N′O′ just five inches each. The lines OK, O′K′, and the other lines forming the outer border of the dial, are then drawn just as shown, OK and O′K′ being just eight inches long, the same as MN and M′N′. The lower lines in the figure, which are not very important, are to complete the squares. You must mark the lines NO and N′O′ with the figures VI, these being the lines reached by the shadow at six o'clock in the morning and evening. The points where the VII, VIII, and other hour-lines cut the lines OK, O′K′, MK, and M′K′ can be found from the table on [page 78].