In Fig. 198 each dot represents two hundred people. A dot in the suburbs may therefore represent all the people in one square mile of territory. If a map were first made with two hundred dots for the two hundred people, the one dot actually used on the final map would have to be placed not at the geographical center of the area represented, but at the center of gravity of the two hundred dots which it replaces.

In Fig. 199 various combinations are shown of pins, beads, etc., of use in map work. Data for map presentation are frequently so complex that ingenuity is taxed to show the facts on any map of a size commercially available. A great variety of effects may be secured, however, by means of the devices shown in Fig. 199. The exhibits given in the illustration are as follows:

1. Long pin with small size glass head, available in many colors.

2. Long pin of brass wire for use with beads as shown in No. 9.

3. Long pin with glass head used in conjunction with a piece of sheet celluloid cut into the shape of a flag.

William D. McAbee in the Survey

Fig. 197. Relative Soot Deposits in Indianapolis, March, 1912

The greatest soot tall is in the vicinity of railroad tracks Carefully selected samples of snow were melted and the soot of twenty-four hours weighed after the water was evaporarated. Spot maps of this kind can be quickly made by using short map pins pushed in till the pin heads touch the map