The main difficulty with the celluloid method is that the inks will rub off if the celluloid is handled too much with moist hands. Ease of erasure has some advantages, however, for tentative schemes can be rubbed out or changed at will, simply by using a damp cloth. After the final adoption of the plan, tracing cloth can be laid over the combined map and celluloid sheet, and the important features of both may be traced for blue-printing or for general reference later. Where maps must have constant use, it is sometimes convenient to have them mounted between two sheets of clear celluloid. The celluloid-manufacturing companies are prepared to do this kind of mounting to order.

Contrasts in map areas as shown in Fig. 170, by superimposing one portion of the world on top of another portion, or by placing two portions side by side on the same scale, are of considerable value.

MAP PRESENTATIONS

In geography books, and in other places where maps are most commonly seen, contrasted territories are frequently drawn to such different scales that a true idea of their proportions cannot be obtained. The relative proportion can be indicated best when care is taken to have the several areas drawn to the same scale and placed in a good position for comparison.

Fig. 171 is shown here only as an example of the almost unlimited range which the application of graphic methods may have. The average person would never think of charting a football game, yet the graphic method certainly gives the information more concisely than could words alone.