Page:The National geographic magazine, volume 1.djvu/319

 process was repeated with each of the succeeding contours until all were placed and glued into their proper positions. At this stage the model presents the relief in a series of steps, each step representing a rise corresponding to the contour interval. The disadvantages of the method lie in the fact that unless the greatest care is exercised in making the photographic prints there will be considerable distortion, owing to the stretching of the paper in different directions, and consequently much trouble in fitting the contours. If care be exercised in having the grain of the paper run in the same direction in all the prints, trouble in fitting the contours will be much reduced, but the distortion in one direction will remain. In our experience this distortion amounts to about two per cent.; in other words, a model that should be fifty inches long will in reality be fifty-one inches; but, as this error is distributed over the whole fifty inches, it is not too great for an ordinary model. If greater accuracy be required, it can be secured by transferring the contours to the card-board by means of tracing or transfer paper. The great advantage of the photographic method lies in the fact that when the model has been built up, with all the contours in position, it presents a copy of the map itself, with all the details, drainage, etc., in position, instead of blank intervals between the contours. Such details and drainage are a great help in the subsequent modeling.

The next step in the process is to fill in with clay or wax the intervals between the contours. I have always found wax more convenient than clay for this purpose as, unless the surface coating is a thick one, the clay is difficult to keep moist. To obviate this difficulty, some modelers have used clay mixed with glycerine instead of water; this, of course, does not become dry, but the material is, at its best, unsatisfactory. The filling-in process is the most important one in relief map making, for it is here that the modeler must show his knowledge of, and feeling for, topographic forms. Some models seem to have been constructed with the idea that when the contours have been accurately placed the work of the modeller is practically done. This is a great mistake. The card-board contours are only a means of control, occupying somewhat the same relation to the relief map that a core or base of bricks, or a frame of wood, does to other constructions as, for example, an architectural ornament or a bust. It is sometimes necessary to cut away the contour card; for, as has been already explained, a map is more or less generalized, and