Page:The National geographic magazine, volume 1.djvu/314

 before the American Institute of Mining Engineers in 1887. The method was published in 1838. Upon a contoured map as a basis cross-section lines are drawn at small and regular intervals, and, if the topography be intricate, corresponding lines at right angles. The sections thus secured are transferred to thin strips of some suitable material, such as cardboard or metal, and cut down to the surface line,—the strips themselves thus forming the cross-sections. These cross-sections are mounted upon a suitable base-board, and the cavities or boxes are then filled up with some easily carved material, such as plaster or wax. The top is then carved down to the form of the country or district,—the necessary guidance being obtained by the upper edges of the strips that form the cross-sections. It will be readily seen that this method is a very crude and laborious one. It necessitates in the first place a good contoured map upon which to draw the sections, but sacrifices much of the advantage thus gained because only a number of points on each contour line are used, instead of the entire line. It is no better, although actually more laborious, than the later method of driving contour pins (whose height above a base-board may be accurately measured,) along the contour lines, and then filling in. A slight modification of the latter method can be used to advantage when no contoured map is available, and when the points whose elevation is known are not numerous enough to permit the construction of one. In this case the only control that can be secured is by means of a number of pins driven into the base-board at those points whose elevation is known. The remainder of the map is then sketched in. This method is perhaps as satisfactory as any, when the material upon the map is scanty. Another method, however, growing out of the same scantiness of material, is in some cases to be preferred, especially for large models. The map is enlarged to the required size, and a tracing of it is mounted upon a frame. Another deep frame, just large enough to contain the mounted tracing, is made, and laid upon a suitable base-board upon which a copy of the map has been mounted. Upon this base-board the model is then commenced, in clay or wax. The low areas are modeled first,—horizontal control being obtained by pricking through the mounted tracing of the map with a needle point, and vertical control by measuring down from a straight edge sliding on the top of the deep frame. This system is rather crude, and only useful where the material upon the map is very scanty, but it gives excellent control.