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the topography when combined with a given horizontal scale. In the 

 illustration selected it was found that sixteen hundred feet to the vertical 

 inch gave the most effective result. Inasmuch, then, as the contour 

 interval used on the topographic sheet was one hundred feet, and we 

 wished to adopt in the construction of the model the scale mentioned 

 above, it follows that sixteen sheets of strawboard, one-sixteenth of an 

 inch in thickness, placed one upon another, would provide the vertical 

 scale desired. This determined, each contour, beginning with the lowest, 

 was then traced on separate sheets of strawboard, carefully cut out, 

 piled in their proper succession and location, and taclied to a well sea- 

 soned wooden base or platform. The model at this stage presented a 

 terrace-like appearance. This objectionable feature so often seen on 

 geographical models, was easily obliterated by covering the entire surface 

 with a sheet of clay, taking care of course to preserve as much of the 

 details of relief as was shown on tlie original map. A plaster negative 

 was next made from the original and from it a final positive was prepared. 

 After thorough drying, the surface was painted a dead white. The 

 partings or the contacts between adjacent formations as indicated on the 

 geologic sheets referred to above, were carefully plotted and drawn on 

 the white surface, in well defined black lines, sufliciently broad to be 

 clearly photogi'aphed on a scale small enough to be transferred to a 

 lantern slide. Befoi'e talcing this step, however, another addition was 

 made to the model. Two cross sections expressing the structural geology, 

 one from east to west and the other from north to south, the former 

 located on the south end and the latter along the east side of the model, 

 were prepared. The outline of the topography along the respective sec- 

 tions was also traced on each section and cut out. These sections- were 

 then fastened to the end and side of the model in their proper vertical posi- 

 tion, so that the relief, partings and structure were correctly correlated. 

 The model was then photographed in a tilted position so that both sections 

 could be clearly seen and the relief at the same time well expressed by 

 obtaining moderately strong light and shade. "It is especially important 

 that the lines of contact be clearly brought out, as they determine the 

 limits of the formations to be subsequently colored. A slide was next 

 made from the negative and sent with a copy of the Harpers Ferry Atlas 

 sheet to a photographic artist, with instructions to color the slide, adopting 

 of course, so far as might be feasible, the same scheme of colors as appear 

 on the geologic sheets. 



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