line serves to fix the position of the next fiber-board sheet, as in Figure 24-47C 

 When all the contours have been cut and the sheets are firmly nailed down, the 

 skeleton of the model will look like Figure 24-47Z). This skeleton should be 

 partly waterproofed with several coats of shellac, after which it may be covered 

 with plaster of Paris, papier-mache, or a similar material. Care must be taken 

 to use only a sufficient thickness of the covering material barely to cover the 

 edges of the fiber-board contours, or the accuracy of the model may be impaired. 



If it is desired to show the structure contours on the finished model, pins 

 may be stuck in the edges of the blocks before the covering material is applied. 

 The pins will later serve as guides for sketching the contours and can easily be 

 pulled out when they have served this purpose. 



The solid model is used principally as an aid in teaching. Unlike the peg 

 model, it cannot readily be altered to incorporate structural data that might 

 become available at a later date, and for this reason the solid model is generally 

 used for illustrative purposes. 



'!'llllllllllllHillllllllllllllliii i|M!.', i millllllilll l lllllllllll« 



Figure 24-48. Models of parallel structural cross sections. 



Section Models 



The section model is a series of parallel cross sections drawn or painted 

 on any thin, rigid, boardlike material (fig. 24-48). These sections in turn are 

 set in properly spaced slots in a solid base. A thick, hard cardboard is satisfac- 

 tory for the sections, although transparent material such as cellulose acetate or 

 Plexiglass is sometimes used because it is then possible to view one cross section 

 through another. The section model is more practicable than the solid model 

 in that any one of the sections can easily be removed and revised without affect- 

 ing the others. 



Miscellaneous Models 



Various types of working models have been made for the purpose of 

 studying the behavior of fluids in permeable reservoirs. Others have been con- 



508 



