/ I 



Figure 15. — Centering for placement of finished stonework at west portal, 1874. At top-right are the 

 sheds where the lining brick was produced. {Photo courtesy oj State Library, Commonwealth of Massachusetts.) 



were sufficient to counteract the earth pressure, and 

 were spaced according to the degree of support 

 required. In more extreme conditions, a solid lag- 

 ging of small poles or boards was set outside the 

 frames, as shown in the model, to provide absolute 

 support of the ground. Details of the framing, the 

 windlass, and all tools and appliances were supplied 

 by Agricola, with no need for interpretation or 

 interpolation. 



The basic framing pattern of sill, side posts a. id cap 

 piece, all morticed together, with lagging used where 

 needed, was translated unaltered into tunneling prac- 

 tice, particularly in small exploratory drifts. It 

 remained in this application until well into the 20th 

 century. 



The pressure exerted upon tunnels of large area 

 was countered during construction by timbering 



systems of greater elaboration, evolved from the basic 

 one. By the time that tunnels of section large 

 enough to accommodate canals and railways were 

 being undertaken as matter-of-course civil engineer- 

 ing works, a series of nationally distinguishable 

 systems had emerged, each possessing characteristic 

 points of favor and fault. As might be suspected, the 

 English system of tunnel timbering, for instance, was 

 rarely applied on the Continent, nor were the German. 

 Austrian or Belgian systems normally seen in Great 

 Britain. All were used at one time or another in 

 this country, until the American system was intro- 

 duced in about 1855. While the timbering commonly 

 remained in place in mines, it would be followed up 

 by permanent masonry arching and lining in tunnel 

 work. 

 ( (verhead in the museum Hall of Civil Engineering 



216 



BULLETIN 240: CONTRIBUTIONS FROM I III MI SI IM OF HISTORY AND TECHNOLOGY 



