Figure 34. — Beach's Broadway Sub- 

 w.iv. Advancing the shield by 

 hydraulic rams, 1869. MHT 

 model — l'j" scale. (Smithsonian 

 photo 49260-E.) 



precision not attainable with screws. Vertical and 

 horizontal deflection was achieved by throttling the 

 supply of water to certain of the rams, which could 

 be individually controlled, causing greater pressure 

 on one portion of the shield than another. This 

 system has not changed in the ensuing time, except, 

 of course, in the substitution of mechanically produced 

 hydraulic pressure for hand. 



Unlike the driving of the Tower Subway, no ex- 

 cavation was done in front of the shield. Rather, 

 the shield was forced by the rams into the soil for 

 the length of their stroke, the material which entered 

 being supported by the shelves. This was removed 

 from the shelves and hauled off. The ram plungers 

 then were withdrawn and a 16-inch length of the 

 permanent lining built up within the shelter of the 

 shield's tail ring. Against this, the rams bore for the 

 next advance. Masonry lining was used in the 

 straight section; cast iron in the curved. The junc- 

 ture is shown in the model. 



Figure 35. — Vertical section through the Beach 

 shield used on the Broadway Subway, showing the 

 horizontal shelves (C), iron cutting ring (B), 

 hydraulic rams (D), hydraulic pump (F), and rear 

 protective skirt (II). (Scientifit American, March 5. 

 .870.) 



230 



BULLETIN 240: CONTRIBUTIONS FROM THE MUSEUM OF HISTORY AND TECHNOLOGY 



