250 



TUNNELING 



and the use of hydraulic jacks. Greathead had also designed 

 for this work a special crane to be used in erecting the cast-iron 

 segments of the lining. 



While these works had been progressing in England, Mr. 

 Beach, an American, received a rmtent in the United States for 

 a tunnel shield of the construction shown by Fig. 126, which 

 was first tried practically in constructing a short length of 

 tunnel under Broadway for the nearly forgotten Broadway 



FIG. 127. Shield for City and South London Railway. 



Pneumatic Underground Ry. This shield, as is indicated by 

 the illustration, consisted of a cylinder of wood with an iron- 

 cutting-edge and an iron tail-ring. Extending transversely 

 across the shield at the front end were a number of horizontal 

 iron plates or shelves with cutting-edges, as shown clearly by 

 the drawing. The shield was moved ahead by means of a 

 number of hydraulic jacks supplied with power by a hand 

 pump attached to the shield. By means of suitable valves all 

 or any lesser number of these jacks could be operated, and by 



