THE HOLLAND TUNNEL — GRAY AND HAGEN 579 



In the time of Caesar Augustus, or perhaps even earlier, the lio- 

 mans built a notable tunnel through the Tosilipo hills, between 

 Naples and Pozzuoli, about 3,000 feet long and 25 feet wide. In 

 order to light this tunnel, its lioor and roof were made to converge 

 gradually from the ends to the middle : at the entrances it was 75 

 feet high. The Komans were the greatest tunnel builders of an- 

 tiquity. During the Middle Ages tunnel building was chiefly for 

 military purposes. Every great castle had its private underground 

 passage from the central tower or keep to some distant concealed 

 place, through which to make sorties, receive supplies, or escape in 

 time of need. 



With the advent of gunpowder and of canal construction, a strong 

 impetus was given to tunnel building in its more modern aspect of 

 commercial or public utility. Previous to 1800, canal tunnels were all 

 through rock or hard ground. Then, in 1803, a soft-ground tunnel 24 

 feet wide was excavated for the Saint Augustine Canal in France. 

 Timbers were laid to support the roof and walls as fast as the earth 

 was removed, and the masonry lining built closely following. From 

 this experience the various systems of soft-ground tunneling since 

 employed have developed. 



The use of shield and metal lining marks the greatest develojjment 

 in the art of soft-ground submarine tunneling. The shield was in- 

 vented and first used by Sir Marc Isambard Brunei in excavating the 

 first tunnel under the river Thames at London, begun in 1825 and 

 opened in 1843. In 1860 Peter William Barlow used an iron lining 

 in connection with a shield in driving the second tuimel under the 

 Thames at London. 



The modern tunnel shield is a steel-plate cylinder whose forward 

 edge acts as a cutting edge. Its rear end, extending backward, over- 

 laps the tunnel lining of cast-iron rings. Inside the shield, hydraulic 

 jacks act against the tunnel lining as a thrust block so as to push the 

 shield ahead when pressure is applied. A partition prevents earth 

 from entering the shield except as permitted through suitable open- 

 ings. As the shield moves forward, the lining is erected under the 

 protection of its rear. In submarine tunneling compressed air 

 pumped into the forward end of the tunnel counterbalances the pres- 

 sure of the water which tries to enter. 



In 1906 the Legislatures of the States of New York and New Jersey 

 ci-eated for each State a bridge commission to investigate the feasi- 

 bility of constructing a bridge over the Hudson River, uniting New 

 York City with Jersey City. Legislative recognition was thus given 

 to an increasingly vital problem — some means to supplement the 

 ferries plying between these two ports. 



Further legislation, enacted from time to time, continued the life 

 of these commissions. In 1913 they were authorized to consider the 

 possibility of a vehicular tunnel. Finally, on April 10, 1919, author- 



