242 TUNNELING 



CHAPTER XXI. 



SUBMARINE TUNNELING (Continued). THE 

 SHIELD SYSTEM. 



Historical Introduction. The invention of the shield system 

 of tunneling through soft ground is generally accredited to Sir 

 Isambard Brunei, a Frenchman born in 1769, who emigrated to 

 the United States in 1793, where he remained six years, and 

 then went to England, in which country his epoch-making in- 

 vention in tunneling was developed and successfully employed in 

 building the first Thames tunnel, and where he died in 1849, a 

 few years after the completion of this great work. Sir Isambard 

 is said to have obtained the idea of employing a shield to tunnel 

 soft ground from observing the work of ship-worms. He no- 

 ticed that this little animal had a head provided with a boring 

 apparatus with which it dug its way into the wood, and that its 

 body threw off a secretion which lined the hole behind it and 

 rendered it impervious to water. Toduplicate this operation 

 by mechanical means on a large enough scale to make it ap- 

 plicable to the construction of tunnels was the plan which 

 occurred to the engineer ; and how closely he followed his ani- 

 mate model may be seen by examining the drawings of his 

 first shield, for which he secured a patent in 1818. Briefly 

 described, this device consisted of an iron cylinder having at 

 its front end an auger-like cutter, whose revolution was in- 

 tended to shove away the material ahead and thus advance the 

 cylinder. As the cylinder advanced the perimeter of the hole 

 -behind was to be lined with a spiral sheet-iron plating, which 

 Avas to be strengthened with an interior lining of masonry. It 

 will be seen that the mechanical resemblance of this device to 



