SUBMARINE TUNNELING 



253 



Longitudinal Section.. 



FlO. 130. Elliptical Shield f-r (Mirny S.'w.-r Tunnel, PmrU. 



Mr. Raynald I^gouez, in his excellent book upon the shield 

 system of tunneling, considers that tunnel shields may be di- 

 vided into three classes structurally, according to the character 



