Figure 43. — Rear view of St. Clair shield showing the erectoi arm placing .1 cast-iron lining segment. 

 1 he three motions ol 1 In- arm — axial, radial, and rotational, were manually powered. (Smithsonian 

 photo 49260-C.) 



Hudson River Tunnel. The heavy traffic .it this 

 vit.il U.S. Canada rail interchange was ferried incon- 

 veniently across the wide St. Clair River, and the 

 bank and river conditions precluded construction of a 

 bridge. A tunnel was projected by the railway in 

 that year, the time when Haskin's tribulations were 

 at their height. Perhaps because of this lack of prece- 

 dent for a work of such size, nothing was done 

 immediately. In 1884 the railway organized a tunnel 

 . ompany; in 1886 test borings were made in the river- 

 bed and small exploratory drifts were started across 

 from both banks by normal methods of mine tim- 

 bering. I he natural gas, quicksand, -i\^\ water 

 encountered soon stopped the work. 



It was at this time that the railway's president 

 visited Greathead's Citj and South London workings. 

 The obvious answer to the St. Clair problem la) in 

 the successful conduct of this subway. Joseph 

 Hobson, chiel engineer oi the Grand Trunk and of 

 the tunnel project, in designing a shield, is said to 

 have searched for drawings of the shields used in 

 the Broadway and Tower Sulmavs oi 1868-9, but 



unable to locate any, he relied to a limited extent 

 on th<- small drawings of those in Drinker's volume. 

 There is no explanation as to why he did not have 

 drawings of the City and South London shield at that 

 moment in use. unless one considers the rather un- 

 likely possibility that (ireathead maintained its 

 design in secrecj . 



The Hobson shield followed Greathead's as closely 

 as any other, in having a diaphragm with closable 

 doors, but a modification of Beach's sharpened 

 horizontal shelves was also used. However, these 

 functioned more as working platforms than supports 

 for the eai th. I he tna< hine was 21 % feet in diameter, 

 an unprecedented size and almost twice that of 

 Greathead's current one. It was driven by 24 

 hydraulic rams. Throughout the entire preliminary 

 I Onsideration of the project there was a marked sense 

 of caution that amounted to what seems an almost 

 total lack of confidence in success. Commencement 

 of the work from vertical shafts was planned so that 

 if the tunnel itself failed, no expenditure would have 

 been made for approach work. In April 1888, the 



PAPER 41: TUNNEL ENCJINEERINt; —A MUSEUM TREATMENT 



237 



