316 



ENGINEERING. 



aroused the dormant engineering spirit with- 

 in him, and, being a ferryman, he went to work 

 with such materials as the region supplied 

 to substitute a bridge for his own advan- 



proof stratum of strong clay existed, and it 

 was only necessary to shore up the roof tem- 

 porarily with strong timbers, to secure it 

 against possible subsidence. In view of the 



BRIDGE BUILT BY A MEXICAN PEON, 



tage. His first attempt was on the suspension 

 plan, the cables and suspenders being twisted 

 from wild vines, passed over rude timber tow- 

 ers and anchored to bowlders. No iron was em- 

 ployed. Mr. Flagg frequently used the bridge, 

 being engaged at the time in constructing a 

 railway near by. A freshet swept the bridge 

 away before it was a year old but the peon, 

 nothing daunted, went to work again and 

 made a second one, as shown in the illustration, 

 evolving from his own head several of the 

 best principles of modern bridge-construction. 

 The piers were made with cribs of driven 

 stakes bound together and filled with stones. 

 The suspension- cables were of twisted vines as 

 before, bound together with lighter vines. The 

 stringers of the main span in two pieces were 

 tied together at A and supported there by the 

 cable the only direct use required of it. The 

 towers were natural forked timber, the cables 

 resting in an upper crotch, and the corbels, 

 B B, in a lower crotch. The stringers were 

 further supported by cantilevers, C, which 

 were loaded with stones near the abutments 

 to balance the central strain. The rest of the 

 abutment construction is apparent from the 

 sketch. The roadway was only wide enough 

 for the passage of one animal at a time, and 

 proved strong enough for all the demands that 

 were made upon it. It was in use for about 

 18 months, when a heavy freshet carried it 

 away, and it is not known whether the experi- 

 ment has been repeated or not. 



The Mersey Railway Tunnel. This highly cred- 

 itable example of subway construction was 

 opened to the public with due ceremony by the 

 Prince of Wales on Jan. 13, 1886. Its progress 

 was briefly described in the " Annual Cyclo- 

 paedia" for 1885, but certain features of the 

 completed structure deserve more detailed de- 

 scription. As a whole the work is notable 

 rather for its magnitude and careful elabora- 

 tion of detail than for any especial difficulty 

 of construction. With the exception of about 

 a hundred yards under the river, the tunnel 

 runs through solid sandstone rock, and no 

 precautions had to be taken against caving. 

 Even where the rock fell below the crest of 

 the tunnel it was found that a nearly water- 



magnitude of the expected traffic, great care 

 was taken to provide for perfect drainage and 

 ventilation. To this end, drainage - headings 

 were run in both directions from the central 

 level at a slight incline shoreward, as far as 

 the working shafts at each end, which were 

 sunk to a sufficient depth, and provided with 

 powerful pumps. These served to keep the 

 main headings free from water during the prog- 

 ress of the work, and in their finished state serve 

 the same office for the completed tunnel. The 

 infiltration of water, however, through solid 

 sandstone and the lining of blue Staffordshire 

 brick, is very slight and will probably cease alto- 

 gether through the gradual deposit of silt. 

 The ventilation heading runs beside the cen- 

 tral level and thence rises at a steep incline to 

 the fresh-air shafts one at either end of the 

 tunnel where two huge fans, respectively 

 thirty and forty feet in diameter, maintain a 

 perpetual current and change the air in the 



TERMINAL ARRANGEMENTS OP MERSEY RIVER TUNNEL. 



tunnel about once in seven minutes. Tl 

 general arrangement of the three tunnels (air, 

 drainage, and traffic, at their landward termini 

 is shown in the illustration. 



