. The railroad engineer's practice . so many additional faces to workfrom, they should be filled up on the completion ofthe tunnel, as they interfere with the ventilation. It isfound that the ventilation of the Iloosac tunnel is verybad since completion. It has a shaft in the middle whichis left open, and acts well in removing the smoke fromone end, but not from the other ; the clear end depending,it is believed, on the direction of the wind. When the tunnel is through earth or rotten rock, it willrequire to be arched. This is done with either brick or 48 stone ; in the London clay, which swel
. The railroad engineer's practice . so many additional faces to workfrom, they should be filled up on the completion ofthe tunnel, as they interfere with the ventilation. It isfound that the ventilation of the Iloosac tunnel is verybad since completion. It has a shaft in the middle whichis left open, and acts well in removing the smoke fromone end, but not from the other ; the clear end depending,it is believed, on the direction of the wind. When the tunnel is through earth or rotten rock, it willrequire to be arched. This is done with either brick or 48 stone ; in the London clay, which swells on being exposedto the air, it required a thickness of 5-i inches of brickworkto withstand the pressure ; 18 inches to two feet is theordinary thickness. The Metropolitan Railroad, in Lon-don, sroes under warehouses eishtv feet hi^h with a thick-ness of fourteen bricks, laid in cement, with a layer of con-crete on top. The proper form for the intrados of a tunnel throughsand, or some such substance which acts only by its weight,.
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