──────────┼────────┼───────┼─────────┼─────────┼─────────────────────── North │ │ │ │ │Maximum in gravel River, │ │ │ │ │ 10,000 cubic feet per N. Y., │ │ │ │ │ man per hour. Penn. │ │ │ │ │ Generally ranged R.R. │ 98│ 20│ 37│ 26│ between 1500 and 5000 ──────────┴────────┴───────┴─────────┴─────────┴─────────────────────── Entrance and exit to the tunnel are gained through air locks. These are sheet iron cylinders concreted into the lining of the tunnel or shaft. Air-tight iron doors are provided at both ends, which open inwards towards the tunnel. On entering the lock from the outside the door to the tunnel is found tightly closed. The outside door is then closed by hand, the air valve is opened and air is admitted to the lock until the pressure on the lock side of the tunnel door equalizes that on the tunnel side and the tunnel door is swung open by hand. When the lock is open to the tunnel the pressure in the tunnel keeps the outside door closed. In order to leave the tunnel the process is reversed. Materials are passed through the lock by the lock tender or tenders who pass through the lock with the material if the pressure is low, or who manipulate the air outside of the lock if the pressure is high. If pressures of 30 to 40 pounds are being used, two or even three locks may be necessary. EXPLOSIVES AND BLASTING[97] =171. Requirements.=—The desirable features in an explosive to be used in trenching and tunneling in rock are: (1) stability in make up so as not to deteriorate in strength or to become dangerous during storage, (2) imperviousness to ordinary variations in temperature and moisture, (3) insensibility to ordinary shocks received in transportation and handling, (4) not too difficult of detonation, (5) convenient form for transportation and loading and for making up charges of different weights, (6) the non-formation of poisonous gases when fired, (7) imperviousness to water and usefulness in wet holes, (8) power without bulk, etc.