. Shield and compressed air tunneling . SLhVJ:) •M) L as the adopted nioasuromont and the mark on the monumentmarked by a piincli mark. The oj)eration is repeated for theother monument and tlieii tlie Hue of the second tumiel is parallelto the first. 71. Method of Transfer by Transit Sights.—In place of wiresused for transferring the line from the surface to the tunnel,transits have been used. These instruments are made so thatthe telescope can be used when pointing is achieved by mounting the telescope on an overhangingsupport so that, when vertical, the telescope cle
. Shield and compressed air tunneling . SLhVJ:) •M) L as the adopted nioasuromont and the mark on the monumentmarked by a piincli mark. The oj)eration is repeated for theother monument and tlieii tlie Hue of the second tumiel is parallelto the first. 71. Method of Transfer by Transit Sights.—In place of wiresused for transferring the line from the surface to the tunnel,transits have been used. These instruments are made so thatthe telescope can be used when pointing is achieved by mounting the telescope on an overhangingsupport so that, when vertical, the telescope clears the horizontallimb and the leveling plates, or the instrument can be made withthe telescope mounted in the ordinary way but having a slot ^gjmme/if Mont/Ttenf. Fig. 155.—Method of transferring line from surface to tunnel where the shaftis on the line of the tunnel and large enough to enable the tunnel monument tobe visible from the tf)p of the shaft on the opposite side. cut in the horizontal plates so that it may be swung to a verticalposition. These instruments should afford a useful check onthe accuracy of the work done with the wires. In very deepshaftsto be perfectly sure that the wires are not touching someobstruction and to check the oscillations are quite difficultmatters. If the shaft is very large in plan it may be possible totransfer the line by using ordinary transits, using two transitssimultaneously and transferring the line diagonally (see Fig. 155). 72. Work Done at Night.—As in most of this survey work, thetransfer of line from the surface to the tunnel is best done atnight, the sights being lit by electric light boxes placed behindthem. 392 SHIELD TUNNELING 73. Trial Run.—It is well to make a trial run in the day time
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