112 TUNNELING 



ing at 70 F. At Iselle, air is blown into the entrance of 

 heading No. 2 at the rate of 14,100 cu. ft. per minute by two 

 fans driven from the turbine shaft. This air travels from the 

 fans along a pipe, 18 ins. in diameter, till a point 15 yds. up 

 the tunnel is reached, where beyond a door the pipe narrows to 

 form a nozzle 10 ins. in diameter. This door is kept open to 

 allow the outside air to be induced up the tunnel, as the head- 

 ings are at present only 2,500 yds. long, giving a resistance of 

 not quite sufficient power to cause the air to return. The fresh 

 air then travels up No. 2, crossing over the top of the " gallery 

 of direction," from which it is shut off by doors, to the last 

 cross gallery, returning by No. 1, and finally leaving either by 

 the " gallery of direction " or by No. 1. A system of cooling 

 the air and driving it on by means of a large number of water- 

 jets will be installed in No. 2 where that heading crosses over 

 the " gallery of direction," but at present there is no need for 

 it. 



The average temperature at the face is 73 F. during the 

 drilling operation, 76 F. after firing the charges, and a max- 

 imum of 80 F., lately attaining to 86 F. on the south side, 

 with 80 F. and 85 F. before and after firing. The tempera- 

 ture of the rock is taken at every 110 yds. in holes 5 ft. deep, 

 and shows a gradual increase according to the depth of over- 

 laying rock, to the conductivity of the rock, and to the form of 

 the mountain surface. The maximum hitherto reached on the 

 north side is 68 F., while on the south side, although a smaller 

 distance has been traversed, it attains to 79 F., due to the 

 more rapid increase in depth. Moreover, the temperature of 

 the rock is observed at the permanent stations, 550 yds. from 

 the entrances, in its relation to that of the tunnel and outside 

 air, and though on the north side that of the rock varies almost 

 as quickly as that of the tunnel air, on the south it is influenced 

 very much less. 



A few statistics may be of interest with regard to the prog- 

 ress of the last three months (taken from the trimestrial report 



