=Plant.=--The plant used by the contractors for Section IV. of the subway comprised a central power plant located about 4000 ft. from the work. This was on 42nd Street near the East River and furnished power for the work on both Sections IV. and V. The buildings consisted of an engine room 63 × 30 ft. and a boiler room, 42 × 28 ft. In the former room was located one Rand-Corliss air compressor, 22 × 40 × 48 ins., having a capacity of 5000 cu. ft. of free air per minute; in the latter room there were two 200 H.P. water tube boilers. There were also the necessary equipment of feed water pump, air condenser pump, etc. The compressors discharged into a 20 × 5¹⁄₂ ft. receiver of riveted steel through a 7 in. pipe. The air from the receiver was carried by a 10 in. pipe 3.277 ft. to the corner of Park Avenue and 41st Street, and was thence run south along Park Avenue in an 8 in. pipe, from which 3 in. branches led to the four headings of the work. =Ventilation.=--The ventilation of the tunnel caused very little trouble. In cool weather the natural draft of the shafts and the air discharged from the drills served to keep the atmosphere wholesome. In warm weather, artificial means were necessary to clear the workings of foul air, particularly after blasting. They comprised at each end a 4 ft. American exhaust fan drawing air from a 12 in. riveted galvanized iron pipe, which extended to the working faces. =Illumination.=--The tunnel was lighted by electric lamps which extended even to the working face. During the blasting, however, all the lamps and wires within 100 ft. from the front were removed and gasoline torches were used; they were also employed before the electric lamps and wires could be replaced, to light the tunnel during the operation of clearing the débris. CHAPTER XI. TUNNELS THROUGH HARD ROCK (Continued).--EXCAVATION BY HEADINGS. EUROPEAN AND AMERICAN METHODS. The more common method of tunneling through hard rock is to begin the work by a heading, instead of by a drift. This heading may be of small dimensions, and the remainder of the section may also be removed in successive small parts, or it may be the full width of the section, and the enlargement of the section be made in one other cut. [Illustration: FIG. 61.--Diagram Showing Sequence of Excavation in Heading Method of Tunneling Rock.]