Two pairs of Stirling boilers, with a total capacity of 2,000 h.p., were installed. As a rule, at times of maximum demand, three of the boilers were in use; after the Central Shaft was stopped, two were generally sufficient, until, toward the latter part of the excavation, the losses in the transmission of the air made it necessary to keep three going. Eight compressors (the six old ones with two brought from Weehawken), were installed in the new power-house. All were of the same type, namely, Rand, straight-line, steam-driven, 24 by 24 by 30-in., each with a nominal capacity of 1,250 cu. ft. of free air per min. Seven of these were generally worked to their full capacity in order to keep up the necessary supply of air. The maximum requirements of air at this end were primarily estimated as follows: Central Shaft, four headings 24 drills. Hackensack, two working faces 20 drills. ---------- Total 44 drills. Cubic feet of free air per minute. 44 Slugger drills (25 by 174) require 4,350 2 Steam shovels 1,600 Pumps and machine-shop, say 1,000 4 Hoisting engines, placing concrete 2,000 4 Derricks 2,000 ------ Total 10,950 The theoretical capacity of the whole eight compressors was: 1250 × 8 = 10,000 cu. ft. of free air per min. It was considered that not more than two-thirds of the above equipment would be working at the same time; the actual requirement, therefore, was taken at about 8,000 cu. ft. of free air per min., thus leaving a margin of one spare compressor. As actually worked out, there were probably never more than eight drills working at any one time at the Central Shaft, and this work was entirely suspended in June, 1907, before there was any demand for power in connection with the tunnel lining. The heaviest actual requirement, therefore, was approximately as follows: (_A_) _Previous to June 25th, 1907:_ Cubic feet of free air per minute. 40 Drills (22 by 174) 3,828 2 Shovels 1,600 Pumps and machine-shop, say 1,000 2 Derricks 1,000 ----- Total 7,428 (_B_) _After November, 1907_ (_after completion of enlargement of Central Shaft headings_): Cubic feet of free air per minute.