Mowbray established a works on the mountain and shortly developed a completely new blasting practice based on the explosive. Its stability was greatly increased by maintaining absolute purity in the manufacturing process. Freezing the liquid to reduce its sensitivity during transport to the headings, and extreme caution in its handling further reduced the hazard of its use. At the heading, the liquid was poured into cylindrical cartridges for placement in the holes. As with the Burleigh drill, the general adoption of nitroglycerine was immediate once its qualities had been demonstrated. The effect on the work was notable. Its explosive characteristics permitted fewer blast holes over a given frontal area of working face, and at the same time it was capable of effectively blowing from a deeper drill hole, 42 inches against 30 inches for black powder, so that under ideal conditions 40 percent more tunnel length was advanced per cycle of operations. A new fuse and a system of electric ignition were developed which permitted simultaneous detonation and resulted in a degree of effectiveness impossible with the powder train and cord fusing used with the black powder. Over a million pounds of nitroglycerine were produced by Mowbray between 1866 and completion of the tunnel.
[Illustration: Figure 12.--WORKS AT THE CENTRAL SHAFT, HOOSAC TUNNEL, for hoisting, pumping and air compressing machinery, and general repair, 1871. (_Photo courtesy of State Library, Commonwealth of Massachusetts._)]
[Illustration: Figure 13.--HOOSAC TUNNEL. AIR-COMPRESSOR BUILDING on Hoosac River near North Adams. The compressors were driven partially by waterpower, derived from the river. (_Photo courtesy of State Library, Commonwealth of Massachusetts._)]
[Illustration: Figure 14.--WEST PORTAL OF HOOSAC TUNNEL before completion, 1868, showing six rings of lining brick. (_Photo courtesy of State Library, Commonwealth of Massachusetts._)]
When the Shanleys took the work over in 1868, following political difficulties attending operation by the State, the period of experimentation was over. The tunnel was being advanced by totally modern methods, and to the present day the overall concepts have remained fundamentally unaltered: the Burleigh piston drill has been replaced by the lighter hammer drill; the Doane drill carriage by the more flexible "jumbo"; nitroglycerine by its more stable descendant dynamite and its alternatives; and static-electric blasting machines by more dependable magnetoelectric. But these are all in the nature of improvements, not innovations.