To work these drills, several of which were mounted upon a light, movable frame and operated at the same time, the power of the Deerfield River was brought into requisition by means of a dam built nearly a mile above the eastern mouth of the tunnel. The force thus obtained was used to compress the atmosphere to one sixth of its ordinary volume, giving it a pressure of ninety pounds to the square inch. In this condition of tension it was conducted by means of iron pipes to the drilling machines, The compressed air answered a double purpose: by its expansive force it worked the drills most efficiently, and, as it escaped from them after doing its work, served at the same time to supply the miners with pure air from the outer world, and to expel from the tunnel the noxious gases generated by the explosions. The use of compressed air, adopted here for the first time in this country, was a most important aid to the work of constructing the tunnel, and is now regarded as an indispensable adjunct of tunneling operations upon any considerable scale. It was supposed, when the dam was built across the Deerfield River, that it would secure power enough to ply the drills in all the headings of the tunnel, hut it was found to be sufficient only for the eastern opening, and steam-engines were established at the western drift and at the central shaft for the purpose of compressing the air for those portions.

The construction of the tunnel, under the supervision and management of the commissioners, went on until 1868, at the close of which year they contracted, on behalf of the State, with Messrs. Shanley, of Montreal, to take the work and complete it. The tunnel was to be twenty-four feet wide and twenty feet high in the clear, and to be finished by the 1st of March, 1874. The price to be paid the contractors was $1,594,268; the amount, it will be observed, still increasing as the work went on and less remained to be done.

The Messrs. Shanley prosecuted the undertaking with great energy and skill, carrying it on night and day by relays of men working eight hours at a time, and the final blast which threw down the barrier separating the workmen, and established communication through the mountain from the valley of the Deerfield to the valley of the Hoosac, took place November 27, 1873. It was not, however, until February 9, 1875, that the tunnel was so far completed as to allow the passage of cars. The first freight train front the West, consisting of twenty-two cars loaded with grain, passed through the tunnel April 5, 1875. Passenger trains began to run from Boston to Troy in October of the same year. But it was not until July 1,1876, that the tunnel was officially declared to be fully open and ready for business.

Thus was accomplished a work which had been prosecuted so long and attended with so many delays, and which had been absorbing such vast sums of money, that it had wearied the patience of the public, and become, in the estimation of many, a gigantic folly. At times the State and all parties, probably, would have abandoned the undertaking, so endless did it seem, and so endless was the expense necessary for carrying it on. Nothing, apparently, but the fact that the State had already sunk so much money in the enterprise induced her representatives to vote further sums, in the hope of making what had been spent of some final benefit to the public.

But the work was great and difficult beyond the expectation of any. When it was begun, none such for magnitude had been undertaken here or in Europe. Experience was wanting, and experience only could make known the difficulties to be encountered.