MECHANICS AND USEFUL ARTS. 45 



passes away at once by small tubes into the iron reservoir (which is of 

 the shape of a pontoon). Each contains nearly 5,600 cubic feet of 

 air, is about 33 feet long, and nearly 6 feet in diameter. All these 

 operations are effected in much less time than it takes to describe 

 them ; and the opening and shutting of the valves of alimentation and 

 discharge are effected opportunely, without the intervention of human 

 aid, thanks to the automatic action of a small apparatus, regulated 

 suitably by a column of water. 



The air in passing from the upright column into the reservoir loses 

 some of its density, and has only a pressure of four and a hah atmos- 

 pheres. The bottoms of the reservoirs are filled with water, which is 

 displaced as the compressed air enters ; this is supplied by the smaller 

 tubes, which descend the side of the mountain between the larger 

 ones, that is, they have five large water tubes on either side of them. 

 These reservoirs are furnished with water-gauges, by which the quan- 

 tity of air and water in them may be seen and regulated at any time 

 by the mechanical engineer in charge of the works. 



The cylinders being always kept sufficiently full, according to the 

 judgment of the engineer, the apparatus is constantly in working or- 

 der ; and the operations of boring the holes for blasting in the head- 

 ing, and a constant supply of fresh air in the tunnel, are a natural 

 consequence. But before proceeding to describe the operations of the 

 boring-machines, and how they are put in motion, we may mention 

 that 7,924 strokes of the machinery per day produce about 51,000 

 cubic feet of compressed air, which flows away into the cylinders of 

 the boring-machines, and eventually into the tunnel, which it venti- 

 lates admirably. 



The principle of the machinery for boring is extremely simple. 

 The machinery consists of two essential parts, one the apparatus 

 for driving the cutting tool into the rock and withdrawing it, so that 

 the blow may be repeated while at the same time it is made to take a 

 whole turn on its axis, now towards the right and now towards the 

 left ; and the other, the machinery for advancing the bed on which the 

 cutting tool rests as the work goes on, so that the auger or borer may 

 constantly advance as the hole is being bored, and may be kept well 

 up to the surface it has to operate upon. Both portions of this ma- 

 chine are worked by means of the compressed air. 



The first or cutting part of the machine is a long borer, which in 

 reality is fixed upon a continuation of the piston-rod, working in a 

 cylinder into which the compressed air is admitted fore and aft alter- 

 nately, so as to drive the piston forward against the rock and withdraw 

 it by^a back and forward motion, while a turning motion is given at 

 the same time. The edge of the borer is in the shape of the letter Z, 

 so that, in turning and returning, the substance of the rock is ground 

 away in the hole, as well as pulverized by impact. 



Six of these borers, with the machinery for driving them forwards, 

 six reservoirs containing water, which is forced in a constant jet into 

 the hole while the boring is being performed, so as to facilitate opera- 

 tions and clear away the debris, and apparatus for lighting by gas, are 

 mounted on a railway-carriage frame, and the whole works in a head- 

 ing 1 1 feet 6 inches by 1 1 feet. It drives holes in the rock varying 

 from 2 to 3 feet in about twenty minutes. These holes can be bored 



