TUNNEL ling through loose rock, with subsequent tim- bering and arching, is shown in figs. 6 and 7 ; it is the one most used in America, and is ex- peditious, though probably more expensive than the European systems. The timbers 1 and 2 are put in to support the roof and sides when the top heading (which is generally preferred through rock) is driven; the "legs" (2) are occasionally braced by a bar (3), which is sup- FIG. 6. ported by a raker (4), while the sides are being dressed down when the tunnel is enlarged and arched. The apace between the timber and the rock 'above, and between the masonry and the timber (which latter in this work should be left in place), is packed tight with fragments of stone, to prevent a sudden fall or stress be- ing brought to bear on the masonry. Tunnel- ling through solid rock by hand labor is still, in Fio. 7. many cases, held to be more economical than by machinery. It is certainly so, as yet, in the case of small tunnels throug'h a comparatively soft rock, where the necessary cost of a plant of air drills and compressors would be in excess of the economy in time gained by their use. In driving a tunnel through rock, an advanced heading is first driven either at bottom or top- and this may either be of the full width of the proposed excavation, or narrower. The head- ing is always the most difficult and expensive part of the work ; for whether it be driven at top or bottom, the miner, in removing the re- maining portion of rock, of course has much less resistance to contend against in blasting. Removing the top rock or the lower " bench " is more like open-air quarrying. Longer holes can be drilled, and heavier charges of powder used. At the present day, however, most heavy tunnel work is carried on with the aid of machine drills, driven by compressed air, which, on being liberated after acting as a motor, serves to ventilate the work. Since the introduction of machinery, the rate of driving attained in tunnelling has been greatly increased. Machine drilling was born of the necessity for some more rapid method of ex- ecuting certain works, deemed almost too heavy to be accomplished by ordinary means. These were, in Europe, the Mont Cenis tunnel (see CENIS, MONT), and in America, the Iloosac tun- nel in Massachusetts. Various types of drills have been invented and tried abroad ; among them the Sommeiller, Dubois-Francois, Sachs, Osterkamp, Brydon Davidson and Warring- ton, Azolino dell' Acqua, Ferroux, McKean, and others. Among compressors that of M. Colladon of Geneva may be particularly noted. At Mont Cenis the air pumps were worked by hydraulic power. The perforators used there were built partly from designs already pre- sented, but improved with original modifica- tions made by the engineers in charge, Messrs. Sommeiller, Grandis, and Grattoni. A descrip- tion of the Sommeiller machines may be found in the Portefeuille economique des machines (1863). The Mont Cenis tunnel was begun by hand labor in 1857, and finished in 1871, at a total cost of about $15,000,000. The follow- ing table, from M. Opperman's NouteUes an- nales de la construction (1869), shows the rate of advance in that work by hand, and the increased rate attained immediately after the first introduction of machinery down to 1865, working throughout with two headings : YEARS. By hnd, mttm. By band and machinery, metre*. By machlBcry alone, metre*. 1857... 88 459 8fi9 813 81 C 58 3 "l 1,8 M 28 1858 1S59 160 1S61 1S62 1868 ]64 1865 The St. Gothard tunnel, also through the Alps, is now (1876) in progress. From a late paper on the subject by Daniel K. Clark, M. Inst. C. E., London, we obtain the following general facts concerning it. The length of the tunnel is to be 16,295 yards or 9J m. The contract prices sum up to a total estimated cost of 1,896,945. Construction was begun in the
