38 TUNNEL ly of Philadelphia, who have made a specialty of tunnel transits. The above described three tunnels have been taken as particular ex- amples, because they are the latest driven at the present time (March, 1876), and are the best examples of the present stage of the art of tunnelling in the United States. A large tunnel in Nevada, known as the Sutro tunnel, has been in process of construction with ma- chinery for some years. (See NEVADA.) It is intended to serve as an adit to the Corn- stock lode. (See "Report of United States Sutro Tunnel Commission," Washington, Jan. 6, 1872.) One of the first tunnels in the Uni- ted States was on the Alleghany Portage rail- road in Pennsylvania. It was built in 1831, double track, 900 ft. long ; contract price, $1 47 per cubic yard ; total cost, 14,857 cubic yards, $21,840. Another early work was the Black Rock tunnel, on the Reading railroad, built in 1836. This was 1,932 ft. long, and the exca- vation proper of the tunnel cost $125,935. According to data furnished by Mr. B. H. Latrobe of Baltimore, there are 44 tunnels on the line of the Baltimore and Ohio railroad and its branches, with an aggregate length of 37,861 ft., or 7 m. 901 ft., the tunnels varying from 80 to 4,100 ft. in length. The Sand Patch tunnel, on the Pittsburgh and Connellsville branch, was begun in 1854 and finished in 1871. The work during this time was intermitted for a total period of nine years, owing chiefly to the financial embarrassments of 1858. It was driven through the old red sandstone, and cost nearly $500,000. The Kingwood tunnel, 4,100 ft. long, was begun in September, 1849, and finished in May, 1852, at a total cost, in- cluding excavation and arching, of $724,000. The Broadtree tunnel, 2,350 ft. long, on the same road, begun in the spring of 1851, was completed in April, 1853, at a total cost (exca- vation and arching) of $503,000. The Chesa- peake and Ohio railroad is 423 m. long, and has 7 m. of tunnelling; the Big Bend tunnel, on the Greenbrier division, is 6,400 ft. long. Of the rates of progress attainable by ma- chine drilling, a fair average can be deduced from three large tunnels driven through dif- ferent kinds of rock. At the Hoosac tunnel, through mica schist and micaceous gneiss, with nitro-glycerine, the progress attained by Shan- ly brothers at the east end in 1869 averaged 139 ft. a month, and in 1870, 126J ft. ; at the west end in 1870, 100J ft. In sinking the cen- tral shaft 1,080 ft. in depth, through rock, the average total progress per working month was 21 ft.,but the 230 ft. sunk by Shanly brothers was driven in 7| working months, or at the rate of 30-7 ft. a month. At Nesquehoning, through conglomerate, the average attained in 12 months' driving was 100 ft. a month ; while through red shale an experience of two months gave an average of 160 ft. a month. Common black powder was used, the consumption in the conglomerate being about 6 Ibs., and in red shale 3 Ibs. per cubic yard of rock broken. At the Musconetcong tunnel the average monthly advance through a very hard syenitic gneiss, pronounced harder by experts familiar with both than any body of rock met in the Hoosae tunnel, was in 1874: east heading, average of 12 months, 115'8 ft. ; west heading, average of last 6i months, when steady work was at- tained, 136-8 ft. At this tunnel a shaft was also driven 110 ft. in depth through soft ground, with timbering, at an average rate of 24J ft. a month. The prices bid at the present day for tunnel excavation vary from $4 to $7 and $8 per cubic yard. But the contract prices are not always a sure criterion as to the final cost ; $6 per cubic yard is a medium bid. Very heavy and expensive tunnel work is often done in con- structing underground railways through cities. In these the plan generally adopted is first to make an open-air excavation through the streets, then build the arches and fill in the ground again. A very heavy tunnel was lately finished under the Lendon docks, passing also under some large warehouses, and needing very careful work. The quantity of water pumped was enormous. The final cost was at the rate of 390,000 a mile. Subaqueous Tunnels. Among these should be particularly noted the first one built under the Thames at London. Except however in view of its vast expense, and the fact that it was the forerunner of mod- ern subaqueous tunnelling, its record at the present day, since the system has been further developed, has no very practical interest. It was begun in 1807, intermitted, and resumed in 1825, under Sir M. I. Brunei, intermitted again, and at last completed and opened for foot passengers in 1843. Its total length is 1,200ft.; final cost nearly 1,200 per lineal yard advanced. (See LONDON, vol. x., pp. 616-617.) A tunnel that has attracted much attention throughout both Europe and this country is the one at Chicago, driven out un- der Lake Michigan, for the purpose of obtaining pure water for the city. This tunnel, begun in March, 1864, and aompleted in March, 1867, was entirely original in plan ; the engineer was Mr. E. S. Chesbrough. A crib was first sunk in Lake Michigan, about two miles from the shore, 58 ft. in horizontal outside measure- ment on each of the five sides, and 40 ft. high. The inner portion or well has sides parallel with the outer ones, 22 ft. long each, leaving the distance between the inner and outer faces of the crib, or thickness of the breakwater, 25 ft. This breakwater was built on a flooring of 12-inch white pine timber, laid close together. The outer and inner vertical faces, and the middle wall between them, were all of solid 12-inch white pine timber, except the upper 10 ft. of the outside, which was of white oak, to withstand better the action of the ice. The outer and inner walls were strengthened and connected with brace walls and cross ties of 12-inch timbers, all securely bolted. The crib was built on land, launched, towed into place, filled with atone, and sunk. An iron cylinder,
