Of the operations which have been completed, those upon Hallett's point reef are by far the most extensive. Hallett's point, the site of fort Stevens, is a Considerable penin sula, projecting into the East river immediately above Astoria. The reef, which was dangerous to vessels, ran along the shore about 720 ft., and projected about 300 ft. into the river. This immense mass of rock was removed by tunneling, a process first pro posed by Mr. G. C. lleithehner, and soon after by gen. Alexander of the U. S. engineers, and Mr. A. W. Von Schmidt, C.E., to whom was given the contract for removing Blos som rock in San Francisco harbor, the first operation of the kind ever performed. See SAN FRANCISCO. Blossom rock was, however, of less size than Way's reef, being only 180 by 100 ft. in area, while the latter was 235 by 195 at its greatest breadth. This, as described above, was removed partly by surface blasting, and partly by surface drilling, but at less cost than for removal. For the removal of the reef at Hallett's point, a coffer dam in the form of an irregular pentagon of 140 ft. greatest diameter was erected on the shore, and in this there was sunk a shaft having a diameter of 105 by 95 ft., and a depth of 33 ft. below mean low water. From this shaft diverging tunnels were excavated in the rock, which is a tough gneiss, with nearly a perpendicular dip (the character of all the rock in Hell Gate). As these tunnels extended they were connected by transverse galleries, and afterwards new diverging tunnels, commencing in the galleries were com menced and extended. There were eight concentric galleries, with the addition of two Additional partial ones at the periphery, the whole number of tunnels being 35. The mouths of the tunnels were from 17 to 22 ft, high and 12 to 9 ft. wide, but dimin ishing quite rapidly to keep under the floor of the river. The galleries, which were about 25 ft. from center to center, also varied in height for the same reason. The lineal meas ure of the tunnels and galleries was 7,425 ft., and the whole area embraced was 21 acres. The ordinary processes now in use for tunneling or excavating rock were employed. See TUNNEL. The mass of rock included in the scope of operation is said to have been something over 110,000 cubic yards; the number of cubic yards of rock removed from the tunnels and galleries being computed to be 47,461 cubic yards. After
the excavation was completed, holes were drilled into the piers, which were left stand ing between the tunnels and galleries, supporting the root', and also into the roof, and charged with nitro-glycerine held in cans into which the electrodes of a galvanic battery were introduced. Water was let into the mine by a siphon into the shaft the day previous to the explosion, which took place on Sunday, Sept. 24, 1876. It is proper here to state, that the blasting was done on this day, notwithstanding there were protests, because of a delay in the delivery of the explosives, and the charging having been completed, gen. Newton was unwilling to endanger the lives of his men by delay. The time of the explo sion was 2 b. 57 m. at high tide. It lasted about three seconds, the vibration of the earth being slight, but perceptible to a considerable distance. No damage was done to property. The nitro-glycerine used was contained in the following compounds: Dyna mite, 28,901 pounds; rend-rock powder, 9.061; vulcan powder, 14,244; making a total of 52,206 pounds, or more than 25 tons. This was contained in 13,596 cartridges. The number of charged holes was 4,427; they were from two to three inches in diameter,. with an average depth of 9 ft., and averaged about 8 ft. apart. They were all connected by about 100,000 ft. of wire, and the blasts were exploded by 64 batteries having an aggregate of 960 cells. Three cartridges were usually placed in a cell, with a priming of dynamite which was exploded by the percussion of fulminate of mercury. connected with the voltaic arc. The success may be said to have been perfect. The blasting at first, or before 1874, was principally with pure nitro-glycerine. Afterwards "mien powder," "giant powder," "rend-rock," and " vulcan powder," were used, and although having less power, were found more effective and economical. The amount appro priated by congress between 1868 and 1876 was $1,940,000, of which nearly $1,717,000 was expended upon the operations at Hallett's point. r!rncrift t