702 BLASTING the longer and shorter axes of the rock, and steam was used in hoisting. The rubbish was dumped upon one side of the rock, from which most of it was washed by the tide. During the month of January, 1870, eight men found room to work. Most of the rock was removed by picks and sledges, only 10 Ibs. of explosive (giant powder) being used in the whole opera- tion. In February 16 men found space to work, and by the 20th of April the dimensions of the cavity were 140 by 60 ft., with a maxi- mum height of 12 ft. Columns of rock were at first left for support, but they were from time to time replaced with upright timbers from 8 to 10 inches in diameter, with the ex- ception of four, which were left standing near the shaft. Preparations were now made to FIG. 6. Vertical Section of Cofler Dam and Eicavntton at Blossom Bock. blow up the shell. The following diagram, copied from the official report, will explain the method of conducting the explosion. Powder Fio. 7. Horizontal Section, showing Charges. was used as the explosive, nitrate of soda taking the place of nitrate of potash in its composition. The quantity used was 43,000 Ibs. The vessels for containing it were 38 ale casks of 60 gallons each, and seven old tanks made of boiler iron, holding about 300 Ibs. of powder each. The explosion was effected by a galvanic battery stationed in a boat about 800 ft. from the rock. A column of water about 200 feet in diameter was thrown into the air to a height of 200 to 300 ft., and pieces of rock and timber were thrown high above the water column. The rock was found to be ef- fectually demolished, although if the excava- tions had been carried to a greater depth much after labor in clearing away rubbish and pro- jecting points would have been saved. The contract was fully carried out by Mr. Von Schmidt, under the immediate inspection of Lieut. W. H. Heuer of the corps of engineers. At New York, the operations of Mr. Maillefert in surface blasting had greatly improved the navigation of the East river; but no compre- hensive plan was projected till the summer of 1866, when brevet Major Gen. John New- ton was assigned by the war department to the duty of examining the obstructions, and making estimates of the work necessary to be done. He submitted three plans, each of which included the removal of the rock at Ballet's point. Some work was done on some of the smaller rocks by Mr. S. F. Shelbourne, who tried experiments with a rotating diamond drill, and afterward constructed a percussion drill of larger size, which was destroyed by a collision before it was brought to the test of drilling. In the spring of 1869 congress appropriated $175,000 for improvements at Hell Gate, and Gen. Newton proceeded to complete the plans for the performance of the work. The re- moval of the submarine rock at Hallett's point was the first work decided upon. This rock, projecting some 300 ft. into the stream, and throwing the tide from Long Island sound against an opposing rock called the Gridiron, makes the navigation at that place very dim- cult. The plan of operation was to sink a shaft upon Hallett's point, and from it excavate tun- nels in the rock in a radiating direction under the river and connect them with concentric galleries ; then, after removing from the inte- rior as much of the rock as possible without danger of letting in the water, to blow up the roof and supporting columns. The work was commenced in July, 1869. A coifer dam in the form of an irregular pentagon, whose great- est diameter was 140 ft., was erected on the shore, and a shaft 105 by 95 ft.. in diameter was sunk to a depth of 32 ft. below mean low water. Diverging tunnels were then commenced, and FIG. 8. Ground Plan of Tunnels and Galleries at Hallett's Point after they were sufficiently advanced concen- tric galleries were excavated, and as the work proceeded their number increased, until at the present time (November, 1872) there are 19 tunnels, some of which are nearly completed, extending from 190 to 240 ft. beyond the shaft, and connected by seven concentric galleries, from which 28,000 cubic yards of rock have been removed. The rock is a tough horn- blende gneiss, and lies in strata of various de- grees of inclination, presenting interesting prob-
