More important, however, is the compressed air rock drill, in which a piston has the drill bit directly on its piston rod and cuts by a reciprocating action. The piston is actuated by compressed air admitted alternately to its opposite sides in an automatic manner by valves. The compressed air conveyed to the drill in the tunnel or mine not only operates the drill, but helps to ventilate the tunnel. As early as 1849 Clarke and Motley, in England, invented a machine drill, and in 1851 Fowle devised a similar machine, having the drill attached directly to the piston cross head. The Hoosac and Mont Cenis tunnels greatly stimulated invention in this field, and among the notable drills of this class may be named the Burleigh, Ingersoll, and Sergeant. The Burleigh drill was brought out in 1866, and was covered by patents Nos. 52,960, 52,961 and 59,960 of that year, and 113,850 of 1871, and the Ingersoll drill, by patents No. 112,254, and No. 120,279, of 1871. [Illustration: FIG. 233.--BLOWING UP FLOOD ROCK.] _Blasting._--The discovery of nitro-glycerine in 1846, followed by its convenient commercial preparation in the form of dynamite, gave a great impetus to blasting. Notable as the largest operation of the kind in the century is the blowing up of Flood Rock, in the path of commerce between New York City and Long Island Sound. The dangerous character of this and other rocks in this vicinity gave long ago to this channel the significant name of Hell Gate. The undermining of the rocks by shafts and galleries is seen in Fig. 233, and the final blowing up of the same in a single blast was the culmination of a series of similar operations at this point tending to safer navigation. On October 10, 1885, 40,000 cartridges, containing 75,000 pounds of dynamite and 240,000 pounds of _rack-a-rock_, were, by the touching of a button and the closing of an electric circuit, simultaneously exploded. In the twinkling of an eye nine acres of solid rock were shattered into fragments by the prodigious force, and a vast upheaval of water 1,400 feet long, 800 feet wide, and 200 feet high, sprang into the air in tangled and gigantic fountains. As the termination of the most stupendous piece of engineering of the kind the world has ever seen, and with spectacular features fitting the enormous expense of $1,000,000, which the work cost, this final scene put an end to the menaces of Flood Rock, and wiped out of existence the worst dangers of Hell Gate. [Illustration: FIG. 234.--CROSS SECTION MISSISSIPPI JETTIES.]