obstructions in Hell Gate recommended a plan which, as to its general features, was substantially the same as that which Lieutenant-Commanding Davis had proposed in 1848. Instead of Maillefert's process of surface-blasting, they recommended that of drilling. This would have had to be done from within a diving-bell, which was impracticable because of the certain disturbance of the apparatus by currents, and the liability to collisions from passing vessels. All of the plans 80 far tried or proposed seemed to have been limited to removing the smaller rocks lying in the channel; the possibility of clearing away the larger reefs was not yet conceived.
In 1866 the department instructed me to make an examination of Hell Gate, and to present a plan and estimate of the operations necessary to improve its navigation. A report was submitted in January, 1867, with an estimate for removing the reefs by blasting, after drilling the surface from a fixed platform above the water. In this plan, or in whatever plan might be decided upon, it was considered essential that the drills, which, to avoid interference from currents, were to be worked within iron tubes reaching from the platform to the rock, should be attached to a framing kept absolutely fixed while the drilling was going on; and that the divers or the machinery necessary to handle and remove from the bottom the rock blasted should be protected from violent currents. It was proposed to accomplish the latter object by means of a system of floats and iron curtains so arranged as to constitute a dam protecting a space of 160 feet long and 40 feet wide, within which the work could go on uninterrupted. Another machine was alternatively suggested, embodying the same principle, to consist of an iron caisson or cylinder, pointed at the ends, open at the top and bottom, and having self-adjustable legs to accommodate themselves to the irregularities of the rock, and to support it and keep it level. The top of this structure was to be above the level of the highest tide, and to be framed over so as to form a platform, on which the machinery could be placed, and from which the operations could be conducted. It was important in constructing the machine to have it adaptable to all the rocks on which it was intended to operate, whatever their size and the shape of their surface, and that it be perfectly stable against the action of the currents without being unwieldy in size. It was furthermore considered necessary to furnish it with a protection against collisions which were sure to occur from the fleets of vessels passing daily, and certain to destroy it, with a loss of life and much property, if it were not protected.
These conditions were fulfilled in the steam-drilling cupola-scow (Figs. 1 and 2). The scow is built very heavy and strong; is provided with an overhanging guard, faced with iron, surrounding it as a protection against collisions; and has a well-hole 32 feet in diameter. The caisson or dome is a hemisphere 30 feet in diameter, composed of a strong iron frame covered with boiler-iron. It is open at the bottom
