Page:Popular Science Monthly Volume 28.djvu/449

Rh them and Rhinelander's Reef. The two surveying-officers, while agreeing as to the desirability of removing or mitigating the obstructions, made different recommendations respecting the manner in which they should be dealt with.

Lieutenant-Commanding Davis recommended that Pot Rock, the Frying-Pan, and Way's Reef be blasted and scattered, and that the Middle Channel be improved by blasting, so as to make a clear channel of sufficient depth for common vessels and steamboats. As the removal of the larger reefs seemed at that time impracticable, he advised that they be faced with sea-walls or piers rising four feet above high water, and provided with spring fenders such as are used at the ferry-docks, so adjusted as to guide vessels coming in contact with them into the channel-ways. Lieutenant-Commanding David Porter, not regarding the deepening of the Middle Channel as practicable, advised that it be filled in with docks, and recommended the removal of a part of the reef at Hallet's Point.

No efficient method was suggested for removing the rocks it was proposed to take away, further than to blast them and leave the fragments to lie where they might fall, or to be washed away by the currents. This method would obviously make but a slight impression on the larger reefs.

A process of surface-blasting was first applied by M. Maillefert in 1851 His method consisted simply in placing upon the rock a charge of gunpowder, usually of 125 pounds, contained in a tin canister, and exploding it by means of the voltaic current. The weight of the water resting upon the charge served to increase the effects of the explosion. No means were provided for removing the broken rock except by breaking it up by successive explosions till it was fine enough to be carried away by the currents. M. Maillefert's operations resulted, by the use of 620 charges containing 74,192 pounds of powder, at a cost of $13,861, furnished through Mr. Merriam, in removing from the rocks to which they were applied the projecting prominences of small area, but were of little effect when, after reaching the main rock, a considerable extent of surface had to be dealt with. The depth of water was increased, on Pot Rock, from 8 feet to 18·3 feet; on Frying-Pan, from 9 to 16 feet; on Way's Reef, from 5 to 14 feet; and on Shell Drake, from 8 to 16 feet. Bald-Headed Billy and Hoyt's Rocks were blown into deep water. The depth on Diamond Reef was but slightly if at all affected, and no effect was produced on Hallet's Point Reef. In 1852, Congress having appropriated $20,000 for the removal of obstructions, Major Fraser, by Maillefert's method, increased the depth over Pot Rock to 206 feet, adding 23 feet of water to the 18·3 Maillefert had gained, with the expenditure of $6,837. The decrease in the ratio of returns for expenditure was occasioned by the increased surface of the rock, due to increase of depth.

In 1856 an advisory council to a commission on the removal of