Dear Sir: In reply to your inquiry as to the relative difference in dredging rock blasted by Nitro-Glycerin and that blasted by Powder, I have no hesitation in saying that I am certain we can dredge twice the number of cubic yards where it is blasted with the Nitro-Glycerin. I think I could speak safely and say three yards to one where the rock is hard. In fact, there are places where we could do nothing with the Powder blasting, when we have no trouble with the Nitro-Glycerin. Truly yours, CHAS. F. DUNBAR, Firm of Lee & Dunbar. RESULT.—Submarine drilling and blasting with Nitro-Glycerin costs 44½ cents per cubic yard. Gunpowder costs 66¾ cents per cubic yard. Nitro-Glycerin used, one ounce and six-tenths of an ounce per cubic yard of rock removed. DIMON’S REEF, NEW YORK HARBOR. General Newton, U. S. Corps of Engineers, who has been entrusted with the expenditure of the annual appropriation for the improvements in New York harbor, having constructed a floating drilling apparatus, with steam power to capstans, four steam derricks, and direct engines to lift the drop-drills, applied to me (1870) first, to enter upon a competitive test, with Nitro-Glycerin as compared with Dualin, and with blasting powder, into which a reel of lightning fuse was inserted, to ensure more perfect and rapid combustion of the powder. These tests were conducted at Hell Gate, under the supervision of Mr. Reitheimer; Mr. H. H. Pratt, with Nitro-Glycerin, on my behalf; Mr. Dittmar with Dualin, and Mr. Gomez, for the powder and lightning fuse blasts, who respectively directed the holes to be drilled, charged them, and fired the several charges. The results were decisive of the superiority of Nitro-Glycerin, over both Dualin, and Blasting Powder, even when assisted by a coil of lightning or fulminating fuse, inserted in the powder. Two points were elicited, as reported by my operator; first the Nitro-Glycerin tore out more work, invariably reaching to the bottom, and sometimes beyond the bottom of the drill hole, whilst its explosion was so instantaneous it did not cause leakage in the roof, as with Dualin. Thereupon I was invited by General Newton, to arrange operations for blasting at Dimon’s Reef, between the Staten Island Ferry and Governor’s Island. Eight holes had been drilled in a circle of twenty feet diameter, with a ninth or central hole, thus leaving an average of eight feet of rock between each drill hole. Finding that the drilled holes were shaped like an inverted cone, owing to the omission of the reamer; that is, whilst the drill, jars, sinker bar, cable and cable clutch of the Pennsylvania oil wells, had been used, the provision for remedying the effect of the worn edges of the drill, had been overlooked, and thus a very disadvantageous form of hole, viz.: