angles to one another. The piers of rock were about 15 feet square and 25 feet apart from center to center. The roof of the cross-galleries, which ran at right angles to the lines of stratification, was blasted down as thin as it would be safe to leave it. Considerable risk was incurred in this part of the work, from the danger of the rock's crumbling, and from the uneven and uncertain thickness of the roof. The average thickness was 18.8 feet, and the minimum thick

a concretion of bowlders and shells formed upon it. Had the excavations at any time broken into a large seam, the mine would have been flooded, and all the work and probably many lives would have been lost. Occasionally small seams were met. One seam was 10 inches wide and 100 feet long; another, from 1 to 4 inches wide and 400 feet long, extending clear across the reef, carried 350 gallons of water a minute. The latter was dealt with after protecting the completed part of the work by building across the gallery a door capable of withstanding the pressure of the water. The seams were all walled, as fast as they were opened, with Portland cement. The total length of the galleries was 21,670 feet.

The galleries were excavated to depths vary. ing with the uneven surface of the reef. The roof was then drilled with holes for the reception of the explosive cartridges. The holes were slanted upward at angles varying from 75° to 45°, and were made from 8 to 10 feet deep-except where the existence of seams open to the river made it impossible to obtain the depth wanted-and of sufficient capacity

Mean High Water

FLOOD ROCK-SECTION THROUGH GALLERY 12.

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FLOOD ROCK-SECTION THROUGH HEADING F.

ness 10 feet. The exact thickness could not be ascertained beforehand, for no soundings could distinguish between the solid rock and

THE CHARGES IN THE MINES.

to receive a rigid two-and-a-half-inch cartridge throughout their entire length. The holes were charged with rack-a-rock as the principal explosive (see page 346). The mixing was done in small batches on Great Mill Rock, in a leadlined trough, and the explosive was packed at once into cartridge-cases 24 inches in diameter and 24 inches long, made of copper 0.005 of an inch thick. To prevent the corrosion of the copper by the chemical action of the sulphureted water running through some

wires, they being held firmly in place by the sulphur. The inner ends of the wires are united by a small platinum wire. The ends of the wires are then surrounded with fulminate, and the two parts of the tube are put together, that containing the wires slipping within the other. The entire fuse is then covered with gutta-percha. The passage of an electric current through the wires heats the platinum bridge to redness, and causes an explosion of the fulminate.

The number of pounds of rack-a-rock put into drill-holes was 240,399; of dynamite, 42,331; total, 282,730 pounds. There were 11,789 drill-holes in the roof and 772 in the pillars, and their total length was 113,102 feet, or more than twenty miles. The whole amount of rock to be broken by the final blast was 270,717 cubic yards, covering an area of about nine acres. The primary charges, the office of which was by their detonation to produce the explosion of the charges in the drill-holes, were placed along the galleries at intervals of 25 feet, on timbers extending from wall to wall, and consisted of two 24-inch dynamite cartridges like those already described lashed to the timber, with one of the "mine-exploders" bound upon them. The entire mine was divided into 24 independent circuits. Within each of 21 of these circuits were 25 fuses or mine-exploders, while three circuits contained 22 fuses each. A wire from the battery on the surface of the rock at the mouth of the shaft led from one fuse to the next, until the 25 fuses were in the same electrical circuit, and thence back to the battery. So far as was practicable, adjacent charges were put on different circuits, so that if any circuit failed through fault in the connections, an explosion of its charges would still be insured through the sympathetic action of the adjoining charges. The whole number of these primary charges was 591. Some of the circuits were nearly a mile long. The battery consisted of sixty cells, all coupled in one series, each of which had an electro-motive force of 1·95 volts and an internal resistance of 0.01 ohm. The plates were 6 inches by 9 inches-four carbon and three zine plates in each cell, separated by a quarter of an inch. The ordinary bichromate solution was used. The poles were constituted of two large mercury cups, into one of which were dipped the 24 lead wires, while the 24 return wires terminated in a third cup. Between this third cup and the remaining pole of the battery stood the apparatus for closing the circuit. It consisted of a stout iron cup containing mercury, in which sat a thin glass tumbler also partly filled with mercury. Two large strips of copper connected the mercury in the iron cup with one pole of the battery, and that in the glass with the cup containing the return wires. To close the circuit through the fuses it was only necessary to break the tumbler so as to let the mercury in it mix with that in the iron cup. To do this at the

proper moment, a quarter-inch iron rod four feet long, terminating at the top in a sinall round disk, stood with its point in the bottom of the glass. It was long enough to pass through the roof of the battery-house. A 30grain platinum fuse, connected with a small battery at Astoria, was laid on the disk and stuck on with a lump of wax. It had been previously determined by experiment that the blow struck by this fuse on exploding, and transmitted by the iron rod, would be so sharp as to pulverize the tumbler and yet not splash the mercury.

The mine was flooded by two siphons of 12 and 16 inches respectively, in fifteen hours and a half, ending at 3.30 A. M., Oct. 10. The explosion was set for 11 A. M., Oct. 10, but everything could not be made ready and tested in time to fire at the appointed moment. The explosion did not take place till 11.13, which caused some confusion in the seismoscopic observations. The whole area of the reef was shattered. The plan of making the excavations large enough to swallow all the débris of the reef and leave a channel deep enough, without further operations, already abandoned at Hallet's Point as more expensive than dredging up the broken rock, was never entertained at Flood Rock. Though the charges all exploded at the same instant, the time and the appearance of the effect above the water-surface varied according to the strength of the rock and the depth of the water. There was no loud report, and no dangerous shock. The breaking of some panes of glass and the shaking down of a few bricks and loose ceilings consti- ' tuted all the damage that was done.

Pending the awarding of a contract for dredging, the work of removing the rock was begun with a scow belonging to the Government, as soon after the explosion as possible. From 15 to 30 tons of rock were hoisted out daily, after being slung by divers on chains. A contract has been let for the removal of 30,000 tons of the rock at $3.19 a ton, the contractor to do his own surface-blasting. This is less than the price for which rock was removed on similar terms at Hallet's Point. The contractor has two grapples at work, and is removing an average of about 120 tons a day. As a whole, the cost of mining a cubic yard of rock has been reduced 34 per cent. from the cost of doing the same work at Hallet's Point. The total cost of the work done on Flood Rock, including the final blast, amounts to $2.99 a cubic yard of the whole amount of rock broken, or $5.66 less than the cost of breaking Hallet's Point. A considerable part of this gain will be expended on the proportionately larger amount of dredging to be done. The net result, however, will show an improvement of not less than 30 per cent. The total cost of the final blast at Hallet's Point was $81,092.24; at Flood Rock it was only $106,509.93, though the blast was 5-6 times as large. The progress with the dredging gives promise that an 18