throwing water on it; but it had already made too great progress, the frequent explosions of fire-damp having become very dangerous. It was then decided to reverse the ventilating current in order to prevent the fire from destroying the pumps. M. Jules Letoret then prepared to introduce carbonic acid into the works, that gas having, as we have already stated, the property of rendering harmless an explosive mixture of carbureted hydrogen and air, if only added to it in the proportion of one-tenth to one-seventh. Several experiments were made for the manufacture and introduction of this gas-the first on the 17th of February, 1844. The effect of the gas was to reduce the number and violence of the explosions, but the fire still continued to burn at the foot of the shaft; the pit was not perfectly closed at the time. On the 23d of February M. Letoret built a furnace, intended to produce carbonic acid, in a continuous manner, from the combustion of coke; the shaft was closed hermetically, leaving only openings for the introduction and outlet of the products of combustion. This apparatus is shown in the accompanying figure, and is of so simple a construction as to require but

little explanation. The furnace was charged with burning coke and charcoal to the depth of 17 inches, that depth being found sufficient to consume all the oxygen in the air passing through the fire, and to produce carbonic acid and nitrogen, (a greater depth of fire will produce at the same time carbonic oxide;) and as this gas issued from the furnace at a high temperature the reservoir of water, h, was inserted through the stopping, so that the gases might be cooled and prevent any chance of igniting the woodwork of the shaft. When the apparatus was set to work on the 24th of February, the fire was visible at the foot of the shaft; on the

Letoret's Furnace for manufacturing After-damp, 1844. 25th the flames and explo

sions had ceased; the introflame.eduction of "after-damp" k.-Iron

a.-Ash-pit. d.-Fire-place. c-Pipes carrying off the Pipe carrying gas into the pit f. g.-Pipe admitting water. bars supporting e. m.-Iron flooring covered with soil. was then stopped, and fresh air was allowed to enter the mine; but on descending into the mine a large fire was discovered, quite red. but with little flame. The work of clearing out the gallery leading to it was com menced, in order to be able to throw water on the fire; but the barometer indicating a diminishing atmospheric pressure, the fear of fresh discharges of carbureted hydrogen induced the abandonment of the mine, into which carbonic acid was again introduced. On the 26th, the fame and explosions having again ceased, fresh air was readmitted, and on entering the mine water was thrown on the fire by means of fire-engines; timbers were set, though with difficulty, on account of the high temperature, and because the rocks, decomposed by heat, disintegrated and fell when water was applied. This work was continued to the 3d of March, when the fire was entirely extinguished and the rocks cooled down. The roof had fallen to the depth of 22 feet. Thus a fire which had threatened to destroy the mine, or at least to prevent its working for months or years, was completely extinguished in the course of ten days.

The above particulars, taken from a "Memoire" by M. Jottrand, in the "Annales des Travaux Publiques de Belgique," though very brief, are yet sufficient to show the manner of operating in such cases, and to establish M. Letoret's claim of invention of this method of extinguishing fires in mines.

I shall now describe Mr. Gurney's furnace and manner of operating. The full par ticulars are given in a parliamentary report on accidents in coal mines, 1849.

The drawing on page 641 shows the arrangement of the apparatus. The furnace was four feet square, the ash-pit air-tight, and the pipe leading from it thirteen inches in diameter. This pipe plunged into a tank of water, B. În order to cool the gas before entering the mine the air was drawn through the fire and forced into the pit by means of three steam jets, E E E, working with a pressure of from thirty to forty pounds of steam in the boilers.

The fire which called this apparatus into use occurred in the Astley pit, (390 feet deep,) near Manchester, England, on the 2d April, 1849. The mine being very fiery,

there were great fears of an explosion. The engineer in charge, Mr. Darlington, wrote to Mr. Gurney-well known from his application of the steam jet to mine ventilationto know if there was any means of "drawing fire-damp out of a closed mine without letting air in." Mr. Gurney, in his evidence before the parliamentary committee above referred to, says: "An idea struck me, (from experiments I had made in passing air through a closed vessel running through the fire, where I found the whole of the oxygen to be combined, and nothing going out but nitrogen and carbonic acid,) if we made a large furnace and connected with the ash-pit, perfectly air tight, a cylinder, and put a steam jet in the cylinder, we might draw air through the fire and drive nitrogen into the mine." Mr. Gurney found that passing air through a fire 18 inches deep would consume the whole of the oxygen of the air; M. Letoret found 17 inches sufficient. It is evidently desirable that the depth of the fire should not much exceed that necessary to effect the complete combustion of the oxygen and the formation of carbonic acid; for when it is increased a portion of the carbonic acid takes up another equivalent of carbon and forms carbonic oxide, a gas which, though incapable of sustaining combustion, being itself combustible, would not act as energetically as the carbonic acid or nitrogen in preventing explosion or combustion.

After injecting this after-damp (mixture of about four-fifths nitrogen and one-fifth carbonic acid) into the pit for two hours, a little white cloud coming out of the upcast showed that the mine was full, which fact was easily proved by the gas coming out of the pit extinguishing burning tow, &c. The quantity of gas injected was estimated at 6,000 cubic feet per minute, and the operation was continued for five or six hours after the gas commenced to come out of the upcast. The fire was then drawn and fresh air forced through the mine by the same pits. After two hours and twenty minutes the cloudy appearance at the upcast disappeared, and a lamp would burn in the gas coming out. The fire was found to be extinguished, though it had been burning for nearly two weeks before commencing this operation. The expense of the apparatus was estimated not to exceed five to ten pounds.

In a fire which occurred about two months later (June, 1849) in the same pit, Mr. Darlington applied carbonic acid made in the wet way-with limestone and sulphuric acid. The fire in this case was walled off and the generating apparatus placed in the lower gallery, and a quarter-inch steam jet placed in a pipe inserted through the stopping in a higher gallery. Mr. Darlington says: "We commenced injecting carbonic acid through the four-inch pipe at 2 a. m., and at 5 a. m. the men were at work." Of course the fire was a fresh one, or the rocks would not have had time to cool in that time. The expense was from £10 to £15, or more than that for filling the entire mine with after-damp in the previous case, where the gas was made with "charcoal, the waste coal round the pits, and a little limestone."

SECTION IV.-BREAKING, CRUSHING, AND GRIND

ING ORES.

CHAPTER LXXVIII.

BREAKING AND CRUSHING.

Ores which reach the surface in large solid blocks require to be broken into fragments that can be easily handled before they can be placed in machines for reducing them to still smaller fragments, or to powder.

The sledge is the simplest and most common tool for this purpose; and it is followed by spalling hammers, until none of the fragments are much larger than the fist. Until within a few years this was the common and only way of breaking up ore into sizes suitable to be fed into the mortars of stamp-batteries, and it is still used where only small quantities are to be broken, and the extent of the operations do not justify the expense of obtaining suitable machines for the purpose.

HEAVY STAMPS.

The first attempts upon the Pacific coast to substitute machine for hand labor in spalling ore were in the direction of stamps of unusual weight, raised by cams to a height of four feet, and allowed to drop upou the mass of rock to be broken. Stamps of this kind, either single or two in a battery, were placed at the superb mills erected near Aurora, at the Real del Monte, and at the Antelope. They weighed 2,000 pounds each. There were no mortars, but a solid bed or anvil was surrounded with massive grates, made of bar iron, through which the fragments could drop. Masses of ore, from one to two feet in diameter, could be rolled in and subjected to a succession of blows. The two heads could break up about two tons an hour, but with an enormous expenditure of power, as is evident when we consider that for each blow a ton weight of stamp was to be raised four feet, and also that the smaller the mass to be broken the greater was the force of the blow. Thus when a mass of quartz, say six inches in height, lay upon the anvil, the stamp fell upon it from a height of three feet six inches; but when a block two feet high, which needed a much harder blow, was upon the anvil, the stamp fell only two feet. Similar stamps were in use at Washoe and at Virginia, but were soon abandoned because of their manifest defects and

cost.

BLAKE'S ROCK BREAKER.

The machine for breaking up rock now most in use is the invention of Mr. Eli Whitney Blake, of New Haven, Connecticut, and is generally known as Blake's Rock Breaker. It was designed at first to break up trap-rock into fragments for macadamizing roads. Its value for breaking ores into sizes suitable for feeding to stamps or jigs was quickly seen, and in 1861 it was introduced into California. Its first operation in the mines was at the Benton Mills upon the Merced River. The ore

delivered there from the Pine-tree vein is noted for its hardness and mas sive character, and it required the constant labor of thirty Chinamen to spall enough to keep the stamps supplied. The same and a greater amount of work was better performed by the machine in a few hours, and effected a saving of seventy-five dollars a day, when sufficient rock was furnished to keep the machine running. From that time it has been extensively used, and is recognized as an indispensable adjunct to every well-appointed stamp mill.

The general construction of this machine has been rendered familiar by numerous figures and publications in the United States and in Europe. It consists, essentially, of a strong iron frame, supporting upright convergent iron jaws, actuated by a revolving shaft. The stones or masses of ore to be broken are dropped between these jaws, and a short reciprocating or vibratory motion being given to one or both of them, the stones are crushed, and drop lower and lower in the converg ing or wedge-shaped space, until they are sufficiently broken to drop out at the bottom. The size of the broken fragments may be regulated by increasing or diminishing the size of this opening between the jaws. But the construction and operation of the machine will be made more clear by the inspection of the annexed figure, accompanied by a description in detail of the various parts.

This figure is a sectional side view or elevation of the machine, rep resenting the parts in place as they would be presented to view by removing one side of the frame. The parts of this figure which are shaded by diagonal lines are sections of those parts of the frame which connect its two sides, and which are supposed to be cut asunder in order to remove one side and present the other parts to view. The dotted circle D is a section of the fly-wheel shaft; and the circle E is a section of the crank, F is a pitman or connecting rod, which con nects the crank with the lever G. This lever has its fulcrum on the

frame at H. A vertical piece, I, stands upon the lever, against the top of which picce the toggles JJ have their bearings, forming an elbow or toggle-joint. K is the fixed jaw against which the stones are crushed.