opening the mine too soon, or before the coal had time to cool down below the temperature at which it will ignite. In some cases, it may even be considered safest and most economical to fill the mine with water. This decision must be taken only after a careful consideration of the position of the fire, the amount of water needed to fill the works to the depth required, the time necessary to pump in that water, the nature of the roof and coal in the works, and the effect which a longer or shorter inundation would have on them, the facility for getting the water out, &c. The inundation of a mine is always an expensive expedient, and should only be adopted as a last resort; yet there are cases where it is undoubtedly the best method to adopt. Each case requires a special study, and the method which might be the most suitable in one may not be adapted to another. The great sources of expense in inundating a mine are the damage caused by the water remaining for any length of time in the works (with certain kinds of rocks-some slates and fire-clays especially,) the falls of roof, causing delay and expense, and the delay and cost of filling with and pumping out the water. And in coal subject to spontaneous combustion it not unfrequently happens that when the water has been pumped out, the wetting of the " gob," or waste," causes it to heat, and even to ignite, before the ventilation can be fully re-established. Every other means should in general be tried before inundating the mine, and the most efficient of these various means are the introduction of steam, carbonic acid, (choke-damp,) and after-damp, which is a mixture of nitrogen and carbonic acid. Steam is available at almost every mine, and is easily applied; it should be carried in pipes, and discharged as near the seat of the fire as possible, in order to prevent its condensation; it is a very efficient extinguishing agent, and from the facility with which it can be employed, it is now commencing to be much used; in many cases, a rubber hose, made especially for a steam hose, is all that is required to carry it for several hundred feet, and it will last as long as the occasion requires in most cases. The greatest disadvantage in the use of steam is its energetic action on some rocks, causing them to disintegrate and "fall;" but where the roof is such that it is not materially injured by steam, this is one of the most convenient, and it is always one of the most effective extinguishing agents we can use. Its action is limited to the expulsion of the air, and as it maintains a high temperature we are generally obliged to inject water, in order to cool the rocks sufficiently to allow the men to work, and also to prevent any possibility of reignition. The following example of its application will prove instructive: In 1857, at the St. Mathew mine, near St. Etienne, France, steam was injected after the mine had been on fire for eight days; this was continued for seventy hours, after which cold water was injected for three days, in order to cool the sides of the shaft, galleries, &c., previous to descending into the mine. The pit was then opened, and a current of air circulated while the men went down. After two days, however, the mine again caught fire, and it became again necessary to close the pit. Steam was then injected during twenty-four hours, and, after an intermission of eighteen hours, cold water was injected for twelve hours. The fire broke out a third time, and steam was admitted for eleven hours, then cold water, after which the men were enabled to enter, and extinguish the fire completely by throwing water on it. I believe the same process was employed at the Yellow Jacket and Kentuck mines, on the Comstock lode, which were on fire a few months ago.

The application of carbonic acid or choke-damp and after-damp is more complicated than that of steam, since the materials for its manufacture are not often on hand. The most usual method of manufacturing carbonic acid is by means of chalk or limestone, or any cheap carbonate easily decomposed, treated with one of the cheaper acids-as sulphuric, nitric, or hydrochloric. The gas produced in this way has the advantage of possessing a low temperature; it not only extinguishes the fire but tends to cool the rocks to a point below the temperature necessary for ignition. Portable machines, known as "fire extinguishers," are convenient means of manufacturing this gas where the quantity required is not very great, and they are to be found everywhere, at a small cost, and are always ready for use. Where the quantity of gas required is very large, as, for example, in filling a mine, one of the cheapest and most convenient methods of producing it is by the combustion of coke or charcoal in a furnace of suitable form, and, as it was the means adopted in the first application of "after-damp" or carbonic acid to the extinguishing of fires in mines, we will devote some space to it, especially as the credit of the invention is commonly misapplied. The first application was made by M. Jules Letoret, in Belgium, in the year 1844. Five years later (1849) we find the same principle applied in England by Goldsworthy Gurney, who takes credit himself for the invention, and is even at the present time credited with it by nearly all the English engineers. It is scarcely possible that Mr. Gurney could have been ignorant of M. Letoret's invention, for we find him perfectly "posted" on the application of his steam jet in the Belgian mines, about the same time.

On the 15th of February, 1844, a fire occurred from an explosion of fire-damp in the No. 2 shaft, Agrappe colliery, near Mons, in Belgium; the pit was 1,171 feet deep, three veins at different levels being worked. Efforts were made to extinguish the fire by

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

Letoret's Furnace for manufacturing After-damp, 1844.

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 25th the flames and explosions had ceased; the introeduction of "after-damp" was then stopped, and fresh air was allowed to enter the

a.-Ash-pit. d.-Fire-place. c.-Pipes carrying off the flame. Pipe carrying gas into the pit f. g.-Pipe admitting water. k.-Iron bars supporting e. m.-Iron flooring covered with soil.

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 commenced, 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 flame 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 particulars 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. In 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 upon 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