are made, so as to keep the current close to the working places of the miners). Branch entries similar to the above are turned off at intervals, and from these again the rooms in which the greater part of the coal is won. Rooms are usually 20 to 30 feet in width, narrow at the mouth, but soon widened. They are about 300 feet in length, and are separated from each other by pillars of coal 15 to 20 feet thick, left standing to support the roof. Whenever any section of the mine is exhausted it is customary to mine out the pillars, beginning with those farthest from the entrance. Another system, called the Longwall work advancing, is sometimes used, where the miner, after getting fairly away from the bottom of the shaft, works out the whole body of the coal, leaving no pillars of coal, but building protecting walls with broken rock and slate along the sides of the roadway, and allowing the roof of the mine to settle on these walls, and to fill up spaces from which the coal has been removed.

If the coal is very soft, only a pick is required to loosen it, but in anthracite mines the pick is little used, the coal being blasted like so much rock. In the majority of bituminous mines, however, it is first necessary to make an undercut three or four feet deep. This is the most difficult of the miner's work, but it is necessary in order to give the powder a chance to act without shattering the coal unnecessarily. Then holes are drilled so as to place the charge of powder close to the roof and over the back of the cut. Coal-cutting machines, driven by compressed air, are now successfully used in many places for making the undercut.

Many of the difficulties and dangers of coal mining have been overcome; but, in spite of all improvements and safety contrivances, many still remain, and among them are roof-falls and fire-damp. Rooffalls are far more dangerous than gas explosions; but, coming singly and at scattered mines, they do not attract the same attention, and are only noticed in tables of statistics.

Fire-damp is a mixture of gases, varying at different places, and consisting principally of light carburetted hydrogen, or marsh gas, but also containing some carbonic acid, nitrogen, and other hydro-carbon compounds, with a specific gravity of only fifty-five per cent as compared with atmospheric air. It is generally without color, taste, or odor, and burns when pure with a yellow flame. It is not poisonous. and can be breathed when forming one-third of the air. On account

of the low specific gravity, it collects most readily near the roof of the mine, but speedily mixes with the air through diffusion. It is detected when in small amount by its effect on the flame of the safety lamp; two per cent is the smallest amount that can be detected with an ordinary lamp, the flame increasing in length and size with the amount of gas. The mixture first becomes explosive when there is six and two-thirds per cent of gas; it is most explosive at ten per cent; and at fifteen and a half per cent it ceases to be explosive and extinguishes the light.

The peculiar principle of the safety lamp was discovered in 1815 by George Stephenson, and also by Sir Humphrey Davy. This principle is that the encasing wire gauze so far cools the burning gas within that the flame does not communicate with the surrounding explosive mixture. This is true so long as every lamp is in perfect condition and there is no sudden movement of the air or carelessness.

In order to dilute the percentage of fire-damp to the least possible amount, centrifugal fans have been used, giving a ventilating current of 150,000 up to 250,000 cubic feet of air per minute.

Fire-damp is not found in all mines, and many parts of our own country have so far been considered entirely free from it. Miners generally contend that drift openings are not liable to fire-damp, but this is not so, and with each year's more extended workings the danger increases.

Loose coal gives off gas constantly, so that the more coal is loosened from day to day the greater the danger. If the mine is allowed to stand a few days, the percentage of fire-damp decreases. There is also an increase of fire-damp with any lowering of the atmospheric pressure. In fiery mines there is a liability to sudden outbursts of gas called blowers, and against these it is hard to take precautions. Since the phenomena of natural gas have been studied these outbursts seem less strange, though perhaps no better understood. It is interesting in this connection to note the experience of the Prussian firedamp commissioners, that the mine gas proves most abundant when the coal is folded on an anticlinal axis not reaching the surface and accompanied by a porous sandstone overlaid by clay slate. This is especially true where the drainage has removed the water and increased the porosity of the sandstone.

The influence of coal dust in colliery explosions was first noticed

by Lyell and Faraday in 1884, and it was recognized as increasing the force of gas explosions. In 1876 Galloway showed by experiment that, while more than six per cent of gas alone was required to make an inflammable mixture with air, less than one per cent of gas was required in the presence of fine coal dust.

Further experiments between 1879 and 1881 led Galloway to believe that an explosion produced by a local occurrence of fire-damp might be indefinitely extended in an atmosphere loaded with coal dust.

In 1876 a paper was read before the North of England Institute of Mining Engineers, by Messrs. Hall and Clark, showing, as a result of their experiments, that the presence of fire-damp was not necessary, but that a blown-out shot in the presence of fine coal dust would cause an explosion. In coal mining a blown-out shot is one where the tamping is blown out by powder without any decided action on the coal, and the effect is much the same as though a cannon were fired in the same position. The force of the powder is expended in projecting the current into the air of the mine, and stirs up any dust that may be present. At the same time it is thought by many that the partial vacuum succeeding a blown-out shot tends to draw the gas from the coal more rapidly than would otherwise be the case.

In the course of the investigations following the Seaham colliery explosion in England in 1880, it was further shown that fine dust, which in itself was entirely incombustible, had a distinct effect in explosions, and made dangerous a low percentage of gas, which of itself would be quite harmless.

In 1883 it was generally admitted by all who had given special attention to the subject that all gas explosions were more violent in the presence of fine coal dust, and that dust would render explosive a mixture of air containing two per cent, or possibly as low as one per cent of fire-damp. But Mr. Galloway was looked upon as an enthusiast, if not a crank, and his claim that coal dust alone could lead to an explosion, in the absence of gas, found little credit. About the same time a Government commission was appointed in France to examine the same subject, and they reported that dust in the absence of gas was not a cause of serious danger.

Partly owing to Mr. Galloway's paper, and partly owing to the renewed attention called to the subject by certain flour-mill explosions, the matter was taken up again in Prussia, and intrusted for

investigation to one section of the Prussian fire-damp commission, which first met in June, 1881, and made its final report in November, 1885. The experiments were made in a gallery of elliptical form, 5 feet 7 inches by 3 feet 11 inches, and 167 feet long, so arranged as to give a chance for observation without danger to life or limb. The speaker gave a detailed example of the manner of conducting these experiments, and the results of a number of the experiments.

Over four hundred such experiments were carried out with the greatest care, and the results were well established.

The most important deductions are as follows: —

First: That with certain classes of coal dust an actual explosion, extending beyond the limit of the dust deposit, may be caused by a blown-out shot, even when fire-damp is entirely absent.

Second: That while the finest dust is usually the most dangerous, the chemical composition of the coal is more important, and that a volatile percentage of from sixteen to twenty-four is the most dangerous.

Third: That a three per cent gas mixture, in the absence of scattered dust, causes no danger in case of a blown-out shot, even though tamped with the most dangerous dust, and that a six per cent mixture is required for actual explosion.

Fourth: That dust in pure air cannot spread a flame from a lamp alone; that fire-damp up to three and three-quarters per cent, without dust, only lengthens a lamp flame; that at four per cent the flame begins to slowly spread, at the rate of one foot per second, and that at six per cent the speed is six feet per second, and incipient explosions take place. Let dust be present, and explosions may be started by an open lamp with only five per cent of gas.

Fifth: That for insuring safety, the dust must be wet down with fifty per cent of its weight of water, not simply moistened,—and that this must be done for a space of fifty feet back from the face of the coal.

Sixth That the Davy lamp as a test for gas is only to be trusted from three per cent up; but that the Pieler lamp can be relied on for detecting one-half per cent of gas, and for estimating mixtures of from one to three per cent.

Seventh That the time required for the full, natural diffusion of fire-damp in a mine gallery of ordinary size is from three to four hours.

These are the principal points settled by the Prussian commission, and it is of interest to us to consider if the subject is to be looked at as practical or only as theoretical. In other words, have we ever had, or are we likely to have, a coal-dust explosion in this country?

The explosion at the Pocahontas mine, which occurred March 13, 1884, was by many persons regarded as a dust explosion, even at that time, and as more light has been thrown on the subject, that theory has gradually received more favor.

Mr. Buck then described the Pocahontas mine, giving a full description of the effects of the explosion.

As a result of the examination by experts the opinion was expressed that the explosion was due mainly to dust. It could not be determined whether its initial cause was a blast or the accidental ignition of a small quantity of fire-damp, but it was believed that the explosion was due either to dust alone, or to dust quickened by an admixture of fire-damp too slight for detection by ordinary means. The speaker said that no gas had ever been noticed in the mine before the explosion except once; that no gas was found on re-opening the mine; and none since, so far as known.

A vote of thanks to the speaker brought the meeting to a close.

MEETING 360.

The Source of Business Profits.

BY PRESIDENT FRANCIS A. WALKER.

The 360th meeting of the SoCIETY OF ARTS was held at the Institute on Thursday, March 24th, at 8 P. M., President Walker in the chair.

After the reading of the minutes of the last meeting and the election of new members, the President announced the subject of the evening to be "The Source of Business Profits."