On the other hand, in winter time water is taken from the mine, and unless this water is replaced by spraying or some other method the walls and roof of the passageways and the dust along the roads will be dried by the ventilating current.

Nearly all coal mines in the United States were formerly allowed to remain dry and dusty in the winter, and a great many are not kept damp to-day. It is true that in most bituminous and lignite mines some artificial watering has been done, but as a rule this watering has not sufficed to replace the water taken out by the air. In the last few years, however, there has been a great improvement in dustlaying methods, and now many mines are systematically dampened.

PREVENTION OF COAL-DUST EXPLOSIONS.

The chance of a coal-dust explosion can be much reduced by care in using explosives. It can be reduced still more by using what are known as permissible explosives-explosives that have passed certain tests by the Bureau of Mines. If these explosives are always used under the conditions prescribed by the bureau, there is little danger of their igniting dust.

The presence of a small or large body of fire damp, the inflammation of which is an important cause in starting dust explosions, can be prevented by thorough ventilation. Many mine operators are satisfied if they send large volumes of air into a mine. At some mines more air enters than is really needed, but not enough reaches the working places to sweep the face or to keep the gob free from inflammable gas.

Leaky stoppings, overcasts, and doors are responsible for the loss of air. A bad error in most coal mines in this country is the use of a single ventilating door in places where double or triple doors would prevent part of the loss of the ventilating current.

It is evident that although the making of dust in coal mines can not be stopped, there are ways of keeping down the quantity of dust. It is also clear that although any bituminous coal or lignite mine that is dry is liable to have a dust explosion, the chances of such an explosion can be greatly lessened. To entirely prevent dust explosions is more difficult, but they can be prevented by carrying out measures to keep the dust from igniting and to keep it in such condition that it can not spread an explosion.

METHODS OF KEEPING COAL DUST FROM BEING DANGEROUS.

There are two ways of keeping coal dust from being dangerous. One is to wet it; the other is to mix or cover it with rock or shale dust, clay, or sand. If the coal dust is wet enough it will not ignite; if there is enough unburnable dust mixed with it, a flame will not spread.

APPLYING SHALE DUST.

The stone-dust treatment for coal dust has been tested at some foreign experiment stations, and is now being systematically tried at the Altofts Colliery in England. The dust, which is finely crushed shale, is thrown on the ribs and timbers of the passageways and scattered in a thick layer on the floor. It is renewed from time to time, whenever coal dust begins to show on top of it. In addition, quantities of the rock dust are placed on narrow boards fastened to the posts of the timber sets, or on horizontal planks over the roadways, or on easily overturned swinging shelves made of boards or canvas. The dust is distributed in this way that it may be thrown into the air in a thick cloud by the shock of an explosion and thus smother the flame. This arrangement for using shale dust is called a "shaledust curtain," and in France is termed an "arresting barrier."

Many foreign experts think the application of rock or shale dust a good method to use in those mines in which the roof may be injured by wetting. In France it has been accepted as the standard method.

A fine example of how well rock dust may be distributed by natural means is to be seen in the northern Illinois longwall field, where the bits of shale that fall from the roof and the pack walls keep the coal dust covered up. Explosions have never happened in this district, though mining has been going on there for over 40 years.

The rock-dust method of making coal dust harmless promises to be of value in the Rocky Mountain coal fields where water is scarce. Certain mine operators in Colorado and New Mexico, following suggestions made by the mining engineers of the Bureau of Mines, are now experimenting with adobe dust and sand as a substitute for rock dust or wetting.

WETTING THE COAL DUST.

The first-mentioned method of rendering coal dust harmless, that of moistening or wetting it, has been tested at the Pittsburgh station, and used in many mines in Pennsylvania, West Virginia, and Alabama. It has proved thoroughly successful when properly carried out. A number of ways of wetting down the dust have been tried. These are by the use of water cars, hose and nozzle, fixed nozzles for sprinkling or spraying water, steam jets, and combinations of steam jets and water sprays.

Another method, the application of a salt that absorbs moisture from the air (calcium chloride), is being tried in a number of mines in West Virginia, and promises to be of value under special conditions.

Dust can be wet down and kept moist by the systematic use of any one of these methods. The great trouble at many mines, including

some in which there have been dust explosions, has been that the method adopted was not thoroughly carried out.

WATER CARS.

It is difficult to wet dry coal dust by throwing water on it. Perfectly dry coal dust will float a long time on water, and pouring water on such dust is like pouring water on a duck's back. Water dribbled on a dry road runs off and is lost in the loose rocks underneath the road or stands in little puddles till it drys up. For these reasons the use of water cars is not successful unless the roads are watered daily. Also, the sprinkling must be done by a force pump on the water car or by compressed air acting on the surface of the water in the car tank, so that the roof, timbers, and ribs can be wet as well as the floor. Pouring water from a bucket or dribbling it along the middle of the roadway from a valve in the tank is not a remedy and is of little value.

HOSE AND NOZZLE.

Systematic washing down by hose and nozzle is an excellent way of laying dust. The water lines should be run along all the haulage ways up to the working faces and should have plenty of taps for attaching the hose. The washing down should be done often enough to keep the dust so damp that it will take up the next water thrown on it. To make fine dry coal dust so damp that it can be molded in the hand, 33 per cent, by weight, of water must be added; to make it muddy, at least 50 per cent of water must be added. The mixture would therefore contain 25 and 35 per cent, respectively, of added moisture.

The hose and nozzle method of laying dust is particularly suited for watering a place previous to firing a shot-a precaution that is required in some European countries. The hose may be used to sprinkle the ribs and roof near the track and also the gob; provided, of course, that such sprinkling does not cause the roof to slack and fall nor the gob to take fire spontaneously, in which case the rock-dust method should be used.

MOISTENING THE AIR CURRENT.

The simplest method of keeping a mine damp is to put moisture into the ventilating current; then the air going through the mine does not dry the dust, but carries moisture to it. There are two ways of putting moisture into mine air. One is by using fixed sprays of water placed at intervals through the mine; the other is by saturating the intake air at the mouth of the mine by means of steam or water sprays.

WATER SPRAYS.

Water sprinklers should give a fine spray, as much like fog as possible, so that the water will be carried along by the air until it is absorbed as invisible vapor. The chief care needed in using sprays is to keep the nozzles from clogging and to prevent men and boys who do not understand the necessity for spraying from damaging the sprays or turning them off to avoid getting wet in passing by.

Clean water should be used for spraying. The nozzles should be of the form least liable to clog and most easily and quickly cleaned. after clogging. All nozzles should be inspected at frequent intervals.

There can be no question of the value of water sprays for laying dust. However, to get the best results from their use some mine official must be made responsible for the condition of the sprays at all times. He must also be required to see that there are sprays enough running to keep the relative humidity of the air currents at, or above, 90 per cent. In zero weather the incoming air may traverse 2,000 to 3,000 feet of entry before its temperature rises above the freezing point. Therefore at mines in districts where the winters are cold provision must be made for draining pipes and nozzles throughout the intake part liable to freeze; and in cold weather the area must be wet down by hose or by water cars, so that it does not become dry. A recent dust explosion started in the haulage road, which was also the intake, at a point only 600 feet from the mouth of the mine.

HEATING THE AIR AND USING WATER SPRAYS.

At certain mines the experiment is being made of heating the ventilating current at the intake to mine temperature and then moistening it by water sprays. This requires a somewhat costly heating plant, for in zero weather the warming of 100,000 cubic feet, more or less, of air per minute takes much heat. This method will moisten the mine air, but it is more expensive and complicated than the following method.

STEAM JETS.

Under some conditions the easiest way of moistening the air current entering the mine is to use exhaust steam. The heat of the steam warms the intake air a little, and the moisture is supplied as vapor and in finely divided form, like fog. Generally the exhaust steam from the fan engine will suffice to dampen the air except in the coldest weather. After a mine has been thoroughly moistened, a slight shortage of water in the intake air for a week will not prove serious, and in most of the coal fields of this country cold waves do not last long. However, if a mine shows a tendency to get dry, some live steam will be needed to supplement the exhaust steam.

Many mining men have thought that the use of exhaust steam would mean the presence in the intake air course of a vapor so hot that the roof would slack and fall, as it does in some entries where a steam main is laid. Such is not found to be the case in practice. The steam expands as it leaves the nozzles and rapidly cools, so that 15 or 20 feet away the air current feels merely warm even in medium weather, and 50 feet away there is no perceptible warmth from the presence of the vapor.

If the part of the entry close to the jets is protected by lining or lagging, the roof will not be perceptibly affected by the heat. Although moisture may have a bad effect on some roofs, generally it is alternate wetting and drying that causes roofs to weaken.

The steam-jet method, if systematically used, will make the ventilating current evenly humid. Many mines that have introduced the method have experienced no bad results. The cost of maintenance is virtually nothing if steam is employed around the mine and the exhaust steam is not otherwise used.

If the ventilating fan is driven by a slide-valve engine, as is usually the case, the exhaust steam therefrom is sufficient for moistening the air in all but the coldest periods; then some steam from other sources may be needed. The efficiency of the method, when properly managed, has been shown beyond question by its trial and adoption in a considerable number of important mines.

Steam jets can not be used on a haulage road when locomotive tenders, mule drivers, or trip riders are employed, since the steam fogs the air for a long distance and makes the haulage difficult or dangerous. Therefore their application is generally limited to mines that have pressure or blowing fans and do not use the intake airways. for haulage; or the steam jets must be used only on "off-shifts" when no haulage is done.

STEAM JETS AND WATER SPRAYS COMBINED,

Usually in mines that make an appreciable quantity of methane, haulage and travel are not allowed in entries carrying the return currents, but only in the intake entries. Where this system is used and no haulage is done except on the day shift, a combination of water sprays and steam jets has given good results. When trips are being hauled over the road the water sprays are used. In cold weather these sprays must be situated so far inside the mine that the incoming air will not freeze them. During the night, or when there is no haulage to the outside, the exhaust steam jets are turned on at the mouth of the intake and the main entries thoroughly dampened. In this way a small number of water sprays help to keep the mine moist during the day shift.