Underground Mine Ventilation

Table of Contents

NECESSITY FOR MECHANICAL VENTILATION
MAIN AIR CURRENTS
DISTRIBUTION OF AIR TO WORKING FACES
COST AND RETURNS
SUPERVISION
CONCLUSION

Something that all of the various and mining techniques have in common board and pillar, stoping, caving, long wall mining; is that they take place in underground environments where fresh air does not naturally occur. Not surprisingly, ventilation is a critical importance to the occupational health and safety of underground workers. A continuous supply of good quality air is absolutely essential to allow personnel to breathe, to dilute toxic and flammable gases, to dilute or carry away dust and aerosols and to provide cooling for the personnel and machinery. In this topic we will look at the various issues that arise with regards with people working underground and the principles behind mine ventilation systems. Ventilation is the control of air movement: its amount, its quality and its direction, to maintain a safe and healthy environment in which miners can work. A lack of proper ventilation can cause lower worker efficiency, decrease productivity, increase accident rates and absenteeism. The hazards which are controlled by proper ventilation in underground mines include; low oxygen content, toxic gases, flammable gases, fumes, humidity, temperature, airborne dust and products of combustion. We are going to look at some of these hazards in detail. A mine ventilation system must provide a insufficient quality in flow to all places where people travel and work and safe oxygen concentration between 19.5 and 23.5 % by volume. Oxygen in mines can be diluted by contaminants such as dust, aerosol, diesel fumes, blasting fumes as well as gases released from the rock strata consumed due to oxidation of reactive minerals such as sulphides and consumed by com busted engines. Gaseous contaminates in mines may be toxic, asphyxiatic or flammable. In areas where toxic or asphxiatic gases are present very limited or zero exposure must be maintained where flammable gases occur concentrations must be prevented from entering the explosive range. Gaseous contaminants may include; methane and other hydro carbons, carbon dioxide, carbon monoxide, oxides of nitrogen, sulphur dioxide , hydrogen sulphide, hydrogen, ammonia and radon and any of these can dispose oxygen leading to asphxiation. Methane and other hydro carbons have a special significance in underground coal mining because these gases are flammable and potentially explosive. We’ll give more consideration to explosive gases in the next topic. In addition to gaseous contaminants there are airborne particles which can include inspirable dust which are particles large enough to see and respirable dust which are particles too small to see. If dust concentrations are too great to be carried away with the moving air, then special filters called scrubbers can be employed to remove the dust. The risk associated with dust will be given further consideration in the following topic. To consider the basic principles of ventilation in underground mine let’s consider some room and pillar workings. In general a ventilation system seeks to direct air in the mine through an entry shaft or tunnel called the intake side and direct it back to the surface via a different path called the return side. Air flows in response to differences in pressure which for underground mines are generated by primary fans situated at the surface. Fans can either push air in at the intake side or suck air out through the return side. Each approach has its own advantages and disadvantages. As a mine develops the extent of open underground space increases however, only current work areas need to be well ventilated and abandon areas reduce the efficiency of the system. To control where the air goes and how much gets there this used area and short cut are sealed off by stoping. These are walls made masonry, concrete, blocks, timbre steel etc directed across openings to direct or channel flow. Even with stopings to direct flow, air flows may not be balanced where there are multiple ays. Regulators, simple fabric shades blocking the airways are used to reduce the air flow to a desired value in any even ay. Where satisfactory pressure or flow cannot be achieved using stopings and regulators, booster or auxiliary fans may be employed as stopings or barriers to access as well as to air flow air doors or air locks are mounted in stopings between intake and return airways. Factors which determine the total primary volume capacity and pressure requirements for a mine include the extent and depth of the mine, the complexity of the mine layout and the stoping and extraction systems together with the size of development openings and the equipment used. To maintain an adequate ventilation system throughout the life of a mine careful advanced planning is necessary as the size and shape of the mine evolves continuously. A well designed ventilation system should be effective, flexible and economical.

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Efficient ventilation of underground mines consists in having such complete control of air currents that there is always supplied at places where men work sufficient moving air to allow working at maximum capacity without injury to health; and in case of underground fire or of surface fire in the vicinity of mine openings, underground air currents may be quickly reversed if desired, or air may be sent into or excluded from any region and fire fumes confined to only part of the mine workings, instead of penetrating practically the entire mine.

Health and safety of workers in mines as well as proper safeguarding and operating efficiency of mining properties are so intimately associated with proper ventilation of mines (both coal and metal) that they are inseparable, yet mine ventilation is only too frequently neglected, especially in metal mines. The metalliferous operator, generally not confronted with inflammable gas or explosive dust which so frequently force ventilation of coal mines, usually disregards air supply altogether except such as may be obtained from compressed air; and generally, even when attention is given to metal-mine ventilation, the air currents rarely are found where they are needed, namely, the working places, especially the working faces.

After several years’ study of miners’ consumption, the writer is convinced that long continued breathing of air impregnated with large quantities of any kind of finely divided dust, such as is found in a large number of the working places of our mines, will ultimately produce some respiratory disease, whether it be asthma, bronchitis, or phthisis; and the progress of these diseases is hastened if there are also present high temperatures, high humidities, or harmful gases or if there is present (and allowed to remain to be breathed) large quantities of the finely divided dust when air temperatures are low, hence favorable to maximum performance of work with accompanied maximum breathing into the respiratory passages of the harmful material. It is generally accepted that coal miners under the age of fifty are much more free of these respiratory diseases than are metal miners, and it is practically certain that this immunity of the coal miner is due largely to the fact that working places in coal mines are much better ventilated than those of metal mines. A close study of dust conditions in metal mines convinces the writer that, except for preventing formation of dust, the most efficient preventive of undue dustiness of air breathed by metal mine workers is the coursing of adequate currents of air past the places where dust is produced or is being thrown into the air—thus replacing the dusty air with dust-free air.

Because of the failure to remove poisonous fumes from detonation of explosives at confined working places, many workers in these places are ill with headache or nausea, which reduces their vitality and ability to work; not infrequently these fumes claim the lives of metal- mine workers. Failure to remove smoke from working faces prevents the worker from properly inspecting overhead conditions and thus preventing many accidents.

When metal mines encounter gases, such as CO2, nitrogen, or, as occasionally occurs, the explosive methane, inadequate ventilation and knowledge of gases usually results in hospital cases or fatalities, or possibly in occasional closing of the place or the entire property until outside aid or information may be secured.

Metal-mine operators generally see no necessity for paying attention to air distribution but, even of those who admit the necessity of ventilation, many hold that they cannot afford such ventilating plants as are found at coal mines; as a matter of fact, they really cannot afford to continue to fail to ventilate even should the altruistic motive be disregarded. Very few metal mines are immune from fire and generally when a fire occurs, either in the mine or at buildings adjacent to surface openings, mines without ventilation control are helpless for hours, possibly days, during which great damage is done; whereas, with efficient ventilation equipment, there is rarely much delay in overcoming a fire soon after its discovery. It is the writer’s opinion that even if ventilating equipment should not be used ordinarily, its installation for use in case of fire would be first-class fire insurance at reasonable cost; in fact, one large western metal-mining company has installed fans chiefly to be able to control possible future fires.

In dead-end working places with stagnant air, the worker, generally on a day’s pay basis, loses a considerable part of each shift waiting for smoke to clear away, or works intermittently from illness due to fumes from the muck piles; or, if the place is hot and humid, he must rest frequently or stop to wring perspiration from his clothes or go to the station or to the level below to get a drink or cool off. Where miners’ consumption or kindred diseases are found (caused largely through lack of ventilation), employment must be provided for a considerable number of pensioners whose physical ability is practically nil yet whose rate of pay frequently is that of able-bodied workers.

The above losses caused by deficient ventilation affect chiefly the mining companies; as much as 10 to 25 per cent, of a shift is lost in cool mines and as high as 50 to 75 per cent, in hot, humid mines. There is no doubt that the dollars and cents equivalent of these specific losses to operating companies amounts to many million dollars annually in our metal mines. In addition to the above heavy financial losses to the operators, the employees on a contract basis lose a large sum in time lost while underground, and both day’s pay and contract workers suffer material monetary loss through inability to work because of illness. In fact, there are few metal-mining companies or their employees who do not suffer material financial loss, actual or potential, by the failure to establish and maintain a positive ventilation system.

NECESSITY FOR MECHANICAL VENTILATION

After about five years spent largely in a study of metal-mine ventilation supplementing over fifteen years spent in more or less intimate contact with ventilation in coal and, to a less extent, in metal mines, the writer believes that no mine (coal or metal) can be efficiently ventilated without the use of mechanical equipment (especially fans). While there are probably well-ventilated mines with natural currents (that is, without the intervention of fans or other mechanical equipment), he has never visited a property of that description.

Natural ventilation is dependent chiefly on the difference in temperature between the outside air and the underground rock and water; the greater the difference, the more pronounced are the quantities circulating. At times of equalization of temperature, there is likely to be little or no air circulation; this is particularly the case if the elevation of the mine intake and the return air courses at the surface are about equal. Mines with hot rock and water temperature (say over 80° F.) are likely to have rather poor or no circulation in summer and fairly good circulation in winter; mines with rock or water temperature 60° to 80° F., will, in general, lack currents in summer and have fair ventilation in winter. Mines with rock and water temperatures below 60° F. will have fair ventilation in the summer or the winter, but the air currents reverse their direction; in the spring and fall, the air currents will be sluggish and their direction will probably change daily. Whether the underground rock and water are hot or cold or whether the surface air is hot or cold, in general, the mining company dependent on natural ventilation has practically no control of the direction of the current, hence, in the case of a fire, it is helpless; and when there is no circulation provided by nature’s forces, there is no method available to provide the necessary air to safeguard the company’s property or the employee’s health and safety.

MAIN AIR CURRENTS

Each mine should have at least one fan, of comparatively large capacity, to control main underground air currents. Preferably, this fan should be located at the surface, in fireproof housing, and should be so arranged that the direction of currents may be reversed in a very few minutes by manipulating a few doors. The most up-to-date practice is the installation of electrically driven, high-speed fans; and where gases, such as CO2 or nitrogen, come into workings from strata, it may be desirable to keep underground air under pressure. Local conditions will determine whether the fan should be operated as a suction or as a force unit, but, in general, the main fans exhaust foul air from mine workings. As a rule, it is desirable that the main operating shaft or tunnel be an intake and that the fan should not be located at the main operating shaft or tunnel. Where possible to arrange, the main operating (haulage) shaft or tunnel should not be used as a main air course, but this recommendation can seldom be carried out.

The main fan should be kept running the full 24 hr. instead of being stopped when the shift leaves. If desirable, the air currents may be reversed with the seasons to take advantage of natural ventilating pressure. Care should be taken to keep main air courses as free as possible of such obstructions as platforms, planks, non-working cages or skips, etc., in shafts or raises, and piles of timber, loaded or empty cars, unnecessary standing posts, etc., in crosscuts and drifts.

Smooth lining shafts that form the main air courses will greatly decrease the friction of the air flow (probably as much as 50 per cent.). If the smooth lining is done by concreting or guniting, the shaft will be practically fireproof as well. Occasionally, it is feasible to remove from main air-carrying drifts or crosscuts, timbers that: are not under pressure; if the rock is gunited, it remains in place and the air flow is greatly facilitated.

Mines in which many men are employed should rarely have less than 15,000 to 20,000 cu. ft. of air per min., and large mines should have approximately 15,000 cu. ft. per min. for each 100 men employed, if the underground rock and water are cool, and at least that amount (probably more) per 50 men if the workings are hot (say over 80° F.). It is desirable to divide a mine into separate air splits each carrying 15,000 to 20,000 cu. ft. per min., each split receiving its pure air from the main intake, coursing it through certain well-defined workings having 50 to 100 men and removing the used air to the main return air course, thence to the surface before it has become excessively contaminated by smoke, heat, humidity, dust, etc. Each split should, at all times, be kept absolutely separate from other splits.

Shafts or tunnels that carry main air currents into or out of a mine and are also used for working purposes should have sufficient cross-sectional area so that the velocity of the air may be kept below 1000 lin. ft. per min. with 1500 lin. ft. as a maximum. Underground levels that carry the air for each split of approximately 15,000 cu. ft. per min. should have sufficient area to restrict the velocity to less than 500 lin. ft. per min., and a velocity of 200 to 300 lin. ft. per min. would be preferable. In figuring the area of underground haulage levels that will be used as air courses, allowance should be made for the effective area absorbed by ore cars, locomotives, etc.

Each mine should be independent of adjoining mines for its ventilation. Intermine ventilation is inefficient at ordinary times, is likely to result in disputes and other inconveniences, and forces one mine to use vitiated air from another. In the case of mine fires intermine ventilation has repeatedly caused deaths to men in the mine that did not have the fire. While, generally, it is not feasible to drive parallel workings, as in coal mining, to insure air circulation at working faces, frequently a little forethought in planning metal-mine work, especially in well-established mines, would greatly facilitate handling of air circulation. In many mines, a lateral is driven in the foot wall parallel to and only a short distance from a drift on the vein. The lateral, however, is not driven until long after the completion of the drift; if the two were driven practically simultaneously (with the drift slightly in advance of the lateral to determine the direction of the latter) the working, faces of both, by use of short crosscuts and line brattices (as in coal mining), could be supplied with fresh circulating air. Similarly, raises, especially when driven in the vein, could be placed in pairs with a small pillar, between and with occasional break-throughs to allow air circulation.

The utility of overcasts, regulators, and doors is only slightly recognized in metal mining in connection with controlling air currents, particularly in case of fires. The door, in particular, is neglected. Near every main shaft, especially those carrying air, doors should be placed in every intersecting level, so that in case of fire any level may be readily closed, as regards the shaft, fire fumes prevented from spreading, and, the fire itself controlled. All underground doors should be well constructed: and air-tight when shut; if the air pressure is excessive the doors should be in pairs with air locks of suitable length between them, so that when one door is opened the other is closed and maintains the pressure. Every door should have a latch to hold it closed in case the direction of the air current should change. Where automatic contrivances are used with doors, the door should close rather than open when the automatic feature releases. Any door that must be closed by persons after going through it is dangerous, as it probably will not be closed at a time of panic caused by fire or other emergency. Doors constructed to allow considerable air leakage are dangerous at time of fire. All doors should be of tight construction; if a limited delivery of air is desired, a slide or regulator should be placed in the door.

Metal-mine officials rarely recognize the necessity of sealing abandoned workings in order to force air currents to live workings. The practice of allowing intersecting openings—horizontal, inclined or vertical—to remain open to air circulation, or leakage, dissipates air into places where it becomes impregnated with fungus spores, CO2 from timber decay, increased temperature and humidity and possibly dust. Frequently, it is not needed in the places where it goes (though needed badly elsewhere) and after having become vitiated passes into places where men work. Closing places that are not being worked would frequently result in the recovery of much material that is now lost and would remove a common source of danger to life and limb in metal mines in addition to greatly aiding ventilation.

DISTRIBUTION OF AIR TO WORKING FACES

Failure to transmit fresh air currents to working faces is the most noticeable weakness of present-day metal-mine ventilation, and unfortunately this fault is only too frequent even where there is an abundance of fresh air in the main air courses. Only rarely does a metal mine use canvas or brattice deflecting curtains, line brattices, or regulators to force air into blind-end drifts, crosscuts, raises or stopes; yet frequently these agencies could be used to advantage. Compressed air from machine-drill exhaust or from an open hose as a blower is generally relied on to ventilate such places; but it nearly always proves inadequate and is expensive.

The usual compressed-air blowers, as well as exhaust from compressed air drills, release but 75 to 150 cu. ft. of air per min., which is by no means sufficient to carry away the gases, humidity, and dust so nearly universally found in confined working places in the metal mines, as well as the high temperatures frequently met in those places. The quantity of air thus available is not comparable to that brought by ordinary circulating methods to the face of working places in coal mines (usually 5000 to as much as 15,000 cu. ft. per min.), nor to the 700 to 5000 cu. ft. per min. readily obtainable in metal mines by the use of small fans connected to galvanized-iron or some form of flexible tubing.

For the ventilation of blind-end workings with hot rock or water, hence with hot humid air, during the past few years, small electrically driven fans (generally with motor direct connected to fan shaft) have come into extended use for forcing air through canvas or some flexible tubing to the working faces. From 700 to 5000 cu. ft. per min. are thus readily placed near the face and the worker not only has sufficient comparatively fresh cool air to remove the gases and dust, but the comparatively high velocity (generally several hundred linear feet per minute) supplies evaporation to overcome harmful effects of highly heated and excessively humid air, body perspiration, etc.

After much experimentation, by the cooperation of manufacturers of fans and mining companies, it has been found that, when used with flexible tubing, the ordinary commercial fans are not as efficient as fans of similar construction but only one-half or two-thirds the width. Such fans deliver essentially the same quantity of air as the fan of commercial size with about 30 per cent, saying in power. One company adopted as standard 8-in. flexible tubing with a fan driven by a 3-hp. motor when the distance for air delivery is not over 200 ft.; 12-in. diameter tubing with a fan connected to a 5-hp. motor when the distance is not over 500 ft.; and 16-in. tubing with a fan driven by a 10-hp. motor for distances over 500 ft. In each case air velocity is about 2000 ft. at the end of the pipe.

One mining company found driving 200-ft. raises in hot rock (between 95° and 110° F.) an almost impossible undertaking because of hot humid air and dangerous siliceous dust, but using direct-connected fans with 3-hp. motors to force air from a point near the foot of the raise through an 8-in. flexible tubing into the raises, keeping the tubing close to the worker, cleared those places of dust and hot air and converted the raises into comfortable places. This, together with placing a large surface fan and adopting a system of splitting the air currents, assured the practicability of working this property for many years, even if hotter rock is encountered with greater depth.

A pipe introduced on the Rand for ventilating headings has some advantages over galvanized-iron and canvas pipe, according to S. de Smidt. This pipe is made of concrete, reenforced with wire netting, in 6-ft. or 8-ft. lengths, and is 1-in. thick for 12-in. pipe. Sleeves about 4 in. wide are used, to cover the joint between the pipe; soft clay is pressed into the joint, and from time to time inspected and tightened. The sleeves have the advantage of permitting gradual bends, and if necessary, special curved shapes may be made. The pipe is laid on the floor, which is another advantage as it can be covered with loose material and thus protected from falls of rock. The greatest advantage, however, is that, the concrete being a non-conductor, there is less heating of the air in traveling long distances to a hot level. If conditions favor exhausting the air through the pipe, in the case of gases being given off at the face, either from the strata or from blasting, this pipe can be used, whereas long lengths of cloth pipe would not be as satisfactory. A movable length of cloth pipe at the end of the concrete pipe might be necessary for bringing the pipe close to the face; and this short piece, if used for exhaust purposes, could be reenforced with ribs.

In many mines, especially in open stopes, there is sufficient air circulation to remove smoke, gases, and possibly the most dangerous dust from working faces, but the velocity is less than 25 ft. per min., hence not sufficient to afford relief to perspiring workers if the temperature and humidity are high. To meet this situation, a compressed-air driven fan has been devised along the principle of the small electrically driven fans used in business offices. This fan weighs about 50 lb., is readily connected to ordinary compressed-air hose, consumes about 20 to 30 cu. ft. per min. of compressed air and gives a velocity of about 1500 lin. ft. at the fan with a velocity of 200 to 500 lin. ft. per min. 20 to 25 ft. from the fan. After a fan of this description was placed in a stope with saturated air 92° F. the miner worked continuously in apparent comfort; whereas previous to its installation, he went out of the stope regularly for a 15 to 20-min. rest after having worked about that length of time. This arrangement consumes only about one-fourth as much compressed air as the ordinary compressed-air blowers and affords much greater relief with only a fraction of the operating cost of the blower.

COST AND RETURNS

The cost of installing a ventilation system in a metal mine is variable, yet under ordinary conditions it should not be particularly burdensome; generally, if any considerable number of men are employed, the savings effected greatly overbalance the cost. The recently perfected ventilation installation at the Colorado mine at Butte, Mont, cost over $70,000, but shortly after its installation it was found that the saving in electrical power for compressing air was about $18,000 per year. In addition to several indirect benefits realized, the average efficiency of all underground employees was increased at least 50 per cent. In a conversation with the writer, Mr. Bruce stated that with the mine working to its normal capacity (two full shifts), the various savings effected through the ventilation installation would readily return its entire cost of over $70,000 at least once, and probably twice, annually.

This installation was abnormal in that it demanded the driving of nearly ½ mi. of underground workings in addition to the placing of a fan. In general, a substantial ventilation installation for a comparatively large metal mine can be established for less than $15,000 and generally for not much over $5000. It requires roughly 1.5 to 2 hp. to circulate 1000 cu. ft. of air per min. through an ordinary metal mine and about 3 hp. to force 1000 cu. ft. per min. through tubing to ventilate blind ends.

If equal quantities are considered, it will cost about 100 times as much to ventilate by compressed air as by fan; in some cases, the ratio will be as high as 200 to 1. However, as only about one-tenth to one-twentieth as much compressed air is supplied as when ordinary air-circulating methods are used, the cost of ventilating blind ends by compressed air is from ten to twenty times as great as by fan methods, with the additional disadvantage that rarely is ventilation by compressed air jets or blowers efficient.

A most uncomfortable part of a mine, in which about thirty men were employed on each of two shifts in dead air with a temperature over 90° F. and saturated with moisture, could be worked only by paying a bonus of about $1 per man per shift to workers who were able to deliver about one-third to one-half efficiency; moreover, the labor turnover was several hundred per cent, monthly, a combination giving slow progress and extremely high costs. Two electrically driven fans, each connected to about 600 ft. of flexible tubing, were installed, each fan consuming about 7.5 hp. at a total monthly power cost of about $30. The tubing cost about 50 c. per ft. in place, or about $600. On account of hot, humid, wet conditions, the tubing had to be replaced about every four months so that it cost about $150 per month, making the operating cost of the ventilation system about $200 per month. After the ventilation system was put into effect, the bonus was withdrawn but the amount of material moved per man per day was almost exactly doubled, and the labor turnover was reduced to essentially that of other properties. The saving due to the withdrawal of the bonus amounted to over $1500 per month, which not only cares for the monthly cost of operating the ventilation system but more than wipes out the entire first cost of the system each month.

SUPERVISION

The installation of a ventilating system in an established metal mine having extensive workings should be entrusted to an engineer familiar with ventilation problems. After the fan has been placed in position (and it should invariably be arranged that the direction of the air currents may be reversed with minimum delay and effort), the supervision of fan, doors, regulators, and other equipment and all situations with respect to air circulation should, at least for a large mine, be placed in the hands of one person who would have few, if any, duties other than those pertaining to ventilation. This person will have sufficient duties to keep him busy. He should keep in touch with proposed development work and have ready ventilation plans together with necessary equipment, supplies, etc. He should take measures to prevent return vitiated air from mixing with intake air shafts; in some so-called well-ventilated mines 50 to 75 per cent, of the air sent to the workers is recirculated vitiated air. The placing of small fans connected to tubing can be supervised so that these installations are made fireproof (even if the installation is only temporary); the fan-intake air should be kept free of return air and the tubing should be kept in place and in repair, and the end kept sufficiently close to the working face to be of benefit to workers. Doors should be located correctly to control air currents both at ordinary times and in case of fire; they should be free of leakage, be kept closed positively by latch or otherwise, and, if necessary, disciplinary measures should be enforced to compel all persons to respect doors and rules governing them. Fans should be inspected and kept clean and in repair, such apparent trifles as occasionally cleaning fan blades of grease and mud have a vital effect on fan delivery. Places not working temporarily or abandoned permanently should be sealed by canvas, gunite, concrete bulkhead, or otherwise, to prevent loss of air needed in active workings and to prevent vitiated air from abandoned places from mixing with air to be used in active workings. Sprays of cool water in intake shafts, drifts, and crosscuts, or in pipes, transmitting air aid in forwarding air currents by cooling them and eliminating dust from them. A map showing ventilation features, including location of fans, doors, overcasts, etc., as well as direction of air currents should be kept up to date. In a comparatively small mine, supervision of ventilation may be given to one person such as surveyor, or a technically trained shift boss, who could also perform his other duties; it is poor policy not to have specific supervision over ventilation or to have the responsibility divided among several shift or other bosses.

CONCLUSION

This paper has been able to touch on only a comparatively few points in connection with ventilation of metal mines, considering safety and health only to a slight extent, with intent to concentrate on suggestion of desirable methods to obtain efficient metal-mine ventilation, with a slight amount of attention to costs and results translated into dollars and cents. There are many phases of the subject that deserve a separate paper, such as the relative advantage of various shaped shaft cross-sections and various methods of smooth-lining and fireproofing of shafts, shaft stations and other places; the various features entering into matter of selection and placing of main fan, whether on surface or underground, whether direct connected or belt-driven, whether to provide for reversing of air currents, etc. Another subject of vital moment is whether a main working shaft, drift, or tunnel, shall be the intake of the return. In conclusion, it is suggested that those interested in metal-mine ventilation carefully read Mr. Bruce’s paper heretofore mentioned; He shows that it pays to ventilate, even when the preliminary work is expensive; he also gives data showing the utility of circular or octagonal shafts for air delivery against rectangular shapes; gives the advantage of smooth-lining over ordinary timber exposed shafts; and interesting frictional coefficients, costs and other data.