Anemometer and its Use. — The anemometer is the instrument which is in general use for measuring the velocity of the air current in mines. There are various forms of anemometers, which, while differing in detail, are all constructed upon the same principle. A series of vanes are placed in an oblique direction round an axis, which is pivoted in front and supported in a bearing behind. This axis carries a small worm which gears into a wheel to which is attached the counting mechanism. When the instrument is held in an air current, the light vanes, which are set like the sails of a windmill, are driven round carrying the axis and the worm wheel with them. The counting gear is usually thrown out or in by means of a lever operating a small clutch. The instrument is fitted with one or more dials, and the pointers which run round them indicate the distance the air has travelled. The larger dial is usually graduated to read feet and a smaller one on the same dial plate to read hundreds of feet. In some cases dials are provided to register thousands and ten thousands, but instruments reading units and hundreds are quite sufficient for any ordinary work. Since the anemometer records the distance travelled by the air it is necessary to use it in conjunction with a watch or some timing arrangement to obtain the velocity or rate of motion of the air, which is distance travelled per unit of time.

Usually when the anemometer is tested to standardise it, the readings are found to be slightly incorrect. In such cases a correction card is supplied with the instrument showing what has to be added or deducted at different speeds in order to get approximately true readings. Such a correction card applies only to the instrument with which it is issued. An anemometer of the ordinary type is shown in Fig. 65.

When making a measurement the reading at which the instrument stands is recorded. It is then placed in the current, and as soon as the vanes have got up to the speed of the air the catch is thrown into gear. The instrument is now allowed to run for a minute or more, and the catch disengaged, and the new reading noted. If run for one minute, the difference of the two readings will be the velocity in feet per minute, if run for two minutes the difference must be divided by two, if for three minutes by three, and so on. In order to get approximately accurate results, a number of readings should be taken at different parts of the cross section, the average of these will be the most acceptable figure for the velocity.

The instrument should be held with its blades facing the air current, and all measurements made at places where the area of the airway can be accurately determined, and where there is as little obstruction as possible.

Distribution of the Air. — After passing into the mine by way of the downcast shaft, the air must be guided into the workings, otherwise it would simply take the shortest path to the upcast shaft. To guide the air in the desired direction, and to prevent short circuiting, Stoppings, Doors, Air Crossings, and Bratticing are used. The fresh air passing into the workings or a section thereof is called intake air, and the roadway through which it passes is called the intake airway. Air after passing round the workings is called return air, and the roadways through which it passes are called return airways.

Stoppings. — Where communicating passages exist between the intake and return airways they are built up by means of brick walls or closed by wood partitions. Walls built with brick or stone and mortar are called permanent stoppings, while those built with wood are called temporary stoppings. Stoppings are built tight so as to seal up the passage between the two airways, and do not provide for traffic between them.

Doors. — Should it be necessary to pass from the intake to the return, the passage is closed by two wood doors which open towards the intake. These doors are placed some distance apart, so that men, horses, or tubs can pass through one door and let it be shut before the other is opened. In fitting up doors, it is necessary to set the framework with a slight cant so that the door when opened will shut with its own weight. It is not desirable to have doors on main haulage roads, but if circumstances render it absolutely necessary then automatic doors which are opened by the tubs and closed by gravity may be used. Regulating doors are sometimes used to regulate the amount of air passing into a section. This class of door is made with an opening near the centre, which is covered with a sliding shutter. The position of the shutter, and consequently the area of opening, is varied until the desired amount of air is passing through. Near the face where the subsidence of the roof makes it undesirable to use doors until the roof has completely settled, single or double screens of brattice cloth are made to serve the same purpose. These screens are nailed to a crown and hang down to the floor. If used where the air current is not too strong and kept in good condition, they are both convenient and effective.

Air Crossings. — In the ordinary course of working an extensive colliery, conditions may arise which make it necessary to carry the intake and return air currents across each other. In such cases it is essential that the two currents should not mix, and to prevent this the two airways must be separated by an airtight partition. The arrangement adopted to carry one air current over another is called an air crossing. One form that is often used consists of two upright walls built alongside one of the airways and covered over with wood. One current of air passes underneath, the other goes over the top. This class of crossing is shown in Fig. 66. Where a more substantial form is desired an arch may be built over the airway at the site of the crossing, one current passing through the arch, the other over the top.

When the mine is one where the possibility of explosion exists, it is better practice to drive one air course over the other, leaving a fair thickness of strata between. An air crossing of this kind is shown in Fig. 67, and although much more costly to make than the other forms described, is altogether more satisfactory, being explosion proof and entirely free from leakage.

Brattice. — In many parts of the mine, especially in pillar and stall workings, the air has to be carried close up to the working places by means of a temporary partition set up so as to divide the roadway into an intake and return. A partition erected for this purpose is called a brattice. Wood, or a combination of wood and screen cloth, is generally used for short distances, while for long drifts and similar places a central division is formed by building a brick wall, usually 4½ inches thick with 9 inch butts at intervals of 6 feet or so. Sometimes air-pipes of light sheet iron, or boxes made of wood, are used in place of brattice. Generally in such cases an auxiliary fan is placed in the mine and forces air through these pipes to the point where it is required. When brattice is used in places where the coal or ground has to be blasted, the blasting may displace some part of it near the working face. Should this occur, care must be taken to have it immediately restored, especially if small quantities of firedamp are given off, as neglect of this precaution may cause accidents.

Laws of Ventilation. — In order to produce a current of air, a difference of pressure between the two ends of the airway must be set up. This pressure difference is called the ventilating pressure, and is measured by means of a water-gauge, which has already been described. It is this difference of pressure which imparts motion to the air, causing it to flow from the region of higher to that of lower pressure. The velocity or rate of flow of the air depends upon the pressure urging it on, and the resistance offered to its flow by the airway. The pressure required to overcome the resistance of the airway varies.

(1) Directly as the rubbing surface of the airway.

(2) Inversely as the area of the airway.

(3) Directly as the square of the velocity of the air current.

The actual pressure required depends on the coefficient of friction between the sides, roof, and floor of the airway and the moving air. The value of this coefficient changes as the character of the roof, sides, and floor changes; but where calculations involving its value have to be made it is customary to take what represents a fair average.

Power. — In order to produce a ventilating current, work must be done in moving the air. The unit of work is the foot-pound and is the work done in moving a load of one pound through a height of one foot. When work is done at the rate of 33,000 foot-pounds per minute, the agent performing the work is said to be doing it at the rate of one horse-power. When a pressure of p pounds per square foot acts over an area of A square feet, then the total pressure is pA pounds. If this pressure moves the air through a distance of V feet in one minute, then the work done is pAV foot-pounds and

The beginner should strive to distinguish clearly between the pressure and power of a ventilating current. Pressure is the force producing motion. Power is the product of this force and the distance it moves through in a given interval of time. As already explained, if the force be measured in pounds and the distance through which it moves in one minute be measured in feet, then the product is foot-pounds per minute, or otherwise power. The engineer's unit of power is called the horse-power and is the power expended in performing 33,000 foot-pounds of work per minute.

A study of the laws of ventilation shows us among other things how to obtain the best results with the minimum expenditure of power. This is done by splitting the air, as it is termed. By splitting we obtain (a) Shorter routes for the air travelling, and lessened frictional resistance. (b) Lower velocities in each of the several airways, so that lower pressure and less power are required to circulate a given quantity of air through the mine. (c) Fresh air for each separate district. (d) Greater freedom from danger should an underground fire or an explosion occur, as the effects are more likely to be confined to the ventilating district in which it occurs.

Splitting should not be carried too far, or the velocities of the separate air currents may be reduced until they are too low to sweep out gas.

A fair velocity for the air to travel is : In shafts 800 to 1000 feet per minute; in main airways 700 to 900 feet per minute; and in working places from 120 to 300 feet per minute. Much depends on the area of the airways, and the amount and character of the noxious gases to be dealt with, but the above may be taken as fairly representative of good modern practice.