influences in a great measure the distance to which the spray is carried.

Laws of Friction, &c.-Before describing how a ventilating current is produced and circulated through the workings, a short description should be given of the laws of friction and of the general rules relating to ventilation. The subject is such a complicated and extensive one that only the briefest summary is possible here. The finest series of papers in the English language are those by the late Mr. J. J. Atkinson,* which, although written so long ago, still remain the standard authority. To these, and others written at the same time by several of his contemporaries, the student is referred for detail of information and reasoning. The principal points dealt with, and brought out by the above series of papers, have been summarised and elucidated by Mr. W. Fairley.†

Currents of air, either on the surface or in a mine, are produced by a difference of pressure, and would flow at a great speed if no resistance were encountered. If v = the velocity in feet per second,

g gravity, or 322, and h = the height from which a body must fall in order to generate this velocity, such height being the motive column,

The water gauge (W.G.), which is the measurement of the pressure required to generate this velocity at which the air travels, if resistance were absent, would be a small one, and may be determined by the formula

where w = the weight of a cubic foot of air at the temperature of the upcast, and h Now

and

=

the motive power.

From this formula it will be found, that if the air has a final' velocity of, say, 50 feet a second, the theoretical water gauge required to produce it is only o6 inch. Fifty feet per second is a velocity scarcely attained in mines, and it is equally rare that the water gauge only shows o6 inch. The difference between the water gauge due to velocity, and the actual water gauge of any mine, is the measurement of the friction which the air meets with in passing through the air

ways.

The three main laws which govern the friction of gases flowing. through pipes are as follows:

*See list at end of chapter.

+ The Theory and Practice of Ventilating Coal Mines.

(1) The frictional resistance varies directly as the rubbing surface; this rubbing surface is found by multiplying the length of the gallery by its perimeter, or, in other words, its circumference.

(2) The pressure required to overcome the friction varies inversely as the area, if the rubbing surface and velocity remain the same that is to say, if two air-ways be taken, one of which is double the area of the other, only half the pressure has to be applied to each square foot of the large one as would have to be applied to the small one, to overcome the same amount of friction in the two ways, provided the velocity of the air and the extent of rubbing surface were the same in each.

(3) The frictional resistance varies directly as the square of the velocity; consequently, if the velocity be doubled, the resistance increases four times. The explanation of this law is a simple one, if it be remembered that if the velocity be doubled, double the quantity of air passes through the air-way in a given time, and meets every resistance with double velocity.

From these laws the following formula is deduced :—

=

where p the pressure in lbs. per square foot, a= the area of the airway in square feet, 8= the area in square feet of rubbing surface, v = the velocity of air in feet per minute, and k is a constant called a coefficient of friction, and is equal to the ventilating pressure required to overcome the resistance that a unit of air with unit velocity would meet with in circulating round a mine of unit area and having unit rubbing surface. The value of this coefficient has never been satisfactorily determined for the irregular passages of mines. Although it is generally admitted that Mr. Atkinson's figures are not strictly correct, yet they are freely adopted. He states that it seems probable that for every foot of rubbing surface, and for a velocity in the air of 1000 feet a minute, the friction is equal to o 26881 foot of air column of the same density as the flowing air, which is equal to a pressure, with air at 32° F., of 0.0217 lb. per square foot of area of section.

The difference between pressure and power must be clearly understood; pressure is the force per square foot producing the ventilation, and power is the quantity passing multiplied by the pressure. The quantity is found by multiplying the area by the velocity.

By transposing and substituting values of the different symbols in (6) nearly every formula can be deduced to work out the problems met with in ventilating mines. Several of the more prominent results obtained may be summarised as follows:

(1) The quantity of air circulating in a mine is according to the square root of the pressure.

(2) In air-ways of the same sectional area, but which only vary in length, the volume and velocity of air currents are inversely proportionate to the square root of the lengths.

(3) The quantity of air passing in air-ways of different areas, other things being equal, is according to the square root of the area multiplied by the area.

(4) The resistance varies directly as the length.

(5) The pressure required to propel air through passages is inversely proportional to the area, other conditions remaining the same.

(6) If any two machines are employed to ventilate a mine, each of which when working separately will produce certain quantities which may be denoted by a and b, the quantity of air that will pass when the two are working together will be √a2 + b2.

(7) The quantity of air passing is according to the cube root of the power applied.*

(8) Since the quantity of air circulating varies as the cube root of the power employed, and as the number of revolutions of a fan also varies as the cube root of the power employed, it follows that the quantity of air circulating depends directly on the speed of the fan.

PRODUCTION OF AIR CURRENTS.-The problem of producing sufficient air, and of so carrying it into every part of the mine that the noxious gases are effectually removed, is one of great importance. By the law in Great Britain and in many other countries, every mine has to be provided with two shafts, or outlets; one of these serves for the introduction of the fresh air, and is called the "down-cast"; the other, for the egress of the current after it has passed round the workings, and is called the “ up-cast."

Natural Ventilation.-No matter what the respective sizes of the two shafts may be, provided that they are connected by a passage and that the density of the air in the two columns is equal, no current is produced. If, however, the densities are different, the pressure of the one column of air will overbalance that of the other. The equilibrium in the two shafts is destroyed by the natural heat of the strata altering the density of the air. As a descent is made towards the centre of the earth, a proportionate rise of temperature is found- that is, after a certain limit is passed. This limit is found at a depth of about 50 feet, where the temperature of the rocks is on an average 50° F., this temperature remaining constant all the year round. From the mean of numerous observations, it may be taken that the underground temperature increases 1 for every 60 feet of depth below the invariable stratum. Therefore, the deeper the mine, the greater the difference of temperature of the air in the two shafts, consequently, the greater the ventilation.

From this cause ventilation is produced without any artificial assistance, and is called natural ventilation. It is, however, so inconstant as to be wholly unreliable, depending to a considerable extent on the temperature of the outside air, and the difference in the levels of the tops of the two shafts. Natural ventilation is due to exactly the same cause as furnace or fan ventilation, only in the latter cases the density of the air is altered artificially. It is present more or less in all mines, because of the heat given off from the strata and from the men and animals working below ground, which rarefies the air

A most interesting paper has been contributed to the Federated Institution of Mining Engineers (vol. ii., 483) by Mr. W. Cochrane on a Duplex Arrangement of Ventilators, the results obtained agreeing remarkably well with the theoretical deductions given in (6) and (7).

and causes it to rise to the surface. The amount of air put into circulation is readily affected by changes of temperature in the external atmosphere. In winter, when the air above ground is cold, it is of much greater density in the downcast shaft than it would be on a hot day in summer, and consequently the natural pressure producing ventilation is greater in winter than it is in summer. In shallow mines, the temperature of the air at the surface in summer may rise as high as that of the air underground, or even on a very hot day may exceed it. In the former case, if the mine relied entirely on natural ventilation, the air current would be stagnant, while in the latter case, the direction of the flow would be reversed from that which went on during the cold weather. While a difference in the level of the two shafts, especially in shallow mines, greatly assists natural ventilation, it is not absolutely necessary.

In deep mines there is always more or less natural ventilation, and as the temperature of the rocks in such cases is more than that of the air at the surface, even on a summer day, the current, when once started to flow by any means in a certain direction, always maintains that direction and only varies in amount, more in winter, less in

summer.

For the reasons given, natural ventilation is never to be relied upon, although it does in many cases materially assist the other means which are used to produce the air current.

Furnace Ventilation. The oldest means of producing ventilation was to artificially alter the density of one of the columns of air by heating it. At first, this was done by merely hanging fire-lamps in the up-cast shaft, to be soon superseded by placing a furnace at the bottom, as by the latter means the greatest effect is obtained. Furnaces may be constructed on two main principles, (a) either an open fire-place with all the air passing over the fire; or (b) contracting the area above the fire, and forcing the greater part of the current through the bars. Neither of these methods, separately, gives the best result. In the former, where a strong current is passed over the fire, its cooling action is so great that the combustion is feeble and a high temperature is not attained; while in the latter, if all the air passes through the bars, not only is carbonic oxide formed in large quantities, but the resistance or drag of the mine is much increased. A combination of the two gives the best results, and is almost invariably employed.

No better illustration of a well constructed and efficient furnace can be given than that at Eppleton Colliery (Fig. 523). The length

of the grate is 60 feet, and its breadth 11 feet; the end of the fire bars are 120 feet away from the shaft. An air passage and firing-hole is provided on each side of the furnace, and also an air passage along each side of the drift going to the shaft. This drift rises 1 in 2, and is lined throughout with fire bricks. With this large grate area, all the air passing over the fire is thoroughly heated, and, in addition, doors are provided at the front, so that the quantity forced through the bars can be regulated. Doors on furnaces are, to a certain extent, necessary, especially on re-starting after cleaning. The current, which is then small, can be forced through the fire, and as it increases, owing to the temperature getting high, the doors are gradually opened, and more air allowed to pass over the fire. The advantage of the side passages is, that not only may firing be entirely done at the side, but the risk of setting the adjoining strata on fire is reduced. A good casing of sand is placed all round these arches as an additional precaution.

At Eppleton Colliery, 24 tons of coal are burnt in the twenty-four hours. During two shifts a number of boilers are at work underground, so that the furnace does not produce all the air circulated. While these boilers are at work one man per shift is employed for firing, but at night two men are necessary; this means four men per twenty-four hours. The quantity of air circulated is 303,000 cubic feet per minute, with 2 inches of water gauge, but only 120,000 cubic feet passes over the furnace.

In fiery mines it would not be safe to pass the return air-current over a furnace, and it has to be fed with fresh air. As the temperature at the bottom of the up-cast shaft is sufficient to ignite gas, the return air-current has furthermore to be brought through a passage called a "dumb-drift" into the shaft at some point above the furnace where the temperature has fallen below the igniting point. Neither feeding the furnace with fresh air nor carrying the return air-current through a dumb-drift increases the efficiency of furnace ventilation, but on the contrary, diminishes it, as not only is the temperature of the aircurrent reduced, but a shorter column of air is heated.

The amount of ventilation produced by a furnace varies as the square root of the difference of temperature in the two shafts-that is to say, if the mean temperature of the down-cast be 50° F. and the upcast 75° F., if the temperature of the up-cast be increased to 150° the ventilation will be doubled, as the difference in the first instance was 25° and in the second 100°; therefore,

The objections to furnaces are the danger of introducing fire into mines yielding fire-damp, the risk of setting adjacent coal on fire, the corrosive effect on all shaft fittings and tubbing, and to the fact that no more than a certain quantity of air can be got out of a given furnace, no matter how much coal is used.

Furnaces are most objectionable where tubbing is employed, as the wood sheeting between the segments is continually being burnt out. Lining with brick-work offers little protection, as when the fires are damped down (for repairs to furnace or drift) the tubbing contracts so much that a large escape of water takes place, which, in some instances,