analogy that one of our very best warnings is the ba rometer, that we hope our own government will follow England's Signal Service Bureau, and send our colliery managers warnings.

CHAPTER XI.

CONCLUSION.

40. THE first method employed to ventilate mines was, we believe, to agitate the air by shaking a cloth, next, natural ventilation by means of upcast and downcast shafts. This method was followed from necessity by the furnace; and even to-day this latter method is considered by some the best, on account of the liability of mechanical ventilators to get out of order, and so stop the current. The furnace is still used in many mines, but is being gradually superseded by the mechanical means now at our command. It is, however, more effective in deep mines than any thing as yet employed, but it is just in this position that it is the most dangerous, as, the deeper we descend, the more gas we are likely to encounter. In shallow mines, especially if worked at a dip, the fan is the more economical of the two systems. The steam-jet, at one time, was

a rival of the furnace. This was followed by allowing water to fall down the downcast; but this system proved inefficacious, as it did not provide enough change of temperature, and the water had to be pumped back again, in most cases, out of the mine. The mechanical ventilators of late years have been numerous; but, of all the number, not one perfect one has been produced. Guibal's Fan seems to be the favorite; and to him is due the simplest, and, with his sliding shutter, the most effective fan. Nixon made a ventilator on the air-pump system, the immense piston of which goes backwards and forwards on wheels. The air is received into a chamber, and forced out by this piston: it works exactly upon the same principle as a pump, drawing the air from the mine by one set of valves, and then forcing it out by another on the back movement of the piston.

Mr. Struve constructed a machine upon a principle similar to that of Mr. Nixon, using two large gas-tanks, arranged with valves. These gasometers moved up and down alternately in water, this means taking the place of a piston. There are also centrifugal machines, receiving the air at the centre, and throwing it off from the ends of the blades: others are made on the windmill plan, each blade, as it revolves, cutting out a definite portion of the air. The Champion Ventilator,

the only really distinctive American ventilator, is so arranged as to be used either as a forcing or exhausting fan. It is credited with very good results. Every new fan which has been built of late has been declared to give at least ten per cent more air for the same amount of power than any fan previously invented. This can hardly be believed, unless it has been proven by placing the new fan in a position where some other fan has been, with the conditions of the mine and the power just the same.

Theoretical comparison of two fans at different mines cannot give any thing like exact results, as the airways and resistances will not be similar in the two mines; and, while one fan may be better than the other, yet, in its position, it may be unable to cope with an inferior adversary more favorably situated. A larger water-gauge may be obtained in the fan-house than in the airway leading to the fan: this is accounted for by the fact that the fan offers more or less resistance to the air, and slightly impedes its discharge. A perfect fan should not do this, at least to any great extent. Theoretically and practically the amount of ventilation obtainable from furnace-action will depend upon the difference in weight of two air-columns. An improper consideration of this subject has led the enemies of the furnace to state that there is a material difference

between the action of furnace and fan ventilation; the former being likened to propulsion, the latter to traction. Were the air propelled, the power expended would be applied to force the air down one of the shafts, which the furnace does not do, but draws the air down, expelling it in lighter form, the same as the exhaust fan.

41. Suppose the exhaustion produced in a fan-drift at 40 revolutions per minute of the fan averages about 1.25 inches of water-gauge, while at 60 revolutions of the fan we have 2.8 inches water-gauge, a rise proportional to the square of the speed. Since the vacuum. increases as the square of the number of revolutions per minute, the quantity of air produced should be the same per revolution at any speed where the conditions. are unchanged, for the volume of air varies as the square root of the water-gauge; i.e., the square root of the lowest pressure bears the same relation to the square root of the highest pressure as forty revolutions. bear to sixty revolutions per minute; or

V1.25: V2.8:40: 60.

Therefore, if it were desirable to pass double the quantity of air though a mine or drift where the existing friction is equal to one inch of water-gauge, without

making any alteration in the underground arrangements, the effect of the change would be to increase the measure of resistance to four inches of water-gauge; also the power required to overcome this friction would be eight times that employed for the original quantity, as, in addition to the friction being fourfold, the volume of air is also doubled. In like manner, for three times the quantity, we have nine times the resistance, and require twenty seven times the power.

The following table is taken from Mr. R. Howe's paper, printed in the Transactions of Chesterfield and Derbyshire Institute of Engineers; the fan under consideration being a Guibal. In the preparation of the table, the following general principles are observed:1st, The quantity of air increases in proportion to the speed of the fan.

2d, The water-gauge increases proportionately to the square of the number of revolutions of the fan.

3d, The horse-power in the air increases as the cube of the quantity.

4th, The steam pressure in the cylinder is in proportion to the square of the piston's speed.

5th, The horse-power of the engine is proportionate to the cube of the number of revolutions per minute, or to the cube of the volume of air.

Illustrations: