a high arch, but there are other things to be taken into consideration. We often meet with mining engineers who have their air-ways driven large and roomy, and avoid, as much as possible, all angles, and everything that will cause any stoppage or extra friction to the current of air while passing through the mine- which is all right; but when they build the furnace, they imagine they must build it a certain shape, and very often block up more than one half of the airway!

And there are others who have read somewhere that a furnace in a well regulated mine should be built with two side chambers, one on each side of the furnace; and, in order to have room for the side chambers, they reduce the size of the furnace, in some cases to one fourth the size of the airway. Now, I claim that the air that passes through the side chambers does not come in contact with the fire, but passes into the upcast, to mix with the heated air that has passed over the fire, decreasing its density, and decreasing the ventilation if the furnace is properly constructed.

Side chambers will, under certain circumstances, add to the ventilation; for instance, if the air-ways are large and roomy, and the furnace is four or five times smaller than the air-ways, and there is a tendency to a strong, natural current, then side chambers would be a benefit, as the power would be lost in the passage of the air through so small a furnace, and by opening side chambers you would give vent to the column of air; but if the same space that is exposed for the passage of air, by opening the side chambers, had been left over the grate bars when the furnace was built, the result would still have been better, as the object of a ventilating furnace is to heat the air in the upcast shaft, and the more this air is heated the lighter it is, and the more atmospheric pressure it will overcome at the upcast shaft, and the pressure remaining the same at the downcast shaft the air is forced through the subterranean passages of the mine to the furnace where it should be heated, in order to keep up the density of the air in the upcast shaft; and as the ventilating power is always as the downcast shaft, no matter what kind of ventilation is adopted, whether it is furnace, fan, steam, or natural, as there is no suction in a furnace, neither is there any in a fan. Therefore a furnace should be built so as to heat as hot as possible the whole volume of air, and at the same time offer as little resist

ance as possible to the column of air while passing through the mine.

Many of the furnaces in this State are too small for good results; the air is too confined in passing through. Roomy air-ways are of little avail with a small furnace. There are instances of this kind in the State, where the air-ways are large and roomy, and the furnace is only about one fifth the size of the air-ways. It is a matter of great importance that care should be taken in building a ventilating furnace for several reasons. Setting the coal on fire must be guarded against, and the furnace should be situated far enough from the upcast shaft to obviate the danger of setting the woodwork on fire, and to avoid as much as possible the friction of the air current at the furnace. The proper place for a furnace is at the bottom of the upcast, because the ventilation depends upon the amount of heat imparted to the column of air, and the larger the column of heated air there is in a shaft the greater the velocity of the ventilating current. The practical power of a column of heated air is in proportion to the depth of the shaft; a deep shaft will give a larger volume of air than a shallow one.

As before stated, fan ventilation is more effective in shallow than in deep mines; but there are a great many drift mines in this State, where they do not use steam power, where the cost of the fan would be greater than a furnace, and the cost of keeping the fan running would be as great as keeping a fire in the furnace, as it requires the constant attention of a man in both cases; but wherever they use steam power, I would recommend a fan, no matter what the distance of the steam from the air shaft, as the fan engine can be placed in the engine house, so that the engineer can look after it, and the power can be transmitted to the fan by an endless rope; this is being done in several places in this State with good results. I would also recommend a fan so constructed that it could be used as an exhaust or force fan; and one of this kind, with the casing put together in a workmanlike manner, will soon pay for the extra expense if there is water in the hoisting shaft; and wherever a fan of this kind has been introduced, mine superintendents and mine bosses say they could not be hired to adopt any other kind of ventilation, and would not go back to the old furnace under any consideration.

There are several different make of fans in use in the State, but those made in the State give as good satisfaction as those that are

brought from the east. Some of the coal companies build their own fans; for instance, the fan at No. 2 slope, at Muchakinock, was made in the blacksmith shop at the mine, is fourteen feet in diameter, is used as a force fan, and gives a volume of air of eighty-three thousand cubic feet per minute when run at one hundred revolutions per minute. There are several manufacturing firms in the State who are making ventilating fans which can be bought cheaper than by sending east for them; by buying of home manufacturers you save the freight charges, which sometimes amount to almost as much as the first cost of the fan.

TABLE OF THE PRESSURE OF AIR AT DIFFERENT HEIGHTS OF THE BAROMETER.

To find the pressure per square inch in pounds, multiply the reading of the barometer in inches by .4908. To find the pressure per square foot in pounds, multiply the reading of the barometer in inches by 70.6752.

GASES MET WITH IN MINES.

The gases generated in coal mines are fire-damp, after-damp, sometimes called choke-damp, black-damp, and white-damp.

Fire-damp is light carburetted hydrogen, and consists of one volume of the vapor of carbon, and two volumes of hydrogen, condensed into one volume. This gas is never met with in the mines of this State.

Black-damp is the carbonic acid gas of chemistry, and is the principal gas met with in the mines of this State. It is composed of two atoms of oxygen and one atom of carbon, and by weight, oxygen 72.73, carbon 27.27, and by volume, one each, and it is rather more than one and one half times as heavy as an equal volume of common air; the specific gravity of common air being 1,000, while that of carbonic acid gas is 1,524.01. This gas is accumulated from several causes: The respiration of men and animals, the combustion of the workmen's lights, the decomposition of timber and small coal in the gobs, the explosion of powder, the excrementitious deposits of men and animals, and it also exudes from the roof and floor of the mine.

Black-damp, in its pure state, is a deadly poison, and will neither support life nor light. When ten per cent of black-damp is diffused through the air of a mine, a light cannot be maintained; but when mixed with a certain portion of pure air, a miner can remain for considerable time after his light has refused to burn; its effect on the miner is such as to produce headache, languor, loss of appetite, and general debility. This gas is mistaken for something else from the position it is sometimes found to occupy in the mines, as a great many miners think that if they are working in a place elevating from the entry, that black-damp will not molest them, as the gas is heavier than common air it would force itself out into the air-way, and would not remain in a room driven at an elevation off the airway; but this is not the case.

Black-damp is sometimes held in suspension in a room elevating from an air-way; for instance, if a room is turned off the air-way, and the current of air is passing the mouth of the room and has no chance to exert any of its force at any other place in the room, then, if black-damp should accumulate, and no car or anything else to cause a current in the room-under such circumstances, black-damp

will accumulate, and remain until a current of air is brought to bear upon it.

But some claim that as black-damp is one and one half times as heavy as common air, that it is not reasonable to suppose that it can be held in suspension, at an elevation from the air-way, by the passing current of air in the air-way. Let us see. Take, for instance, an air-way five feet wide and five feet high; the sum of its four sides would be twenty feet of resisting surface for each foot in length of the air-way. Now, suppose the room-mouth is five feet wide and five feet high; then, the room-mouth would present the same resisting surface as the air-way; and as the room-mouth is five feet high and. five feet wide, it would give an area of twenty-five feet exposed to the pressure of the moving column of air. The atmospheric pressure varies according to the density of the air. For instance, if the barometer reads thirty inches (see table of the pressure of air at different heights of the barometer), the pressure on all surfaces exposed to the air is 2,120.25 pounds per square foot; therefore, on the mouth of the room above referred to there would be a total pressure of 53,006.25 pounds.

But there is another fact to be taken into consideration in connection with air pressure: that if we increase the speed of the air in the air-way, we also increase the pressure in the following proportion: If we double the quantity of air in an air-way, we have four times the pressure, and nine times the pressure will produce three times the quantity, and sixteen times the pressure will give four times the quantity, and so on in like proportion. And if the pressure of 2,120.25 pounds per square foot would give a volume of air of one thousand cubic feet per minute; and if the volume of air is increased to two thousand cubic feet, the pressure would then be 8,481 pounds per square foot, or a pressure on the room-mouth of 122,025.10 pounds; and if we increase the volume of air to three thousand feet per minute, then the pressure would be 19,082.25 pounds per square foot, and at the room-mouth it would be 477,056.25 pounds. And if the volume of air is increased to four thousand feet, we would have a pressure per square foot of 33,924 pounds, and on the room-mouth there would be a pressure of 848,100 pounds. In increasing the volume of air from one thousand cubic feet to four thousand, we have increased the pressure at the room-mouth from 53,006.25 pounds to 848,100 pounds; but as we have made no arrangement for this air