1st. We are here dealing with air, a compressible body which immediately expands to its normal volume at the atmospheric pressure.

2d. That the fan delivers air, not strictly speaking, through an orifice, but from a rectangular passage that presents no appreciable obstruction to the free exit of the air (as the orifice through which water discharge experiments are made.)

3d. Reëntries of air usually occur on at least one side of the fan.

If the evasée chimney accomplished all it is designed to accomplish the Guibal fan should certainly give a much greater useful effect than has yet been obtained from it, but while its action is doubtless imperfect it is certainly a valuable addition to any fan.

Experiments made on many different forms of Guibal fans, principally by English and French mining engineers, show that that the average useful effect obtained in the air is not more than 60 per cent. of the power indicated in the steam cylinder. If we allow 20 per cent. for loss from friction, (with direct-acting engine,) we have still a loss of 20 per cent. due to some imperfection in the fan.

We can attribute a small portion of this loss to leakage, but it is evident that the greater part of the loss is due to some other imperfections. When the fan is used as a "blower" the useful effect should be correspondingly greater, because the principle of the evasée chimney (reversed) can be successfully utilized.

A series of experiments carefully made by Messrs. Gille and Franeau, -a translation of which may be found in vol. xvi, 1866-7 of the Transactions of the North of England Institute of Mining Engineers, has most clearly demonstrated a marked increase in the useful effect due to the addition of the spiral casing, the proper adjustment of the shutter, and the expanding chimney. These experiments were made at the colleries of Crachet and Picquery on a fan with blades inclined backwards from the radius at an angle of forty-five degrees.

Fans of very large diameter are now coming into use at

the fiery collieries of the Wyoming district, but even these are small compared to some in use at English collieries.

As the water-guage produced by a fan is dependent upon the velocity (squared) of the tips of the vanes, as the quantity of air obtained is dependent upon the water-guage, and as comparatively small fans can be run at as high a peripheral speed as large fans, many engineers are opposed to the adoption of very large fans, believing that smaller fans will give the same results at less cost.

A peripheral speed of about three thousand feet per minute (60 revolutions for a 16 foot fan) is generally considered to be about as high a speed as it is well to adopt under ordinary circumstances, but the fans are commonly constructed strong enough to be run up to nearly or quite double this speed in emergencies.

Very large fans are run relatively somewhat slower than small fans.

Atlas sheet No. XIII shows the style of fan now used by the Lehigh and Wilkes-Barre Coal company. It is thirtyfive feet in diameter, with masonry pillars, brick side walls, and plate-iron casing and chimney, so as to be practically fire-proof. It is driven by a direct-acting horizontal engine. Upright engines are largely used for fans of moderate size (16 to 20 feet), but they are disliked by the engineers in charge of them, because it is impossible to keep them clean. Small fans are frequently run by belting, but nearly all mine superintendents prefer direct-acting, to geared engines of any type for this purpose.

Too much importance cannot be placed upon the selection of a fan engine for a large fiery colliery; a break down may at any time cause the loss of many lives and great damage to the mine, and it is, therefore, of great importance to have an engine that will run regularly without close watching and with little risk of breakage, and that can be quickly repaired or replaced in case of serious damage.

There are as yet comparatively few fans more than twenty feet in diameter, a very large number of twentyfoot fans, and a still larger number of sixteen-foot fans are in use; there are very few now used less than ten feet in

diameter, but quite a large number of old twelve and fourteen feet fans are in use.

Many of these latter, as well as some of the sixteen-foot fans, are old open periphery radial-winged fans, to which have been added the spiral casing and flaring chimney of the Guibal type.

The Philadelphia and Reading Coal and Iron company have recently built their fan casings of wrought-iron, and the Hazleton and other shops have turned out similar work, but the number of fire-proof casings is still comparatively small.

However, the feeling among the mining superintendents and engineers on this subject is such that in the future there will probably be few if any new fans erected at fiery collieries with wooden casings.

Water-gauge.

The water-gauge developed by a fan is theoretically (approximately) proportional to the square of the number of revolutions and also to the square of the radius or diameter-in other words to the square of the velocity of the tips of the vanes, (blades). Efforts to obtain an observed water-gauge that shall agree with that obtained by calculation almost invariably fail, and it is doubtful whether we shall ever obtain a formula practically useful as a check on water-gauge readings.

As at present used the water-gauge is rarely, if ever, a reliable instrument. It is commonly placed in a hole in a stopping between the in-take and return airway, either of the mine or of any split, and the hole luted with clay or otherwise made nearly air-tight.

Sometimes the stopping in which the water-gauge is fastened borders directly upon the airway, and in that case a current of air is flowing past, or directly against, the mouth of the gauge, increasing or diminishing the reading by the suction or pressure due to the velocity. With very high velocities this error may reach serious proportions. If the stopping is located midway in a heading, or at a consider

able distance from the airway, the current of air flowing past it may increase the reading.*

It is frequently claimed that water-gauges permanently fixed at certain points might be of value in determining when a roof-fall or other obstruction, or an open door, had impaired the ventilation. While this is doubtless true, it is also true that the miners usually discover at once from the state of the ventilating current when anything is wrong, but unfortunately not always soon enough to prevent an explosion.

The water-gauge is principally useful in determining from time to time that the condition of the airways is properly maintained to get the best results from the fan.

The water-gauge (or in other words the ventilating pressure) rarely exceeds two inches. At a few collieries it runs up to three inches, but as a rule it is from half an inch to one inch and a half. The secret of the large quantity of air passed through some of the mines of the Wilkes-Barre district-two hundred thousand feet and more per minuteis in the small water-gauge obtained by large airways, and the method now adopted at all collieries (when possible) of dividing the ventilating current into as many splits as possible.

In Mr. J. J. Atkinson's essay on the "Friction of Air in Mines" we find a clear exposition of this subject, which may be summed up in these few words: To reduce the friction, and consequently to increase the quantity of air with a minimum expenditure of power, enlarge the airways and split the current into several sub-divisions.

The value of his teachings is probably nowhere more amply demonstrated than in our anthracite mines, notably those of the Wilkes-Barre district, where the ventilating currents are probably larger, with lower water-gauges than in any other mining district in the world.

At several large collieries the water-gauge is little more than half an inch, and there are now comparatively few large collieries in the gaseous portion of the Wilkes-Barre

*As in a water syphon that will suck in air or fluid through any opening.

field at which the water-gauge is more than one inch or an inch and a quarter.

At collieries making only a small amount of gas the ventilating pressure is frequently from one and a half to two and a half inches of water-gauge.

Remarkably high water-gauges with an enormous volume of air have lately been reported from the double fan erected at the Baltimore Tunnel mines of the Delaware and Hudson company. This apparatus consists simply of two seventeen-foot fans of Guibal type with complete spiral casing, eight blades bent backwards from the radius and mounted on double spiders placed in the center of each fan. The two fans are provided with separate casings, are mounted on a single shaft about fourteen feet apart from center to center, and are driven by belting (6 and 12 foot pulleys) by a pair of 16" X 30" engines.

The policy of building two fans on one shaft or of placing two fans over one upcast is not indorsed by the mine superintendents and engineers throughout the region. Two fans so built must give practically about the same effect obtained from one fan twice as wide as a single fan*, and it has already been shown that the width is a factor of small importance. Fans built wide enough to insure a good degree of stability, are always wide enough to readily pass any current that can ordinarily be drawn from a mine.

The results claimed for this fan, and for other double ventilators, are viewed with distrust by nearly all anthracite colliery superintendents and engineers.

At a few collieries the experiment of placing two fans on one upcast has been tried, -the results have been found to be exactly what we should naturally expect,-no appreciable benefit.

As the quantity of air circulated through a mine is directly governed by the ventilating pressure (water-guage) and as this depends upon the speed of the tips of the vanes, and not upon the number of fans or width of the blades, it is evident that with two fans of equal size running at equal

* Except that the fan resistance, which is always very small in well built fans, is somewhat less.