When the air is coursed back through the breasts and cross-headings, which is the most common plan, the shutes and travelling-ways are all closed, except at the last breasts in the panel, and the air is coursed back through the breasts into the main upcast, but as none of these stoppings can be made air-tight, a small split of air enters each breast. To secure the best results by decreasing the friction of the air, the cross-headings connecting breasts nearest the main upcast should, therefore, be relatively larger than those near the limit of the workings, but when the air is returned through an independent air-course the reverse is evidently true.

When this latter plan is adopted the air is forced into the workings (Fig. 52) by brattice sheets (or doors and regulators) hung across the gangway, or by regulators in the travelling-way which is bratticed off on one side of the main shute.

In flat or slightly-inclined gaseous workings, the shute*

* An improper use of the word; the English terin "bolt-hole seems peculiarly appropriate,-we might call such passageways breast entries.

opening into the breast is closed by a door opening near the gangway in wagon breasts, or in buggy breasts, by a brattice with a draw-hole closed with brattice cloth, under which the coal is dumped to fall on the platform below.

When the bed is thick or steeply inclined, accumulations of gas near the roof, or in holes in the top caused by rooffalls, are prevented or dispelled by deflecting the current by brattice or brattice cloth carried up from the last crosshole or from the juggler manway which is within a few feet of the face.

Ventilation by blowing.

Ventilation is, with few exceptions, effected by suction, either by furnace or exhaust fan. At the collieries worked under the supervision of Col. D. P. Brown of Lost Creek, superintendent of the Lehigh Valley Coal company's collieries in that neighborhood, we find the ventilation effected by fans used as blowers. There can be no doubt that as large a ventilating current can be produced by a fan when so used as by suction ventilation; and it seems probable that better results may be obtained by blowing than by suction, if we measure the value of these results by the number of cubic feet circulated per minute. The great loss of power caused by projecting the exhaust current of air against the atmosphere at high velocity, is avoided, and while a certain loss of power results from throwing the current into the mine at such a high velocity, some of the power so expended is utilized by imparting a certain momentum to the air in the downcast.

The special advantage claimed by Col. Brown for this method is that the headings of gangways and airways, etc., can be more thoroughly ventilated, and the cross-headings

need not be so close together to insure thorough ventilation in gaseous workings.

Thus in Fig. 55 showing the ventilation of an airway and gangway heading by suction, the air current comes slowly up the gangway, turns around the brattice cloth hung at a b into the cross-heading e, and slips around the brattice cloth cd into the return airway. The brattice cloth cd must be hung in this position to allow coal from the airway to be dumped through e and loaded (under the brattice cloth ab) into wagons on the gangway; while the brattice ab is so hung to allow the wagon to pass in to the face.

Ventilation of the same headings by blowing is shown by Fig. 56.

It will be observed that the ventilating current coming in through the airway, passing along behind the brattice, is confined to a narrow passage and acquires great momentum (velocity), which carries it well into the face; the same result is obtained on the gangway, both headings being thoroughly ventilated up to the face.

The chief disadvantage in ventilating by blowing is that the foul air, explosive gases, powder, smoke, and steam are carried out through the gangway. As the gangway is the main haulage road, this constitutes a serious objection to the use of blowing fans, and more especially in gaseous workings where safety lamps are required in driving headings and opening new workings, because the explosive gas (diluted, of course, with a large quantity of air, but which may become explosive) is blown out through the gangway and traveling-ways where the employés are using naked lamps. At the same time it is claimed that this system furnishes an additional safeguard against accident, as it absolutely requires that there shall be sufficient ventilation

to make the gangways perfectly safe-and when the gangway air is safe, all of the working places (by this system) must necessarily be free from danger.*

It is, however, a very serious drawback to the proper working of any mine to have the mainways filled with foul air, clouded with powder smoke, and perhaps steam.

The style of fan preferred by Col. Brown is roughly shown

by Fig. 57. In some respects it resembles the Schiele fan, but is doubtless more efficient.

The fan has eight wings curved backwards from a point one-half the radius distant from the center. Diameter 16 feet, width 5 feet. The shroud or casing is spiral throughout. The fan exhausts (blows) into an intake airway 10x12 feet.

Several methods of measuring the amount of air are in use. They all depend upon correctly ascertaining the velocity *Except from accumulations near the roof, etc.

The circle marking the outer periphery of the fan blades should be erased. The figure shows the fan in section, the inner circle representing the intake in the fan shroud. The outer circle is a mistake of the draughts

man.

of the air current, which is then multiplied by the crosssectional area of the passage to find the volume of air circulated per minute. The area of the airway (whether gangway, intake, upcast, or return airway) is obtained by careful measurement. A measurement made at one point is not sufficient to insure accuracy (except for anemometer measurement), and it is advisable to take the mean of measurements made at several points, the smaller measurements being taken in preference to the larger ones.

Having determined the size of the airway, the velocity of the air is taken either with an anemometer or by one of the following methods:

1st. Noting the time powder smoke takes to travel a certain distance previously measured, or the distance it travels in a fixed time; or

2nd. Substituting for the gun-powder, ether, bisulphide of carbon or some volatile fluid or gas with strong odor;

3d. Noting the time, or distance traveled in a fixed time, by walking with a lamp at equal speed with the ventilating current, so that the flame remains vertical.

As the anemometer requires very much less time and trouble than any other method and does not require the services of an assistant, it is nearly always used,—but the above methods furnish a valuable means of checking the anemometer readings and of determining what corrections are necessary.

Several different styles of anemometers are now manufactured, (principally by English and French makers,) but the well-known Biram instrument is preferred.

Every anemometer is more or less liable to irregular working, either from the presence of dirt, gummy oil, undue friction, or from wear of its working parts, and for this reason it is considered advisable to make periodic tests to determine its friction constant.

The anemometer dials score the number of revolutions made by the wheel, assuming each revolution to represent one foot of distance; but as the inertia of the instrument does not permit it to start off at full speed, as there is also a certain resistance due to starting friction, and a certain