speeds, the water-guage (barring fan resistance) will not be appreciably greater than with one fan.

At a few collieries where a very high water-guage is necessary, the air is passed through two fans, the exhaust air from one being turned into the in-take of the second, thus effecting double rarefication and obtaining a water guage (nearly) double that produced by a single fan. When this arrangement is adopted, either by placing one fan inside the mine or having both at the surface, it is not essential that they shall either be both of the same size, or run at equal velocities.

When the size of the fans and the speeds of revolution are unequal, the resulting water-guage is approximately equal to the sum of the water-guages produced by both fans.

The law provides (Sec. 9) that "every mine having explosive gas. . shall be divided into two, four, or more panels or districts, each ventilated by a separate split or current of air, and fifty persons shall be the greatest number that shall work in any one panel or district at the same time...

In lift mining it is customary to divide each lift into two splits, one on each side of the slope; with two lifts working there will be four splits, and with three lifts six splits. Counter-gangway workings are usually ventilated by a split from the main gangway below, or by an independent split from the main in-take. (See Page plate No. 14.)

The intake at shaft workings is usually split into two main sub-divisions near the foot of the shaft, running off along the gangways opened in opposite directions from the foot of the shaft. Separate splits are taken off for each counter-gangway, and for each panel of workings operated by a plane or by an inside slope.

When there are several in-takes, as is frequently the case in shallow workings and at slope collieries where the workings are holed through to the outcrop, the separation of the current into splits is readily accomplished.

An excellent example of workings of this class is shown by Atlas Sheet No. XIV.

Ventilation of breasts.

The method of ventilating breasts is sufficiently well shown by Page plates 18, 19, and 20, and by the illustrations composing Atlas Sheets XXI, XXII, and XXIII.

The method shown by Fig. 52 has been used by the Lehigh Coal and Navigation company for single shute breasts in thick steep-pitching coals. It differs from those shown by the illustrations above cited, but has been found unsuitable for fiery workings because any obstruction at the face, or in the juggler manways above the last cross-hole, impairs the ventilation of all the workings beyond the breast in which the obstruction is located.

For this reason it has been partially replaced by the plan shown by Figs. 53 and 54, in which each breast is ventilated by a separate split of air taken off from the gangway through the travelling-way in the main shute, and returned through a cross-hole driven through from the first or main heading into the return airway.

In Fig. 53 (and also in Fig. 52) the airway is represented as being below the gangway. It was drawn in this position

to simplify the illustration. In the cross-section shown by Fig. 54 it is shown in place, -c being the return airway; m the cross-hole driven through into the heading a near the foot of the left-hand juggler manway (airway).

It will readily be seen that when this plan is adopted an obstruction in a breast does not affect the ventilation of any other workings.

This plan is more expensive than many others, and in soft coal is objectionable because the cross-hole (m) weakens the stump, but in fiery workings its advantages will probably prevail over any objections that may be urged against its adoption.

It will be understood without further explanation that in ventilating all steep-pitching breasts the air is either held in the breasts by air-tight stoppings or check batteries, by stoppings with regulators to allow only a small part of the air current to pass directly into each breast, or by door or brattice-cloth ("sheets") hung on the gangway or in the main shutes, and that all other passages (slant shutes, manways, etc.) are closed by stoppings or trap-doors. (See Page plates Nos. 18, 19, and 20.)