METHODS FOB BOOF SUPPOBT.	51
2.	The middle series of beds, viz, Diamond, Rock, Big, New County, and Clark, under the Hyde Park and Providence sections.
3.	The three lowest beds, Dunmore No. 1, Dunmore No. 2, and Dunmore No. 3, under the major part of the city.
METHODS FOR SURFACE SUPPORT.
The methods employed at the present time for supporting the surface over the coal mines under the city of Scranton are of two general classes, which may be termed natural and artificial.
NATURAL OR PILLAR SUPPORT.
The natural method, of course, consists in leaving pillars of coal sufficiently strong to support the weight of the earth and rock that overlie the coal bed. The efficiency and value of these supports depend upon their size. That is, the horizontal area, the height of the pillar (which is fixed by the thickness of the coal bed), the compressive strength of the coal, the regularity of distribution of the pillars, and whether or not they are columnized with respect to pillars in near-by overlying or underlying beds, all have to be considered. In this vicinity the size of pillars has been mainly regulated by the one-third rule previously mentioned.
ARTIFICIAL SUPPORT.
FLUSHING.
There are several artificial methods of roof support, the principal and most effective of which is known as the flushing method. In this method coal culm and other fine refuse is washed into the mines through pipes by means of a stream of water, thus filling the desired portions of the mine.
This method was first used at Shenandoah, Pa., by the Philadelphia & Reading Coal & Iron Co. Afterwards it was introduced at Plymouth, Pa., and now has been adopted and is in practice over the whole anthracite region. Only culm is used, and the method has been adopted mainly for the purpose of protecting those parts of the mine or of the surface which it is necessary to support in order to maintain the mining operations.
Under Scranton considerable flushing has been done at various places, as indicated in the chapter on “Present mining conditions.” Foreign engineers, after inspecting the process in this country, have adopted it in Europe. There, much extended and amplified, it is now an essential part of the more recent mining methods by which the engineers are able to recover all the coal; the excavated spaces are filled by the flushing method with crushed rock, sand, gravel, and soil obtained from quarries opened for the purpose, and also with ashes and city refuse, some of which is transported long distances over the surface to the flush pipes. Foreign engineers