VERTICAL SECTION AT FOOT OF SHAFT, WITH ASCENDING CARRIAGE.
At the mouth of the shaft and projecting into it are the “wings,” “keeps,” or “cage rests,” which are pressed against the sides of the shaft by the ascending carriage, but spring back into place underneath it and support it while it is at rest. When the carriage is ready to descend the wings are withdrawn by hand levers.
The safety carriage is now in general use in at least one hoisting compartment of every shaft. This carriage is built of wrought iron instead of wood; it has a bonnet or roof as a protection against objects falling down the shaft, and it has safety clutches or dogs to stop the carriage and hold it in place in case of accident by breaking ropes or machinery. Operators are required by the act of 1885 to provide safety carriages for the use of their employees, and also to keep movable gates or covers at the mouth of each shaft to prevent persons and materials from falling into the opening.
Where mining is done by shaft there is seldom any other way provided for the passage of workmen in and out than the way by the carriage. A small shaft for the admission of air is sometimes driven down to the highest part of the seam, and ladders are placed in the opening on which men may climb up and down, but these ladders are seldom used save in an emergency. It is made obligatory upon operators, by the act of 1885, to provide two openings to every seam of coal that is being worked; these openings to be at least sixty feet apart underground, and one hundred and fifty feet apart at the surface. The object of this rule is to provide a way of escape for workmen in case of accident to the main outlet.
It is seldom necessary, however, in these days, to sink a separate shaft in order to comply with this provision of the law; the underground workings of the mines having such extensive connections that often not only two but many openings are accessible from each seam.
As to the comparative cost of the different methods of entry, the drift is of course the cheapest. In this method the very first blow of the pick brings down a fragment of coal that may be sent to market and sold. For this reason the sinking of a slope is less expensive than tunneling or shafting, because the excavation is made in the coal. It may be said to cost from twenty-five to fifty dollars per linear yard to sink an ordinary double track slope, from fifty to seventy-five dollars per linear yard to drive a tunnel of average cross-section to accommodate two tracks, and from three hundred to five hundred dollars per linear yard to sink a shaft with four compartments. Of course circumstances, especially the character of strata, may greatly increase or lessen these limits of cost. Indeed, it has happened that a shaft in process of sinking, which had already cost many thousands of dollars, has been necessarily abandoned because an intractable bed of quicksand has been encountered.
The experienced coal operator, knowing the advantages and disadvantages of each of these methods of entering a mine, and the adaptability of each to his particular coal bed, will find no difficulty in making a selection from them. Indeed, there may be, and usually is, practically, no choice. The selection of a site for the opening is ordinarily attended with but little more freedom of choice. The outcrop, if there be one, the topography of the surface, the outline of the coal seam, the accessibility of the spot, the location of the breaker, all govern in the selection of the site, and usually all point to the one most available spot.
CHAPTER VIII.
A PLAN OF A COAL MINE.
The progress that has been made in the science of mining coal within the last half century bears favorable comparison with the progress that has been made in the other industrial sciences. To-day the ripest experience and the best engineering skill in the land are brought to bear upon the problems connected with coal mining. In comparison with the marked ability employed and the marked success attained in the mining enterprises of to-day, the efforts of the early miners are almost amusing. The pick and the wedge were the chief instruments used in getting out coal. Powder was not thought to be available until John Flanigan, a miner for Abijah Smith, introduced it into the mines in 1818. It is said that when openings were first made for coal in the vicinity of Pottsville shallow shafts were sunk, and the coal was hoisted in a large vessel by means of a common windlass. As soon as the water became troublesome, which was usually as soon as the shaft had reached a depth of twenty or thirty feet, this opening was abandoned, a new shaft sunk, and the process repeated.