At the mouth of the shaft heavy upright timbers are set up, inclosing the opening. These are united by cross-beams, and the whole structure is well braced. In this head-frame are set the sheaves, at a distance from the ground of from thirty to fifty feet, although, when the entire surface plant was under one cover, they were set much lower. These sheaves are huge upright wheels sixteen feet in diameter, over which the ropes pass that connect with the cages. A sheave similar in form to the bicycle wheel is now coming rapidly into use; it is found to bear a greater strain in comparison with its weight than does any other form. The hoisting engine must be in the immediate vicinity of the shaft, and the rooms for this and the boiler, furnace, and pump are usually all under one roof. The iron or steel wire ropes extend from the sheaves in the head frame to the drum in the engine-room, around which they are coiled in such a manner that as one is being wound up the other is being unwound. Therefore as one carriage ascends the other descends by virtue of the same movement of the engine. Since the breaker may receive coal from two or more openings it must be so located as to be convenient to both or all of them. If the ground slopes sufficiently the breaker may be so built that its head will be on a level with the head of the shaft. This will save breaker hoisting. When coal is brought out by a slope the track and grade of the slope are usually continued, by an open trestlework, from the mouth of the opening to the head of the breaker. Wherever it is possible to do so, the loaded cars are run by gravity from the mouth of the opening to the breaker, and the empty ones are drawn back by mules. Sometimes they are hauled both ways by mules, and sometimes a small steam locomotive engine is employed to draw them back and forth. The coal breaker is an institution that is peculiar to the anthracite coal fields of Pennsylvania. Its need was made manifest early in the history of anthracite mining, its development was rapid, and it has now come to be wholly indispensable in the preparation of anthracite coal for the market. It is very seldom indeed that one sees this coal in the shape and size in which it was mined. All anthracite coal for domestic use is now broken, screened, and separated into grades of uniform size before being placed upon the market, and this work is done in the coal breakers.