good practice. A set composed of four sticks is the same as a threestick set, with the addition of a sill laid across the floor, which serves to support the ends of the legs and is designed to resist upward pressure of the floor. Sometimes the legs are tenoned to fit into a mortise in the sill, but this is unnecessary if the angle the leg makes with the vertical does not exceed 15°. Figure 19, E, shows this method. 114. Q. What method may be used to give rigidity to three-piece sets on pitching beds or slopes?

A. Notched round timber at least 12 inches in diameter may be placed at the required distance apart (3 feet is used in the mine represented in fig. 20). Round timber 4 inches in diameter should be cut to fit snugly between the sets and nailed to act as braces. Six braces are used, about 20 inches from the inside notch at each end of the crossbar. Braces are also placed between each pair of legs about 20 inches from the floor and 20 inches from the notch of the crossbar. The legs should be put in hitches in the bottom, deep enough to prevent them from "kicking" out.

ELEVATION

SIDE ELEVATION FIGURE 20.-Method of timbering with a three-piece set in inclined beds or on slopes.

115. Q. What is a crib or cog? A. Cribs-called cogs in some mines-are formed by placing timbers to form a hollow square. Two timbers are placed on the bottom horizontal and parallel to each other. Another pair of parallel timbers is placed at right angles to these on the ends of the first pair. Sometimes intermediate timbers are used also. The crib is built up by placing each layer of timbers at right angles to the one below until the roof is reached. Frequently the interior of the crib or cog is filled with rock or similar waste material. Cribs or cogs are used to support large areas of roof, especially in dipping coal beds. They are also used to prevent slides of material or caved ground above men working in pitching beds on dips, usually above 45°.

116. Q. What is the essential difference between the use of props or cribs in pillar workings?

A. Props are used to provide temporary support, while cribs or cogs are used to provide more nearly permanent support to the roof. Cribs or cogs also sustain much more weight than props or even timber sets.

117 Q. How are cogs placed in steeply dipping coal beds?

A. Cogs higher than 12 feet are unstable, and a section of the top coal must be left in place to provide a cushion or support above the cog.

118. Q. What is a battery in pitching bed?

A. A battery is a line of cogs placed about 5 feet apart in the pillar
workings to hold back caves. Timber or lagging is placed between
and above the cogs built to the roof, and a cushion of coal or caved
material is placed above the battery to prevent any caved material
from falling or moving. If the bed is less than 5 feet thick, posts
or stulls are often used instead of cogs to form a battery.
How should props be set in place on a slope?

119. Q.

A. On a slope props should be set a little out of right angle with the floor, so that the angle on the lower side of the props at the floor is greater than 90° and perpendicular to the weight they support.

120. Q. What should be done to prevent props on a slope from being knocked out by falling or sliding material?

A. A safe plan to prevent the props from being knocked out on a slope is to dig a recess in the floor in which the props are set. which shows props set in a recess in the floor.) (See fig. 21, On moderate slopes, where material is not likely to slide, props may be placed without a recess, as shown in figure 22. Props should be tightened down the pitch with the wedge pointing down the pitch.

FIGURE 21.-Props set on slope, recessed in floor.

FIGURE 22.-Props set on floor without recess.

121. Q. What are some of the usual methods used in placing timber in rooms? A. Rooms vary in width. Some are made narrow-14 to 18 feet-and some are made wide-20 to 40 or more feet. The nature of the immediate roof in general governs the width most economical for operation and the quantity and method of timbering needed. The immediate roof over some coal beds can be supported only by crossbars and props set close together; where this method is required in rooms, the expense of timbering often makes it necessary to change the method of mining.

122. Q. What is meant by rows of props?

A. Single props set in line along the rib of the coal or along the tracks constitute a row of props. When the props are set along two lines parallel to the rib or track they are called a double row.

123. Q. What are some of the methods of placing props in rows in a room? A. The methods of placing props in rows in rooms can be shown best by diagrams, and some of the methods used are indicated in figures 23, 24, 25, and 26. Where the track is in the center of the room four rows of props set staggered can be used (see fig. 23). Figure 24 shows a room with the track at one side of the center and five rows of props. Figure 25 shows a room with the track near the side of the room and four rows of props. Figure 26 shows three rows of props with the track at one side. The character of the roof determines the method to be used.