the bottom by the bank, as in the case of quick or running sand, a set of rangers and braces should be put in place immediately under the lowest set already in position as soon as the excavation is low enough to permit it. As the excavation and sheathing are carried down this last set of rangers should be driven down, always being kept level crosswise of the trench and just above its bottom, until it is the proper distance below the preceding set, when it is driven no further, but another set is started under it. If the bank is tolerably stable the stiffness of the sheathing-plank can be relied upon to keep it in place below the second ranger until the trench is sufficiently deep to permit placing the third ranger in its proper position without any further driving.

Each brace should be exactly beneath the braces in the tiers above.

There is considerable friction between the sheathing and the bank on one side and rangers on the other, and after two sets of rangers are in the driving becomes quite difficult and the upper ends of the plank become battered and broomed and the plank broken, sometimes even when they are protected by caps. Ordinarily 10 or 12 feet is the greatest depth to which plank can be driven economically. It then becomes necessary to start a new course of sheathing, which is placed inside the upper course, its back resting against the rangers of this. The second course is driven and held in place by rangers and braces as was the other, and may be succeeded by one or more other courses each 10 or 12 feet high. When a new course of sheathing is started it is advisable to temporarily fasten planks horizontally in front of and behind this sheathing near the top, by a nail at each end driven into a sheathing-plank, to keep the plank in line and steady them while driving.

In placing each course after the first one an opening must be left at each vertical line of braces, since the sheathing

cannot be driven there. If these openings give trouble they may be closed by slipping into them, behind the rangers, 5-foot lengths of plank when the trench has reached that distance below the first course of sheathing, and driving these to keep pace with the other sheathing. When the trench is 5 feet deeper still another 5-foot length of plank may be slipped behind the ranger on top of the former length, and so on. A short piece of plank, at least, should be kept in the bottom of this opening to keep the planks on either side the proper distance apart.

Another method of closing these openings is to cut a plank just long enough to reach from the bottom of one ranger to

FIG. 14. SHEATHING UNDER BRACES.

the top of the next and a little wider than the opening. This is placed over the opening against the face of the sheathing, and between the rangers, to which blocks are nailed to hold it in this position (see Fig. 14).

In some cases it will not do to leave this space open for even a foot above the bottom of the trench, as in quicksand. It may then be advisable to use a somewhat different system of rangers, as follows: In placing rangers for the first course of sheathing, where one ranger is ordinarily placed two will be placed, one in front of the other but separated from it by a small piece of plank at the end of each brace. The front ranger may be but a 2-inch plank. The second course of

sheathing is slipped between the two rangers and when it is all in place except where the spacing-blocks interfere the braces are driven along about a foot, the spacing-blocks knocked out, and sheathing dropped into the spaces they occupied. Generally plank behind a brace cannot be driven, owing to the friction, but when the one next to it has been driven the brace can be moved over in front of this and the former then driven.

Where there is more than one course of sheathing, or whenever the bottom of any course is not kept at the bottom of the trench, all braces in each vertical line should be tied together by cross-bracing of plank nailed to them; otherwise one side of the sheathing may drop, loosening the braces and causing a complete collapse of sheathing and trench. The author has seen several serious accidents due to the neglect to use such cross-bracing.

The sheathing is usually of hemlock plank, although pine would be better, being less brittle. Maple and other hard wood has been used in a few instances. The plank is usually 2 inches thick, although heavier may be advisable in deep, wide trenches or where it is desirable to use as few rangers and braces as possible. It should never be less than 2 inches thick. Ten or 12 feet is the usual length, although 18 or more is sometimes used. But the great amount of friction between such long plank and the earth makes it extremely difficult to drive the last 6 or 8 feet, the top of the plank being usually broomed or broken in the attempt. For the same reason the width of the plank does not usually exceed 6 or 8 inches. All the sheathing in a given course should be of approximately the same length. Sheathing-plank should be sharpened to a chisel edge, the flat side being placed against the bank, and the edge which will not be in contact with the plank last driven should be bevelled, that the plank may hug the bank and keep a close joint with the one previously

driven. The bevel may be 3 to 5 inches long. The top of the sheathing-plank should be bevelled on each edge, to lessen splitting and binding and to permit of using a drivingcap, which is advisable if the sheathing drives hard, to keep the plank from brooming.

For driving the sheathing a hardwood maul is ordinarily

FIG. 15.-DRIVING-CAP AND MAUL.

used, about 6 inches in diameter and 15 inches long, with a wrought-iron hoop banding each end.

If a large amount of sheathing is to be driven in deep trenches a steam-hammer pile-driver may be used to advantage. This does not broom the pile, and by using it sheathing 18 feet long or more may be driven. It is particularly applicable to sand and elastic soils.

If the ground is such as to require sheathing from the

very beginning of the excavation it would be difficult to keep vertical sheathing standing and in line while the trench is only 1 to 3 feet deep, and it would greatly interfere with casting

out the excavated material. It will be better in such a case to erect skeleton sheathing, with only one set of rangers and braces and short uprights, behind the uprights placing plank laid horizontally. When this construction has been carried down 5 or 6 feet vertical sheathing can be started and continued as above. But even then if the vertical sheathing is more than 8 feet long it will be necessary to use platforms or staging, unless a sheathing-plank can be omitted every 5 or 6 feet and the earth cast out through the opening thus left. On account of the difficulties just described it is better, if the trench is so deep as to require more than one course of sheathing, to place shorter sheathing in the top course for instance, 6-foot sheathing and then 12-foot in a 15- to 18-foot trench.

Some contractors use horizontal sheathing altogether, the verticals being only 3 or 4 feet long, several being placed one above the other. Most American contractors, however, prefer the vertical sheathing.

The size of the rangers may vary between wide limits, but in any one trench they should all be the same length, and when in position the ends of all should come opposite or under each other. Two-inch plank may be used for rangers in ordinary loamy or clayey soil and shallow trenches, and the braces placed with sufficient frequency to prevent their bellying too much. This would in many cases bring the braces so close to each other as to interfere with the work, and it will then be advisable to use 4 X 4 or 4 × 6 material. The author prefers these in any case, as being stronger, but neither costing nor weighing more, than 2-inch plank. If excavatingmachinery is used the braces should be at least 5 or 6 feet apart, and the rangers of 4 X 6 or 6 X 8 timber. The deeper the trench the heavier should be the rangers and braces.

The braces should be heavier also the wider the trench,