=134. Drag Line and Bucket Excavators.=—A drag line excavator is shown in Fig. 94. The back of the bucket is attached to a drum on the engine by means of a cable passing over the wheel in the end of the long boom. The front of the bucket is attached by another cable directly to another drum on the engine. In operation the bucket is raised by its rear end and dropped out to the extremity of the boom. It is then dragged over the ground towards the machine, digging itself in at the same time. When filled the bucket is raised by tightening up on the two cables, swung to one side by means of the movable boom, and dumped. [Illustration: FIG. 94.—Drag Line at Work on Trench for Drain Tile. ] Drag line excavators will perform as much work as steam shovels under favorable conditions. They are less expensive in first cost and operation, and are equally reliable but they are not adapted to the more difficult situations where steam shovels can be used to advantage. Drag lines are suitable only for relatively wide trenches in material requiring no bracing, and in a locality where relatively long stretches of trench can be opened at one time. The bucket excavator differs from the drag line in that the bucket can be lifted vertically only and the types of buckets used in the two types of machine are different. The bucket may be self filling of the orange-peel or clam-shell type, or a cylindrical container which must be filled by hand. A drag line can be easily converted into a boom and bucket excavator. Boom and bucket excavators are well adapted to use in deep, closely braced trenches and shafts. =135. Excavation in Quicksand.=[85]—A sand or other granular material in which there is sufficient upward flow of ground water to lift it, is known as quicksand. Its most important property, from the viewpoint of sewer construction, is its inability to support any weight unless the sand is so confined as to prevent flowing of the sand, or unless the water is removed from the sand. Excavation in quicksand is troublesome and expensive and is frequently dangerous. The material will flow sluggishly as a liquid, it cannot be pumped easily, and its excavation causes the sides of the trench to fall in or the bottom to rise. The foundations of nearby structures may be undermined, causing collapse and serious damage. These conditions may arise even after the backfilling has been placed unless proper care has been taken. The greatest safeguard against such dangers is not only to exercise care in the backfilling to see that it is compactly tamped and placed, but to leave all sheeting in position after the completion of the work.