For digging narrow ditches and trenches, particularly for sewers, water mains, gas-pipe lines, etc., a number of very interesting machines have been invented which are of two general types, the endless chain and the wheel excavators. The first type consists of a traction engine with a ladder at the rear that drops down into the trench. About this ladder runs an endless chain which carries a series of scoops or buckets. These are dragged up against the breast of the work and carry the excavated earth to the top of the ladder where it is dumped upon a transverse belt conveyor and carried to one side of the trench. With the larger sizes of chain excavators, trenches may be dug to a depth of twenty feet and the width of the trench may be from two to six feet wide. The wheel type of trenching machine is similar in principle to the chain excavator. Instead of the chain of buckets, it has a wheel fitted with buckets. In one prominent type the wheel has no hub or spokes, but consists of a rim that is supported on and is turned by four friction rollers mounted in a rigid frame. This frame is mounted on a boom which can be raised or lowered as desired. Such machines can commonly dig to a depth of a dozen feet or so. A ditch differs from a trench in the fact that its side walls are sloping instead of vertical. A trench is usually a temporary excavation for the laying of pipes or conduits while the ditch is left open and serves for irrigation or drainage. There are chain ditching machines in which the chain of buckets runs laterally across the course of the ditch, dipping into the earth to form a V-shaped channel. In connection with the draining of the Everglades of Florida, a peculiar wheel type of ditcher was built. The wheel is of gigantic proportions, consisting of a series of radiating shovels or scoops shaped to conform to the outline of the ditch. As the wheel revolves it scoops up the mud which slides down toward the hub of the wheel and is carried to one side by a traveling conveyor. FLOATING EXCAVATORS