=131. Cableway and Trestle Excavators.=—Cableway and trestle excavators are most suitable for deep trenches and crowded conditions. They should not be used for trenches much less than 8 feet in depth. They differ from the continuous bucket excavators in that the actual dislodgment of the material is done by pick and shovel, the excavated material being thrown by hand into the buckets of the machine. A machine of the Carson type is shown in Fig. 92. The machine consists of a series of demountable frames held together by cross braces and struts to form a semirigid structure. An I beam or channel extending the length of the machine is hung closely below the top of the struts. The lower flange of this beam serves as a track for the carriages which carry the buckets. All the carriages are attached to each other and to an endless cable leading to a drum on the engine. This cable serves to move the buckets along the trench. The buckets are attached to another cable which is wound around another drum on the engine and serves to lower or raise all the buckets at the same time. In operation there are always at least two buckets for each carriage, one in the trench being filled and the other on the machine being dumped. There should be a surplus of buckets to replace those needing repairs. The machines may be from 200 to 350 feet in length, and the number of buckets which can be lifted at one time varies from one to a dozen or more. On trenches over 5 to 6 feet in width a double line of buckets is sometimes used. The entire machine rests on rollers and straddles the trench. It is moved along the trench by its own power, either by gearing or chains attached to the wheels, or by a cable attached to a dead-man ahead. The Potter trench machine differs from the Carson in that only 2 buckets are used at a time and these are carried on a car which travels on a track on top of the trestle. The movement of the buckets and the car are controlled by 2 dump men who ride on the car and who can raise or lower the buckets independently.