The organization needed to operate these machines is: a lockman who locks and unlocks the buckets on the cable, a dumper, as many shovelers as there are buckets on the machine, and an engineman who is usually his own fireman. From 50 to 400 cubic yards of material can be excavated in a day with one of these machines, dependent on the character of the material and the depth of the trench. H. P. Gillette in his Handbook of Cost Data reports that about 190 cubic yards were excavated per day with a Potter machine. The machine was 370 feet long. Six ¾-yard buckets were used, 4 in the trench and 2 on the carrier. The trench was 10½ feet wide and 18 feet deep in wet sand and soft blue clay. The organization consisted of an engineman, a fireman, 2 dumpmen on the carrier, and from 17 to 21 excavating laborers depending on the kind and the amount of the excavation. In general the capacity of such machines is limited by the amount of material which can be shoveled into them by hand. =132. Tower Cableways.=—These are essentially of the same class as the trestle cableway machines. They differ in that the carriage supporting the buckets travels on a cable suspended between 2 towers instead of on a track supported on a trestle. As a rule only one bucket is handled in the machine at a time. They are used in sewer work only in exceptional cases as the towers must be taken down and re-erected each time that there is an advance in the trench greater than the distance between the towers. =133. Steam Shovels.=—The use of steam shovels for the excavation of sewer trenches is becoming more prevalent because of their growing dependability and durability as compared with other machines, their adaptability for small trenches, and the relatively large number of widely different uses to which they can be put. In excavating a trench the shovel straddles the trench and runs on tractors, wheels, or rollers on either side of it. The shovel cuts the trench ahead of it. As a result it is difficult to set sheeting and bracing close to the end of the trench while the shovel is operating. Steam shovels are therefore not suitable for excavation in unstable material, unless the sheeting is driven ahead of the excavation. It is only in the softest ground that ordinary wood sheeting can be driven ahead of the excavation. Steel sheet piling is more suitable for such use. Fig. 93[84] shows a shovel at work on a trench in Evanston, Illinois. Shovels are equipped with extra long dipper handles to adapt them to trench excavation. The dipper handle in the picture is longer than the standard for this type of machine. The method of supporting the shovel can be seen in the picture under the machine and the method of bracing and of finishing the trench by hand work are also shown. The excavated material is taken out in the shovel and dropped on the bank or into wagons.