At all of the above-mentioned cases it seems to have worked out that it Was found advisable to deliver some stone at night. This is natural as it results in economy of time if the cableway can work regularly with loads originating at one point, i.e., no time is lost in traveling between a point where an empty skip has been left and a point where another load must be picked up. Thus in 100 round trips say with concrete from one originating point to the dam and return with an empty skip, there is involved hooking onto, hoisting, conveying, lowering and unhooking 100 loads and the same for 100 return empty skips; while if fifty loads of concrete alternate with fifty loads of stone there is involved, in addition to the above, 100 trips empty between the points of origin of the stone and concrete, or probably 25 per cent, more time required at best. The interference with the continuity or regularity of the operations of the attendants probably results in further loss.
Roughly comparing the above, we find that at Roosevelt four derricks in eight-hour shifts attained an output equal to six Cross River derricks in ten-hour shifts, or 50 per cent, more per hour. The Roosevelt cableways did a larger per cent, of their work at night, and not as much during the day as at Cross River. The Cross River and Olive Bridge cableways apparently made about the same performance. The Olive Bridge derricks, however, laid on an average 25 per cent, more masonry per hour than the Roosevelt derricks, and twice as much as the Cross River derricks.
The low Cross River derrick record for cyclopean masonry containing 33 per cent, stone is probably due to some peculiar condition attaching to that particular work as normally a less percentage of stone to set should operate to increase the yardage per derrick hour. It is probable that the capacity at Cross River was limited by the cableways. Regarding the time required by a derrick to handle stone and concrete, it may be said that usually more time is required to hook onto a stone than onto a skip of concrete. The stone must be held up by the derrick while being washed and occasionally while a loose fragment or a fin is being knocked off, some stone must be lifted and set a second time, particularly if they are being bedded in mortar; further, the average stone is undoubtedly smaller than the average skip of concrete, and so involves a larger number of operations for the same yardage. As illustrating how this may affect the yardage of a derrick, attention is invited to the following record on the Medina dam.
On the Medina dam the concrete was sent out in 2-cu. yd. bottom dump buckets and was handled by five derricks. The masonry contained only 10 per cent, of plums. During June, July, and August, 1912 (not including Sundays), in seventy-nine ten-hour days 107,550 cu. yd. of masonry were laid or 27.2 cu. yd. per derrick hour, with a best monthly record of 40,303 cu. yd. This is a remarkable performance. It is due undoubtedly to the small percentage of stone handled, and the consequent uniformity in the operations of the derrick; it also implies that the derricks were kept well supplied. The derricks were supplied partly by two cableways and partly by cars running on a double track laid along the toe of the dam. No figures are available as to the relative yardage handled by each, so no comparison can be made regarding the cableway performance.
At the Gatun locks of the Panama Canal were installed eight Lidgerwood cableways, mounted in pairs on four sets of traveling towers. The span was 800 ft.; the conveying distance when placing concrete in the lock walls was from 150 ft. to 500 ft.; and the vertical distance 40 ft. to 100 ft. below the origin of the load. "About 2900 cu. yd. of concrete have been placed in the locks in one day of twelve hours by the battery of eight (four duplex) cableways in addition to handling forms and ironwork for the day's work. Three duplex (six) cableways have placed 2700 cu. yd. in ten hours. One duplex (two) cableway has placed 64 cu. yd. in thirty-two minutes. Thirty complete round trips have been made in one hour with one cable-way." The concrete was handled in 2-cu. yd. dump buckets.