in the trench and covered with earth back-filling should not be relied upon. Rock may be piled in front of and over the log, but a better plan is to bury in the trench a platform of stout timber, inclined backward about 45° from the vertical, to which the cable is fastened. The hoisting- and conveying ropes are driven by an engine located at one end of the cable. Like derricks, the cable-way is not adapted to trenches which move forward rapidly, as the moving and resetting of it take considerable time and labor. In the trestle-machine the buckets travel upon an overhead track which is supported at intervals by trestles spanning the trench. Generally from 6 to 20 buckets are in use at once, one half of which are being filled while the remainder are being carried to the dump and emptied. In some machines the track forms a long loop, one side of which is for going and the other for returning buckets. There are then three sets of buckets, one going to the dump, one returning, and one set being filled. To obtain the greatest efficiency of the machine the number of men casting into each bucket should be just sufficient to fill it during the time occupied in removing, emptying, and returning a set of buckets. Such machinery is economical when the cost of runningincluding all labor but that of the men digging in the trench -and of repairs, plus the rental or interest on first cost of the machine, is less than the cost of "staging" it out (as the use of platforms is called) plus that of back-filling. If the backfilling is to be hand-tamped this last item should not be included, since if a machine is used the material must be spread after dumping. A good trench-machine is usually economical when either depth or breadth of trench exceeds 8 to 10 feet in ground capable of rapid trenching; but this economical least dimension increases with the decrease in the rapidity of excavation possible. Where for some reason the excavation can proceed but slowly the use of machinery is not advisable for economical, though it may be for other, reasons. Whatever the machinery employed it should work successfully although the sheathing extend at least 6 feet above the surface along each side of the trench, should be able to drop a bucket anywhere in the trench, each bucket being always under perfect control, and no cable or rope should hang within 6 feet of the ground. It is better that it should have no cross-ties or other parts extending across the trench within 6 feet of the ground, and that it should not obstruct the street for more than 2 feet on each side of the trench. For deep trenching through city streets the use of excavating-machinery is strongly recommended as of advantage to both city and contractor. Most makes of excavating-machinery can be either rented or bought. For a village or small city the former is generally preferable if the work on which it is to be used can be pushed. But if it will be needed for more than one season it may be preferable to buy instead. Probably the best-known and most extensively used trenching-machinery are the Carson, of Boston, Mass., and the Moore, of Buffalo, N. Y. Other machines are described in Engineering Record, vol. XVI, page 123, vol. XXXIII, page 100; and in Engineering News, vol. XXIV, page 268, vol. XXV, page 547, vol. XXXVII, page 50. ART. 72. SHEATHING. Just when a trench can be relied upon to stand without sheathing and when it cannot is something that only experience can teach. Sheathing is expensive, but not so expensive as excavating a trench which has begun to cave, to say nothing of settling for injuries and death of laborers. If earth has been piled upon a bank which afterwards caves it may be necessary to re-excavate more material than all that which would have been excavated had no caving occurred, and all of this must be removed to some distance because there is no bank upon which to pile it. Not only that, but the soil is liable to continue to slide into the trench, making it almost impossible to keep the bottom uncovered. If, after caving has begun, sheathing is used the difficulty of placing it is greatly increased. A trench which if sheathed would have given no trouble may become a most discouraging hole into which many times the cost of sheathing must be placed in the form of labor before the sewer is built therein. The author's experience has been that it does not pay to take many chances with unsheathed trenches. He would use at least skeleton sheathing in every trench more than 8 or 10 feet deep, in any trench in gravelly and sandy soil, and whenever the least sign of caving appears. Wherever the street is paved a plank should be placed horizontally on each side of the trench about 6 inches below the surface, and braces driven between these not more than 6 feet apart. Sheathing is usually placed as follows: A plank (a, Fig. 13) is placed upright in the trench against. the bank, another (b) 12 feet from this, and two against the other bank and directly opposite these. Against each two and near the street-surface is placed a horizontal ranger (cd and ef), both at the same level, and between them at each end a brace is driven, long enough to be a tight fit. Two other rangers (gh and kl) are placed, one on each side of the trench, from 4 to 6 feet below the others, and braced. Sometimes these lower rangers are placed first. The ends of the rangers come in the middle of the uprights, the braces only an inch or two from the ends of the rangers. The next set of rangers abut against these and are braced in the same way. Generally an additional upright and braces are placed midway of each ranger. This forms skeleton sheathing. If the sheathing is to be close, plank are slipped behind the rangers and in contact with each other, and one or more additional braces are placed at equal intervals between each tier of rangers. For bracing only, the rangers and braces are used without any vertical sheathing. These are ordinarily placed a foot or two from the surface, or just beneath and in front of an exposed water-main or other conduit. When a series of rangers and braces are placed one just below the other horizontal sheathing is formed. As the trench is deepened the sheathing should be driven so that its lower end is as near as possible to the bottom of the trench, unless rock or some firm soil be previously reached. In quicksand or running sand the bottom of the sheathing should always be kept at least one foot below the bottom of the excavation. This is essential if the work is to be done without considerable loss of money and perhaps of life. As many men as are necessary to insure this should be kept constantly at work driving the sheathing. No two planks behind the same ranger should be driven at once, as the latter would in that case be apt to follow them down, which it should not do. If there is a tendency for the sheathing to be forced in at |