to be worked in terraces practically require spuds. In working deepground in terraces from the top down or the bottom up, it is claimed that there is less danger of injury to the bucket line and cleaner work done if spuds are used. However, with lines a dredger can be handled more readily, and moved quicker than with spuds, so that under similar conditions of deep-ground working heavy caves can be avoided. It is also claimed that by means of the head-line tailing can be disposed of to greater advantage, and more evenly and that the sand. pump is unnecessary. There have been failures of both spuds and lines, but it is probable in working hard ground, that spuds hold the dredger to its work better than do lines, there being more resilience or give in the lines, which, however, relieves the buckets from much unnecessary strain.

Shovel dredgers are of necessity operated by spuds owing to ever changing center of mass, and must sacrifice quickness of movement to stability.

In the interior work where the dredger is to operate in an artificial excavation or in an artificial body of water there are two methods of procedure. When some distance from a stream the dredger is assembled in a slight excavation made sufficiently large to float the scow or boat. Water is pumped in or if possible, run through a ditch from some source of water supply. The excavation is then enlarged by operating the dredger, the tailing being conducted clear of the excavation by a long sluice-way. This process continues until the pit has been extended to or nearly to bed-rock, which is cleaned and as work is continued serves as dumping ground for the boulders and tailing. The method employed varies considerably with kind of dredger and manner of handling the same, but following the floating of the dredger and selection of a proper place for the disposal of the tailing there is little difference between it and ordinary river-working.

Interior work may be carried on in a valley which can be flooded by the construction of a dam across the stream traversing it. The dredger is assembled at the same time that the dam is being built, and when all has been made ready the dam is closed. Ultimately the dredger will float and begin operations, having the advantages of quiet water, and a means of controlling the water-level, and consequently the height of the dredger above the surface of the gravel and the bed-rock.

Tailing banks formed by suction and dipper or shovel dredgers Min. and Sci. Press, Vol. 91, p. 160.

occupy more space than with the bucket type owing to the manner in which they are placed in the bank. When the coarse material and boulders are put upon the bottom of the pit, the fine material being deposited later there is a tendency for a more or less thorough mixing during settlement; but when the process is reversed, the boulders being placed on top of the finer materials, the unfilled space between the boulders causes an increased height of the resulting bank, which in turn necessitates raising the dumping point of the tailing. Care should always be taken to keep a full boats' length between the face of the gravel bank and the tailing bank.1

When the channel in river dredging has a fair grade, say 25 feet or more to the mile, tailing can be readily disposed of by working upstream, but when this is impossible or inconvenient, considerable trouble may result. However, downstream work is not infrequently resorted to, and with moderately swift currents may cause no serious inconvenience for short distances. Occasionally brush dams are employed in impounding and controlling the débris, especially if there is a large proportion of sand present.2

When large quantities of sand are encountered in the gravel, occurring both above and below water-level, it may be taken from the sluices and discharged by means of a centrifugal pump behind the bank of tailing. This method of disposing of the sand has been found to be very satisfactory.3

The gravel on being excavated and elevated is discharged into a revolving screen, within and extending its whole length is a large perforated pipe, by which means and by other jets playing on the in- and out-side of the screen, the gravel is thoroughly washed, the finer material passing through the perforations while the larger gravel and boulders are discharged at the outer and lower end of the screen, and fall overboard. In some dredgers all undersize material passes directly into a tank or sump, from which it is raised by a centrifugal pump and delivered to the sluices. In this arrangement the upper end of the sluice-box rests on a turn-table, the base of which sets on the dredger. Connection is made between the pump and sluice-box by means of a short length of hose. The sluice-box is adjustable so that the tailing may be evenly distributed on the waste bank.1

1 Min. and Sci. Press, Vol. 83, p. 216; Ibid., Vol. 81, p. 582.

2 Min. and Sci. Press, Vol 80, p. 120.

Ibid., Vol. 82, p. 36.

Min. and Sci. Press, Vol. 83, p. 194.

Occasionally shaking screens are employed instead of the revolving forms; which should be used, is determined by the hardness of the gravel and the presence of clay.

Definite information regarding the condition of the gold contained in the gravels to be dredged should be obtained in order that the proper appliances may be chosen for its collection. The principal considerations are: First, shape of particles, whether in grains or flakes; second, size of grains or particles; third, presence of clay; fourth, presence of materials which interfere with amalgamation, such as arsenic, etc.; fifth, amount of platinum in sands; and sixth, character of gravel whether hard or soft.1

When tables are used on dredgers the gravel from the screens usually passes directly to them. These tables are arranged on either side of the screen, especially if it is of the revolving type, and are covered with cocoa matting alone or with cocoa matting and expanded metal. From the tables the gravel is run through sluices, placed longitudinally with the dredger, which are also provided with riffles, usually of the expanded metal forms. So-called "beehive" riffles are often used next to the screen, and consist of a nest of riffles in layers placed under the upper portion of the screen. In certain dredgers the sluices discharge into a sump from which the material is elevated and discharged overboard by a centrifugal pump, but a gravity disposal is more usual.

Experience shows that most of the gold is saved at the upper end of the tables, in some cases even the first six inches.

Hungarian or ordinary cross-riffles are also used with mercury on the tables, which in dredgers are made of iron to prevent warping.2

Tailing is handled by various contrivances which must, however, extend far enough over the stern of the boat to discharge it so that it will not interfere with the dredging operations. The tailing stacker, now commonly used, works very satisfactorily, and consists of an endless belt conveyor of some description, but preferably the rubber belt form.

The capacity of a dredger is regulated largely by its ability to save gold, and in order to insure this the tables should be as wide as possible and have frequent drops in their surfaces. It is claimed that the capacity of a dredger can be increased fully 6 per cent, by using

1 Min. and Sci. Press, Vol. 91, p. 160.

2 Min. and Sci. Press, Vol. 83, p. 229; Ibid., Vol. 90, p. 252, and Ibid., Vol. 91, p. 160.

closed-linked chains.

Further, by their use the load on the motor or engine is steadier, and the feed to the screen is more regular.1 Boulders exceeding 200 pounds in weight should not be allowed to enter the screen.

The range in capacity of buckets in endless chain dredgers is from three to five cubic feet or from 25,000 to 45,000 cubic yards per month. What was claimed to be the largest dredger in the United States as late as 1905 was that of the Folsom Development Company, at Folsom, California, which has a capacity of 125,000 to 140,000 cubic yards per month. The ordinary life of a well constructed boat should not be less than 12 to 15 years.2

The buckets on dredgers at Folsom, Oroville and the Yuba River vary in capacity from three to eight and one-half cubic feet, while a 13 cubic foot machine is contempated.3

The acreage of ground estimated to be available for dredging in California in 1905 was as follows:

California leads in number and extent of dredging operations, while the Oroville district, as indicated above, is of the most importance among the districts of the state. In April, 1905, there were 28 dredgers operating in the district, and at least four more were either being built or their construction contemplated. Beginning in 1899 electrically driven dredgers were introduced into the Oroville district, and since that time all boats previously steam operated have been changed and fitted with electrical equipment.

Electrically operated dredgers are now employed in the Boise Basin, Idaho; many of them having a capacity of 2500 cubic yards

1 Min. and Sci. Press, Vol. 83, p. 204.

* Min. and Sci. Press, Vol. 90, p. 252, and Ibid., Vol. 90, p. 283.

Ibid., Vol. 91, p. 160.

Min. and Sci. Press, Vol. 91, p. 125.