The principle of which the hydraulic miner takes advantage is the great specific gravity of gold as compared with water and rock. To illustrate this quality it may be noted that on a smooth surface inclined at an angle of 1 in 48, subjected to a heavy stream of water, 95 per cent. of the fine gold in gravel does not travel three feet. The loss of quicksilver fed into sluices will vary, even under good management, from 11 per cent. to 25 per cent. of the amount fed to the boxes. Hydraulic mines under favorable conditions are very paying investments. Gravel yielding 10 cents a cubic yard has been worked for 6 cents a cubic yard, at the rate of a million cubic yards a year. On another large claim 600,000 cubic yards were worked for 6 cents a cubic yard, yielding 13 cents a cubic yard. River dredging is another form of gold winning that has been brought to a great state of perfection in New Zealand. Although the dredge has not yet acquired the importance in America that was expected, it is successful on one or two western rivers, and as the subject becomes better understood it is conceivable that American mining engineers will be as successful in devising improved dredges as they have been in all other branches of their profession. In New Zealand the bucket dredge has proved more satisfactory than the suction dredge, although a hasty conclusion would probably give the latter the palm. At Bannack, Mont., the Bucyrus Company has several dredges in successful operation. One is 102 feet long, 36 feet wide, and draws 36 inches of water. It is very substantially made, and weighs nearly 700,000 pounds. Before such a dredge is launched, a dam is built across the gulch to impound sufficient water. As the gravel is dredged and washed, it is dumped astern of the dredge, which, in the case of a shallow creek, moves up to the excavation made by the buckets. The boilers of this dredge are double, and together have 250 H.P. There are 36 buckets, and each one has a horizontal drag of eight feet, a capacity of five cubic feet, and travels at the rate of fourteen feet a minute. After treatment by trommels, or revolving screens, coppers, and sluices, and finally by a centrifugal pump, the now almost valueless gravel goes overboard again, leaving behind 98 per cent. of the gold it once held. The traction dredge is really a land-mining machine, as it is adapted for work on land nearly flat, where but little water is obtainable. The machine travels on bogie tracks. A 50-H.P. boiler supplies the water. A boom, 40 feet long, carries a shovel of 1.5 cubic yards' capacity, and moves 70 cubic yards each hour. Mr. John W. Gray, one of the best authorities, has recently written to the Mining and Scientific Press of San Francisco a most interesting description of the progress made in saving the gold from the streams in New Zealand. He says, in part: