face of the bank two similar cross-drifts are also driven. From the ends and centre of each cross-drift two small "lifters," as they are called, are driven at right angles, extending respectively half way between the cross-drifts and the face of the bank. These places are then filled with powder, hard cement ground requiring from 450 to 500 kegs.

The heads of several of the kegs being removed, the main drift is tamped; and the powder is exploded by means of an electric battery or fuse.

In large blasts several cross-drifts may be required, and in such cases it is customary to fire the powder simultaneously in several different places by electricity. The quantity of powder used is determined by the position, character, and height of the bank, a sufficient quantity only being taken to shatter it.

In some places, with lighter material, two or three hundred kegs of powder will easily do the work that five or six hundred barely accomplishes in heavy cement. At Blue Point, a blast of 2000 kegs was exploded; at the Enterprise Mine, 250-feet banks, a blast of 1700 kegs was fired. The powder is of the ordinary blasting quality. For destroying large pieces of lava, pipe-clay, boulders, trunks and stumps of trees, giant powder cartridges are found very efficient.

It is customary in certain districts to wash off the top or lighter gravel, and subsequently blast the bottom cement. For this purpose shafts, fifteen to twenty feet deep, as may be demanded, are sunk, and a smalier chamber is excavated in the bottom of them. The chamber is charged with five or six kegs of powder, tamped, and then exploded by electricity. Undoubtedly there is a great waste of powder in bank blasting, and the subject is worthy of investigation with a view to future improvement in this particular.

In blasting, it is desirable to thoroughly shatter the material, i. e., to separate rock and cement, so as to facilitate its washing, thus insuring the earliest separation of the gold, by enabling the bulk of the precious metal to come in immediate contact with the quicksilver in the head of the sluices, and affording every opportunity for the most complete scouring and securing of the eroded gold particles.

The following method of bank blasting has been found to give excellent results with banks from 50 feet to 125 feet high, such as are generally encountered in hydraulic mining, and likewise in cement gravel of ordinary tenacity. In the absence of more definite knowledge on the subject, its adoption can be recommended.

The main drift should be run in a distance two-thirds the height

of the bank to be blasted. The cross-drifts from the end of the main drift should be driven parallel with the face of the bank, and their lengths determined by the extent of the ground which is to be blasted. A single T is all that is necessary. The amount of powder* required for charging the drift is from one-half to twothirds of a keg of powder, minimum quantity, per 1000 cubic feet of ground covered by the drifts-i. e., height of bank length of cross-drifts length of main drift = cubic contents. The quantity of powder used must depend on and vary with the position of the bank and the character of the gravel.

Late experiments made with the Judson powder, applied as above directed, have given good results, and, though not definitely determined, the indications at present are that the use of this new explosive will be a great saving in the cost of bank blasting.

The shattering effects of powder, used in the manner and proportion already described, have been roughly estimated from the appearance of the ground subsequently washed at from 225 to 230 cubic feet of ground shattered per pound of powder exploded.‡

Apropos of tamping, one of the attendant costs of bank blasting, it may be well to remark that, as yet, with the present explosives employed, all experience in bank blasting proves that, with a strong tamping, the best results are obtained. With 150, 250, and 350-feet banks a different method of blasting is adopted. The main drift in such cases is driven in from the face of the bank 45 to 50 feet in length. The cross-drifts are run parallel with the face of the bank, and their length determined by the ground to be moved.

In charging these drifts the amount of powder used should be sufficient to blow out the bottom ground (the line of least resistance), the bank then falling by its own weight. The firing § of all blasts is best done by electricity, and where dynamite exploders with platinum wires are used the "compound circuit" is most desirable.

The powder in boxes or kegs is piled up in rows in the drift; two wires, A A and D D, extend along the middle row, the tops of

* Ordinary black blasting powder, 25 lbs. per keg.

The quantity of powder is increased when the banks are strongly bound, or when the gravel is exceedingly tough.

Experiments made with blasts of 250 to 400 kegs powder.

A paper, titled On the Simultaneous Ignition of Thousands of Mines, by Julius H. Striedinger, read before the American Society of Civil Engineers, and published in its Transactions for June, 1877; also in London Engineering, August 17th and September 21st, 1877, contains much valuable information on the subject of simultaneous ignition of mines.

the boxes on which the wires rest being removed. The exploders, bbb, are inserted in giant powder cartridges, and placed on top of the paper covering the powder. (The Judson powder comes covered

with strong paper to exclude moisture.) The wires, A A and D D, are then connected with the wires, Y Y and ZZ' (as shown in sketch), which extend to the battery.

DERRICKS.

In working hydraulic claims, boulders are frequently encountered which cannot be moved by hand. To facilitate their removal a strong derrick is used. The bed-rock derrick now in use has a mast 100 feet high, and a boom 92 feet long. The whole is set in a cast iron box placed on sills. It is held in position by six guys of galvanized iron wire rope, 1 inches in diameter. A whip-block, with inch diameter steel rope, is used for the hoisting tackle. A 12-feet diameter hurdy-gurdy wheel is attached, and, using 30 inches of water, it lifts stones weighing 11 tons. The guys are held by double capstans.

The derrick is not taken down when moved. It can be readily moved one hundred feet in ten hours.

*

EXPERIMENT WITH THE HURDY-GURDY WHEEL, AT NORTH

BLOOMFIELD.

As the hurdy-gurdy wheel is the outgrowth of hydraulic mining, the following table showing its efficiency may be interesting.

*These experiments were made at the N. B. G. M. Co.'s Works, by H. Smith, Jr., C. E., and as they are the only experiments of the kind made with the hurdy-gurdy wheel, they are here given.

The wheel was 18 feet in diameter on outside, and 17 feet 4 inches in diameter to inner line of buckets (17 feet 8 inches in diameter at centre line of buckets). The buckets were 4 inches deep, with flanges on each side.

The work done was measured by a Prony dynamometer, carefully made.

The head given shows the real head in feet at the point of discharge; that is, the head due to a discharge from a pipe of infinite size.