of water is admitted at the inlet valve with each stroke, and is thrown into the receiver at each return stroke, thus circulating through the apparatus, and carrying off the heat given out by the air when com pressed. An old boiler is used as the receiver, and the compressed air is conveyed to the drilling machine in iron gas-piping.

LESCHOT'S ANNULAR DIAMOND DRILL.

The drills about to be described work upon an entirely different principle from those noticed in the preceding pages. The latter are all of the

class of percussion drills, and cut by the force of the blow concentrated at the point of the drill. The diamond drill, on the contrary, is not used percussively, but by virtue of its extreme hardness it is made to cut away the rock by contact and pressure. The drill is pressed firmly, and is rapidly rotated against the rock to be bored.

The application of rough black diamonds to boring and cutting into rocks was made in 1860, by Mr. Rodolphe Leschot, a civil engineer residing in Paris, and formerly a student in the École Centrale. He found by experimenting that such diamonds, firmly set in the end of an iron or steel tube or cylinder, could be made to bore holes in rock to great depths and with a rapidity before unknown.

A short section of a cylinder of soft steel is used for the purpose. It varies from one to three or more inches in diameter, according to the size of the hole to be drilled, and is only about one-quarter of an inch thick. Cavities are drilled in the end of the cylinder, and into these cavities black diamonds of the common or bort variety are firmly set, by hammering up the soft metal around them. They are allowed to project about half a millimetre in front and slightly beyond the outer and inner surface of the cylinder or ring. This ring is secured by means of a bayonet joint or screw thread upon the end of a long hollow rod or drill bar, to which a rapid rotation is given. By pressing or feeding this revolving diamond-mounted ring against the rock to be cut, and at the same time supplying it freely with cold water, the rock, whether of soft or hard materials, of clay, slate, or of flint, is rapidly worn away, and an annular cutting results, leaving a central core of rock, which passes into the center of the drill and drill holder, and may be broken out when the drill is withdrawn, thus leaving a truly cylindrical hole with smooth sides.

The rapidity with which this apparently delicate ring cuts its way into the hardest syenite, quartz, or granite is surprising. The first experiments showed that two men with a machine like an ordinary feed drill could bore 0.025 in depth per hour. The cylindrical core left in the center was 0.031 in diameter and the annular groove 0.043 in diameter; consequently the part cut out was equal to a cylinder 0.012 thick. But this method of feeding or advancing the drill was clearly very defective, no allowance being made for the varying hardness of the rock. Since that time very great improvements in the way of mounting, operating, and feeding the drills have been made.

The wood-cut represents the drill and its mounting, as manufactured by Messrs. Severance & Holt, the assignees of the patent for the United States. This form is known as the "testing or prospecting drill," and it is designed chiefly for testing the character and value of mineral deposits, although it is adapted to a variety of other work, such as drilling holes in quarries for blasting, and for well-boring.

It consists of a small, upright boiler, to one side of which is firmly bolted the castiron frame which supports the engine and swivel, drill-head, gears, and screw shaft, as shown in the engraving. The engine-an oscillator of from five to seven horse poweris shown at A. B is the screw shaft with drill passing through it. This shaft is made of hydraulic pipe from five to seven feet in length, with a coarse thread cut on the outside. This thread, a portion of which is shown in the cut, runs the entire length of the shaft, which also carries a spline by which it is feathered to its upper sleeve-gear. This gear is double, and connects by its lower teeth with the beveled driving-gear, and by its upper teeth with the release-gear (E.) This release-gear is feathered to the feed shaft, (F,)at the bottom of which is a frictional gear fitting the lower gear on the screw shaft, which has one or more teeth less than the frictional gear, whereby a differential feed is produced. This frictional gear is attached to bottom of feed shaft (F) by a friction nut, thus producing a combined differential and frictional feed which renders the drill perfectly sensitive to the character of the rock through which it is

passing, and maintains a uniform pressure upon the same. The severe and sudden strain upon the cutting points incidental to drilling through soft into hard rock with a positive feed is thus avoided. The drill proper (passing through the screw shaft B) consists of a tubular bar, made of lap-weld pipe, with a steel bit or boring-head (D) screwed on to one end. This bit is a steel thimble about four inches in length, having three rows of black diamonds in their natural rough state firmly imbedded therein, so that the edges of those in one row project forward from its face, while the edges of those in the other two rows project from the outer and inner peripheries respectively. The dia monds of the first-mentioned row cut the path of the drill in its forward progress, while those upon the outer and inner periphery of the tool enlarge the cavity around the same, and admit the free ingress and egress of the water as hereafter described. As the drill passes into the rock, cutting an annular channel, that portion of stone encircled by this channel is of course undisturbed, and passes up into the drill in the form of a solid cylinder. This core is drawn out with the drill in sections sometimes of from 8 to 10 feet in length.

The sides of the hollow bit are one-fourth of an inch thick, and the diamonds of the inner row project about one-eighth of an inch, so that the core or cylinder produced by a two-inch drill (the ordinary size for testing) is one and a quarter inches in diameter. Inside the bit (D) is placed a self-adjusting wedge which allows the core to pass up into the drill without hinderance, but which impinges upon and holds it fast when the action of the drill is reversed-thus breaking it off at the bottom and bringing it to the surface when the drill is withdrawn.

In order to withdraw the drill it is only necessary to throw out the release-gear (E) by sliding it up to the feed shaft, (F,) to which it is feathered, when the drill runs up with the same motion of the engine which carried it down, but with a velocity sixty times greater; that is, the speed with which the drill leaves the rock, bringing the core with it, is to the speed with which it penetrates it as sixty to one-the revolving velocity in both cases being the same.

The drill rod may be extended to any desirable length by simply adding fresh pieces of pipe. Common gas-pipe, or, better, lap-welded iron tube, is found to serve admirably for this purpose, the successive lengths being quickly coupled together by an inside coupling four inches long, with a hole through the center of each to admit the water. The drill is held firmly in its place by the chuck (G) at the bottom of the screw shaft.

The small steam pump (C C) is connected by rubber hose with any convenient stream or reservoir of water, and also with the outer end of the drill pipe by a similar hose having a swivel-joint, as shown in the picture. Through this hose a steady stream of water is forced by the pump into the drill from which it escapes between the diamond teeth at the bottom of the bit, (D,) and passes rapidly out of the hole at the surface of the rock, carrying away all the grit and borings produced by the drill. Where water is scarce or difficult of access, a spout is laid from the mouth of the hole to the tank or reservoir and a strainer attached to the connecting hose, so that the same water may be used over and over again with but little loss. This pump also supplies the boiler.

The same parties manufacture another style of prospecting drill, similar in its construction to that just described, but larger and more powerful. It has a horizontal tubular boiler 3 by 7 feet, with flues three inches in diameter, and steam capacity equal to twelve horsepower. The engines are two oscillators of 43-inch cylinder, five-inch stroke, and both attached to the same crank shaft. The whole is mounted upon wheels so as to be portable. It is geared to run with twice the speed of the first-described machine. Its construction and general appearance is shown by the cut. The pump P and water hose H fill the same offices as those in the upright machine. This pattern is mounted on large wheels with broad felloes for easy transportation in rough mining districts, and, like No. 1, is all complete in itself.

It is especially adapted to well boring, draining, and prospecting, and will bore holes from two (2) to five (5) inches in diameter, as desired, and to any required depth. The total weight is about 3,800 pounds.

An open cut or quarry drill is also made. It is similar in its construction to the last-described machine without the boiler, and has two oscillating engines, B B, of five horse-power each, and is geared three to one. This machine gives to its drill rod 900 to 1,000 revolutions per minute, and drills in ordinary rock at the rate of fifteen to twenty

Severance & Holt's Portable Prospecting Drill.

The speed of boring of course depends upon the character of the rock, ranging from five feet per hour in very hard rock to fifteen feet per hour in limestone, sand rock, and shale. The rate of speed in drilling appears to be limited only by the velocity with which it is possible to rotate the drill. It is claimed that with one thousand revolutions per minute the hardest rock may be drilled from eight to ten feet per hour, the diamonds cutting only the one-four-hundredth part of an inch at each revolution, and the drill thus advancing one inch for every four hundred revolutions. This is the finest or slowest "feed," and is used only in flint or rocks of greatest hardness; while in ordinary rock the drill is fed at the rate of three hundred revolutions to the inch-the diamonds, of course, cutting the one-three-hundredth part of an inch at each revolution; and in marble, sandstone, &c., at the rate of an inch for every two hundred revo lutions. The pressure of the bit or drill head against the rock does not depend upon the length and weight of the drill rod, and is no greater at the depth of a hundred feet than when entering the rock at the surface. The variations in the feed are effected by simply changing one gearwheel, requiring less than five minutes' time. The same machine will bore holes from one to four inches in diameter, as desired.

The diamond teeth are the only parts of the tool which come in contact with the rock, and their hardness is such that more than two thousand feet have been drilled by the same points with but little appreciable wear. The cost of resetting the diamonds so as to present new points is very slight, and no special skill is required for the operation.

Other repairs are seldom needed. The diamonds vary in price from six to seven dollars per carat, gold. Some of them are light-colored, translucent stones, nearly one-quarter of an inch in diameter. The usual diameter of bits for drilling blast holes in mines is one and a half inch, carrying six diamonds.

The peculiar shape of the boring bit prevents the drill from running out of line; hence the hole bored, however deep it may be, is perfectly straight and there is no friction of the drill against the rock.

The manufacturers have made another boring head of similar construction, but having the annular opening partially closed and the diamonds so arranged as to bore out the entire hole instead of producing the core. This form of bit, however, is not desirable, as it requires far more power to drive it, consumes more diamonds, and is not available