turned in it presenting two inclined shoulders; c is the valve-chest into which the compressed air enters from one of the sides; dis the main valve, and as it moves to and fro it alternately places the port e or fin communication with the exhaust g; e leads to the port h and to the front end, and f to the port i and to the rear end of the cylinder; j, the auxiliary valve, is a slide-valve made in the form of a segment of a circle, and having a recess in one of its flat faces. It is slightly longer than its arc shaped seat, so that one end of it always projects into the cylinder. The projecting end of the valve is caught by the corresponding shoulder of the piston as it passes, and it is thus being constantly knocked backwards and forwards. By means of its recess this segmental slide valve puts the ports k and I alternately into communication with the port M, which opens into the exhaust. The port k leads to the front end of the valve-chest, the port to the rear end; consequently the two ends are being alternately placed in communication with the exhaust. The compressed air leaking past the piston-like ends of the main valve escapes into the exhaust at one end of the valve-chest, but exerts a pressure at the other end where it is confined, and so throws the main valve over, changing the direction in which the air is being admitted into the cylinder. The piston makes its stroke, knocks over the auxiliary valve, which in its turn releases the pressure at one end of the main valve and causes it to move across once more.

The rotation is effected by a rifled bar, n, as usual; but instead of there being a ratchet-wheel fixed to this bar with pawls attached to the cylinder, the rifled bar carries the pawls which work inside

FIG. 208.

a ratchet-wheel, o, with internal teeth and a smooth exterior (Fig. 208). The pawls are pressed out by springs, p (Fig. 207). So far the action is very like that of other drills, save that the pawls move round inside the wheel, instead of the wheel moving round under the pawls. The special peculiarity of the Sergeant rotating device is the mobility of the wheel if the drill jams in a hole. The ratchet-wheel o lies loose in a recess behind the cylinder, and in ordinary working is pressed sufficiently firmly against the end of the cylinder by steel cushion springs to make the piston rotate without turning itself; but if for some reason the borer jams in the hole and causes a strain upon the rifled bar, the wheel is capable of turning and so preventing a breakage.

The feed as usual is by hand; 7 is the handle working the feedscrew in the feed-nut s.

(5) In the drills of this class the piston performs a double function; it not only acts as a medium for receiving the pressure of the air, but it also itself uncovers or closes the passages by which the air enters or escapes, and so causes a reversal of the stroke without the intervention of any separate valve.

The Adelaide drill (Fig. 209) comes first alphabetically, although

it was preceded in time by the Darlington drill, of which it may be regarded as a modification. 4 A represent the annular port, admitting the air all round the piston, and B, B, are ports in the piston-rod. When the latter are opposite A A, air passes down through the space C in the piston-rod to the rear end of the piston, and drives it forward till it uncovers the port B, which puts this part of the cylinder into communication with the atmosphere. At the same time B, B, have passed beyond the stuffing-box and part of the exhaust escapes in that direction; while this is happening the long shallow annular recess cut in the piston rod is brought to A, the air presses on the small annular space at the front end of the piston and drives it back. It will be noticed that this drill uses the air expansively, for when once B, has gone past A no further supply of power is taken in. D is the rifled bar, E the ratchet wheel, H the feed-screw, and G the feed-nut, similar to the corresponding parts of many other machines.

The construction of the Darlington drill will be understood by referring to Figs. 210, 211, and 212; a is the cylinder; b the piston-rod; c the borer; d d are two openings for bringing in compressed air, either of which may be used according to the position of the drill; e is the inlet hose with a stop-cock; f, drillholder; g, stretcher-bar; h, piston; j, rifled bar for turning piston and drill; k, ratchet wheel attached to rifled bar; l, rifled nut fixed in the piston head; m, wood for lessening weight of piston rod and blocking space; n, portway for allowing the compressed air to pass to the rear of the piston and give the blow; o, exhaust portway. The action of the drill is as follows:-The compressed air is always acting on the front end of the piston, and when the rear end communicates with the outer atmosphere, the piston moves rapidly backwards and uncovers the portway n. The compressed air rushes through and presses against the rear

end of the piston, which has a greater area than the front end, the difference being equal to the section of the piston-rod. The piston is driven rapidly forwards, and the drill strikes its blow. At the same time it uncovers the exhaust port o, and then the constant pressure on the annular area on the front end of the piston produces the return stroke. The number of blows per minute is from 600 to 800. The rotation of the drill is effected by the rifled bar. On the forward stroke of the piston, the bar with its ratchet-wheel is free to turn under a couple of pawls, and consequently the piston moves straight whilst the bar and ratchet-wheel turn. When the back stroke is being made,

the ratchet-wheel is held by the pawls and the piston is forced to make part of a revolution. As the hole is deepened the cylinder is advanced forwards by turning the handle p; this works an endless screw, q, passing through a nut attached to the cylinder; r is the cradle carrying the feed-screw and supporting the cylinder. It is centered on the clamp 8. As this clamp can be fixed in any position on the bar, and as the cradle can be turned on the clamp, it is evident that holes can be bored in any direction.

In driving a level with a Darlington drill, it is usual to fix the stretcher-bar horizontally so as to command the upper part of the face; holes can then be bored with the cradle above the bar or below it. The bar is then shifted low enough to bore the bottom holes. It is found that all the necessary holes can be bored from these two positions of the bar.

The bar, therefore, has to be fixed only twice; the shifting of the machine for boring holes in various directions is managed

by sliding or turning the clamp on the bar and by moving the cradle on the clamp.

Fig. 212 shows the stretcher-bar fixed in a vertical position, which is sometimes convenient.

In order to keep the holes clear, a jet of water, supplied from a hose attached to a 1-inch gas-pipe leading from a cistern at a higher level, is made to play into them during the process of boring.

For sinking shafts, Mr. Darlington has the drill fixed in a cylindrical case with a large external thread, which works in a nut on the clamp. The drill is fed forwards by turning a handwheel attached to the case.

The Marvin Drill* of the Edison General Electric Company is based upon the principle that a spiral coil of wire assumes magnetic properties when a current is passed through it, and becomes capable of exerting a very strong attraction upon a bar of iron placed in a suitable position.

FIG. 213.

The actual working parts of the drill are shown in Fig. 213. A and B are two hollow coils of copper wire (solenoids), through which passes the rod CD. The two ends are made of bronze, but the central portion, E F, is of iron. At the end C there is a socket for receiving the tool, whilst the end D is rifled and works in a ratchet-wheel, and so effects the rotation in the usual way. A current is led to the drill by a cable with three wires, shown separately by G, H, and I, and by means of a very simple revolving armature on the dynamo it can be made to pass, first through one solenoid, and then through the other, in each case returning by the wire H. For instance, we may suppose that the current is passing through the front solenoid; this becomes magnetic and draws the iron core forwards, and so causes the tool to strike a blow. The current is then reversed by the revolution of the armature, and flows into the solenoid B, which in its turn becomes magnetic and draws the iron back, for A has lost its magnetic power. The rear end of the rod C D is made to compress a spring, and so store up force which is utilised in increasing the strength of the forward blow.

The drill makes 600 strokes a minute, and is said to be capable of boring in granite at the rate of 2 inches a minute.

At the present time there are few, if any, electric percussive drills in regular use in mines, one objection to them being their

Electric Percussion Drills," Eng. Min. Jour., vol. li. (1891), p. 609.