having a groove cut in it (Fig. 48), to allow the means for lighting the shot to lie against the side of the bore-hole. This may be either a needle, or pricker where straws are used, or fuse, or wires if electricity is the agent. The farther the tamping is from the charge, the harder it is stemmed. In strong rocks blows are given to the end of the rammer by a hammer.

Hand Machine Drills.-The general type of these consists of a screwed spindle working through a nut, with a socket for the boring tool at one end, having a square on it for the ratchet-handle which communicates power.

The tool commonly employed (Fig. 49) consists of a screw-spindle, a, working through the nut collar b. The boring-bit is of the ordinary auger form, with a V point. The screw is revolved and pressed gradually against the rock by turning the ratchet-handle c, small pieces are broken off, and the hole is bored. When the advance has reached the length of the drill, it is worked back into its sheath again, and a longer one inserted. With an ordinary nut arrangement,

Fig. 49.

as many revolutions have to be made with the screw to replace it in its sheath, as took place during boring. To prevent this waste of time, a split-nut, having lugs on each half tapped with right and left-hand threads, is adopted by the Hardy Pick Co. These lugs are connected by a screw, cut with a right-hand thread at one end and a left-hand thread at the other, and can therefore either be brought in contact with, or disengaged from, the main propelling screw. Consequently the drill and screw can be withdrawn without being wound back.

With this type of drill, a tree or prop has to be set near the face to support one end of the machine. To prevent loss of time, many machines are supplied with a stand, whose length is adjustable, as it is formed of two pieces which can slide upon each other, and be clamped together, the final adjustment being made by an ordinary lengthening screw at the bottom.

In the Elliot machine (Fig. 50) the nut is replaced by a wormwheel, a, in the teeth of which, a square-threaded feed-screw, c, of -inch pitch, takes its bearing. This wheel is carried in a ring, having a hinged joint at one side, and a screw clamp, b, on the other, so that more or less friction can be set up between the screw and the wormwheel. The feed is thus automatic, and the extent is regulated by the tightness, or otherwise, with which the ring is screwed up. If the resistance is excessive, the wheel slips round to a certain extent, and reduces the full advance of the drill, which may vary from inch per revolution to nothing.

If the clamping screw b is slacked, the drill can at once be withdrawn without being wound back.

When boring near the sides or roof, the crank handle cannot be completely rotated, but has to be worked back wards and forwards, and a ratchet employed, thus all the time devoted to one-half the motion

is lost. To remove this disadvantage the crank-handle is not connected directly to the screw, but through the intervention of bevel gearing. Bornét's* machine is so fitted, and, in addition, the nut in which the feed screw works is seated in a spring box, so that with an increase in pressure, when working in hard strata, the feed is equal to the pitch of the screw, less the amount of compression of the springs. When these are fully compressed the nut slips out of its bearings and revolves with the screw, the feed being then governed solely by the spring pressure until the resistance decreases and the nut again occupies its seat.

In thick seams ordinary stands cannot be employed. In the anthracite region of America they are replaced by a clamping device, shown in Fig. 51, attached to the Howellt drill, one of the best

known in that coalfield. To fix the machine a hole, 3 or 4 inches diameter, is first cut into the face, and the clamping-bar a, which is supplied with a number of spikes, is firmly wedged in it. The illustration to a great extent explains this. The auger bit is rotated by bevel wheels, geared down from 1: 3 to 6. A point worthy of attention is that two or three holes can be bored from one position, owing to the sector arm b allowing movement either to the right or left.

The merits of a drill depend upon its weight, the facility with which it may be set and used in different positions, and the wearing. capacity of the machine itself. The rate of boring depends entirely on the form of the cutting tool and the quality of the steel, because, unless the latter is suitable metal, it is no use making it of a suitable form, as that form is soon lost by rapid wear. A great deal also depends upon the men. Unless a certain amount of skill is shown in setting the machine, and properly clamping it in position, as much time is occupied in drilling holes in ordinary varieties of rock as if they were put in by hand.

The most suitable shape of the points of the twisted augers for drilling ordinary rock-binds and coal is shown in Figs. 52 and 53. *N.E.I., xxxvii., 117. + Report A. C. Second Geo. Survey Penn., 172.

This form penetrates with greater speed and less labour than any other pattern, and is easy to repair. The piece cut out of the centre should be a little more than one-third of the width of the point, and of a broad V shape, in order to keep the two outside portions as strong as possible. These should be kept as thick and stiff as the section of the steel will admit. The cutting point should be carefully kept sharp, with a good clearance left at the back. As a rule, the greater the opening in the middle the more rapid is the penetration, especially in coal, shale, and soft sandstone'; but the size of the V opening is governed by the hardness and strength of the rock to be bored. When great pressure is necessary the opening at the top of the V should be narrow.

The best results are obtained in tempering, by heating 1 inch of the points to a blood-red, and then plunging them into coal-tar, as the cutting edge is made extremely hard, the points gradually becoming softer as the thickness of steel increases. Drills so treated can be re-sharpened once or twice on a grindstone, until it becomes necessary to put them in the fire again to

enlarge the points.

To show the advantage of using these drills, the following may be cited:-At a colliery under the author's charge a road was driven for 61 yards, crossing the measures over a fault. The section of the road was 6 feet wide at the bottom, 5 feet at the top, 5 feet high, and it was driven at a down gradient of 2 inches Figs. 52 and 53. to the yard. Time occupied, 5 weeks; rate of progress, 12 yards per week. The cost -was: - Labour, £47 10s.; powder and fuse, £11 18s. 9d.; total, £59 88. 9d. The total cost per yard run was 19:488s., equal to 6·386s. per cubic yard; and the cost of explosives per yard run was 3'9148. The hardness of the measures varied considerably. A small portion could be worked with the pick, but other parts consisted of a hard, gritty sandstone, nearly too hard for the drill. Very little timbering was required, so this did not interfere much. Ventilating pipes and rails had to be laid. The road might be considered a very fair sample of a cross-cut in the Coal Measures. Similar work in another part of the pit, without the aid of a drill, cost £2 a lineal yard.

TRANSMISSION OF POWER.-In considering the question of transmitting power to the machines used in breaking ground, choice is limited to compressed air and electricity; the other means of steam, water, and wire ropes are inapplicable in the majority of cases. Steam is, to a certain extent, out of place in a mine, although, under certain exceptional conditions, it is employed, and gives good results; but its use in confined spaces, where either coal-cutting is in progress or rockdrills are being worked, is quite out of the question.

Compressed Air.-Air may be considered a perfect gas, and obeys the laws relating to such a body. These are:

(1) That if the temperature be kept constant, the volume varies inversely as the pressure; if, for example, the pressure is doubled, the volume will be reduced to one-half.

(2) If the volume be kept constant, pressure varies directly as the

temperature reckoned from the absolute zero (– 273° C. -459° F.). Thus double temperature gives double pressure. (3) If the pressure remains constant, the volume varies directly as the temperature reckoned from the absolute zero. Thus if

the temperature is doubled the volume is doubled.

If the above laws are clearly understood, it will be at once seen that great losses must occur in compressing air. When the volume in the cylinder is reduced by the piston, a considerable rise in tem perature takes place, which can only be produced by an expenditure of power, heat being simply work in another form. If the compressed air were used immediately at the point where it was generated, no loss would take place. This, however, is never done; the heat produced by compression is lost in the transmission pipes, and all the power which produced it is lost also.

The increase of temperature during compression expands the air in the cylinder and increases its pressure, so that the piston is met both by the natural resistance of the air to compression, and by the increased resistance due to expansion by heat. Another loss through this heating is that, at the moment of discharge the air bears the pressure it should do, but as it cools the pressure falls. It has been noted that, in an ordinary compressor, the air was compressed to four atmospheres after the piston had travelled three-fifths instead of three-fourths of its stroke (see first law above), the compressed air occupying two-fifths instead of one-fourth of the space in the cylinder.

A third loss is due to the fact that the sides of the cylinder become heated, and the air on entry is expanded, so that when the piston commences its stroke, a smaller mass or weight of air is in the cylinder, but the increase of pressure due to the temperature makes the pressure normal.

From these considerations it follows that, to secure good results, there should be (1) thorough cooling during compression, (2) the air on introduction should have as low an initial temperature as possible, and (3) the air should be raised to as small a pressure as is practicable in any single cylinder.

The losses, however, do not, unfortunately, end at the compression cylinder. The loss of head in the pipes is considerable unless the pressure is high, and consequently the user is apparently between two stools. The adoption of compound compressors, where the air is first raised to a pressure of about 25 to 30 lbs. in one cylinder, and then passed through a cooling receiver to a second cylinder for final compression to from 85 to 90 lbs. per square inch, reconciles the advantages of low compression as regards the original yield, and of high compression as regards the loss of head.

The advantages of successive compressions are not fully realised unless the action is repeated in an inverse direction at the motorthat is to say, the air must first be expanded in one cylinder, then passed through a warmed receiver in order to bring back the expanded air to its initial temperature, and finally expanded down to atmospheric pressure in a second cylinder. The difficulties of using compound motors with intermediate warming receivers are insur mountable in many of the operations in which compressed air is used in mines, but the difficulties can be overcome if, instead of endeavouring to compound each motor separately, they are com

pounded mutually. This arrangement has been carried out since 1888 at Newbattle Collieries in the following manner* :-The motors are not compound, but the air is conveyed from one engine to another engine, the first being the high-pressure cylinder, the second the lowpressure cylinder, and so on. The motors are coupled up in series, and do not necessarily perform any work in connection with one another. Thus the first installation comprised three pumps, the second one being 70 yards from the first, and the third one about 100 yards from the second. The exhaust air was carried in 6-inch diameter pipes from one motor to the other, and during its passage abstracted sufficient heat from the hot air of the mine to come back to atmospheric temperature before it reached either the second or third pump. A by-pass valve was so arranged that air may be passed directly by the first or second pumps without working them. In the case of hauling engines, where their stoppage might involve the stoppage of the plant behind them, relief valves are fixed to lift at a slight increase in the pressure, and the air then expands down to the next stage.

Air Compressors.-Two systems are in use by which the heat produced during compression is absorbed. In one, water is not admitted into the cylinder, while it is in the other. are called "dry" and the latter "wet" compressors.

The former

In dry compressors, air is cooled during compression by the use of a water-jacket on the compression cylinder, but at the best the action of this is very imperfect, as the area of surface exposed to the cooling action is small, compared with the volume of air compressed, so that only a small portion of the confined air can come into contact with the inner surface of the cylinder. In addition, air parts with its heat to a metal cylinder very slowly, and, with a compressor working at moderate speed, there is not time between the inlet and discharge to effect sufficient reduction in the temperature.

Wet compressors may be subdivided into two classes (a) those where the air is compressed by a piston of water, (b) where a fine spray of water is injected into the cylinder during compression.

The former type, the design of which is due to Sommeiller, is illustrated in Fig. 54. It consists of a piston, a, moving horizontally in a cast-iron cylinder kept full of water. From the extremities of this cylinder spring two vertical cylinders, bb, closed at their upper ends by covers bolted on. The in-take air is admitted through rectangular openings, cc, in the sides of the vertical cylinders, the suction-valve being of leather, while discharge takes place through a conical brass valve, d, situated in the top. The reciprocating movement of the piston causes the water to rise on one side and fall on the other. A partial vacuum is formed above the falling water, which causes the admission valve to open and the unoccupied spaces to be filled with air; while on the return stroke the water is driven back, and the air with it, until compression in the cylinder is equal to the pressure in the receiver, and then the delivery valve opens.

These compressors have been largely employed on the Continent, the idea being that, if the air during compression was in contact with water, all heat would be absorbed. Such, however, is not the case. The air is only exposed to water on one side; a thin film of this soon *So. Wales Inst., xvii., 226.