blows, the other blows being delivered by a second man using a heavy hammer or sledge. The usual, and at the same time most satisfactory method, is for three men to do the work. One man takes up a sitting position and holds the drill vertically between his knees, keeping it in proper position, and turning it partly round, while the other two men strike the head of the boring-bar with sledges, lightly at first, and increasing the force of the blow after the bit has fairly entered the rock. The degree of turn necessary to be given in drilling is one-eighth of a revolution, which prevents the cutting edge falling twice in the same place; by this means the greatest amount of work is accomplished, and a circular form of bore-hole is ensured. By using water in the bore-hole the drilling is greatly facilitated, the time being reduced by one-third; it also keeps the cutting edge cool, and converts the dust and chippings in the bottom of the hole into sludge, which must be cleaned out by means of a 'scraper' from time to time as it accumulates. In very hard rock a drill may have to be changed frequently, which necessitates the use of several of about the same length; and these should always be in good order and near at hand. The depth to which the bore-holes are drilled by the double-hand method is generally from 5 to 6 feet, sometimes 8 feet; for greater depths up to 10 feet churndrilling is resorted to when the nature of the rock admits of this being done. 163. Churn-drilling.-The drilling-bar or steel for this method of boring has a cutting edge of different width at either end of the stock, the blow given to the rock being obtained by the direct impact of the falling drill, which is raised by two men to a height of about one foot, and then allowed to drop. In long boring-tools a sufficient weight for the purpose is generally

FIG. 50.-Long boring-tool.

contained in the steel stock, but additional weight may be obtained if necessary by having a bead or swelling fixed on the stock between, but at unequal distances from, the extremities, as shown in fig. 50.

164. The water which is poured into the bore-hole to facilitate the penetration of the bit into the rock is prevented from splashing about, particularly in shallow holes, by placing a perforated leather or india-rubber washer on the shank of the drill at the surface level of the rock. This object may also be accomplished by winding straw round the steel, which should be fixed as in the case of the washer.

The sludge formed during the process of drilling must be removed from time to time as the depth of the bore-hole advances, since the oftener this is done the greater will be the amount of drilling performed in a given time. The rock being exposed to the action of the cutting edge of the tool and the full effect of the blows transmitted to it by the jumper, will not be cushioned, as

it would be in the case where the sludge is lying between the bottom of the hole and the edge of the boring-tool. The cleaning out of the bore-holes, when not too deep, is effected by a simple tool called a scraper, which consists of a rod of iron varying from to inch in thickness, and of a sufficient length to reach the bottom of the holes. It is provided with a disc attached at each end of the rod of different diameter, so as to make it suitable for the various sizes of holes bored. Instead of having discs at either end, one of these is sometimes formed with a spiral hook or drag-twist, which is very useful, as, by attaching a wisp of hay to it, the bore-hole can be thoroughly wiped dry previous to charging.

165. In the event of the shank of a drilling-tool or jumper being broken. by some cause or other while boring operations are in progress, the portion left in the bore-hole may be recovered by various means. The simplest way of accomplishing this is to guide a running noose of cord by the aid of a second drill on to the broken one, and pull the noose tight, when the portion of the drilling-steel left in the bore-hole can be drawn out. Tongs having an arrangement attached for tightening the legs so as to grip the steel firmly are also sometimes employed for this purpose; while another instrument called a beche, made of iron, and having a hole at the lower end of a slightly conical shape, which fits on to the top of the broken steel, is at times made use of for the same purpose.

166. If, as often happens in many quarries from which road material is taken, water finds its way through fissures into the bore-hole, it becomes necessary to line it with clay when the explosive agent employed is blasting powder.

This is performed by putting the clay into the bore-hole in a plastic state, inserting an iron tool slightly less in diameter than the hole, and pointed at one end, called a brill, and by repeated blows forcing the clay into the joints and crevices of the rock. This instrument, which at one time formed an important part in a set of blasting gear,' has now become of less importance by the introduction of waterproof cartridges containing gunpowder, and the nitro-glycerine and gelatinous compounds, which are now much used in blasting operations.

167. Amount of Work accomplished by Hand-boring or Manual Labour. The amount of drilling or the depth of holes which can be accomplished in one day will greatly depend on the hardness of the rock and the size of the drilling-tool used. Joints and fissures when lying at an angle to the direction of the bore-hole give considerable trouble, and much time is consequently lost, reducing the amount of work below what could otherwise be accomplished under more favourable circumstances. For this reason it is advisable to select the best possible position for the bore-hole, while at the same time due consideration must be given to its position and direction in relation to the portion of rock which is to be dislodged from the parent mass.

The amount of drilling which can be accomplished by hand, in the different classes of rock used for road purposes, varies considerably according to circumstances.

The method of calculating and stating the actual amount of drilling performed will be given in cylindrical inches, which is particularly applicable to open quarry work. This system of computing the amount of work done will also be adopted in determining that performed by machine rock drills, which will be described later on. The cylindrical inch has a relative value, and the use of the term is preferable when making a comparison of the amount of work done and the cost of the operation, than by simply stating the number of inches or feet in depth drilled each day. Under the latter circumstances no comparison is possible, as the width of the bit or size of the bore hole may vary considerably according to the depth drilled, while the length of the working day is at times a variable factor in quarrying operations.

Therefore, by adopting this numerical figure as a standard for the amount of work performed in a given time, a comparison can be more readily arrived at between drilling by manual labour on the one hand, and that accomplished by a machine rock drill on the other, while the drilling capacity of the different types of the latter can also, by this means, be easily ascertained.

In Table XVIII. the average amount of work performed by hand drilling is stated in accordance with the preceding particulars. As the length of a day, however, is a somewhat vague definition of time, more especially when the working hours are interfered with by stress of weather, or shortened during the winter months, the hour is adopted from which a day's work of any specified number of hours can be calculated. The figures are taken from the results of actual experience under varying conditions, time being allowed for cleaning out holes and resting.

TABLE XVIII.

Rock Drilling by hand: one man manipulating the drill, two men
striking with sledges. Bore holes vertical downwards.

The depth of a bore-hole in inches, drilled in an hour's time, can be found by dividing the figures given in column No. 1 by the diameter in inches of the drilling-bit used. For horizontal or breast-holes the amount stated in Table XVIII. will be less by about 30 per cent. It should also be mentioned that the figures in the table are calculated for bits capable of drilling a bore-hole of an average diameter of 1 inch; for shallow bore-holes of a less diameter, and which are at times required, the amount will be slightly in excess of the figures given.

168. Cost of Hand-boring.-To arrive at the cost of drilling per foot of depth with drills of varying diameters the cost of labour and incidental expenses require to be determined. Taking the average wages at 3s. 8d. for each man per day of nine hours, and allowing 2d. per foot of depth for sharpening the bits on the harder rocks, loss of weight in steel, repairs to hammers and sledges, etc., amounting to say 1s., 12s. will be the total cost for each day's work of one squad of drillers.

The cost of drilling one foot in depth with bits of different sizes, taking 12s. as the actual cost per day of nine hours, or rather over 1s. 4d. per hour, is shown in the following table, which may be taken as a broad average.

TABLE XIX.

Comparative Statement of the cost of drilling by hand per foot
in depth with drilling-bits of varying sizes.

Should the total wages per day be more or less than that mentioned, then the figures in the table must be correspondingly increased or reduced, on the assumption that the same amount of work is performed.

When the nature of the rock permits of the bore-holes being put in by means of a churn-drill, the cost per foot, after the first 18 or 24 inches, can, by this method of drilling, be accomplished for about 20 to 25 per cent. less than the cost per foot stated in the table above.

169. The remarkable advance made in recent years in the construction of mechanical rock drills is such that the practice of hand-boring, the only method of former days, has been to a great extent superseded.

The practical advantages accruing from the use of rock-drilling machines in quarrying operations are decidedly great, while the rate of progress made possible by their use has been the means of reducing the cost of production, and greatly extending work in connection with the stone industry.

This applies very particularly to the quarrying of material for macadamizing purposes; and the substitution of mechanical power for hand labour in accomplishing this has been very marked in recent years, while the economy effected by its use enables the quantity of material for repairs amounting to double that possible by hand-boring to be procured for the same expenditure. In other words, the economy effected, on any quantity of material quarried, is 50 per cent. by machine over what can be accomplished by hand-drilling and the attendant expenses of blasting.

Machine rock drills are applied in a variety of ways, according to the work required of them; the arrangement with a tripod for its support will alone be described, as it is the only form employed in boring rock from which the material for making and repairing roads and streets is obtained.

Machine rock drills are actuated in the same way as the drills used in hand drilling, namely, by a succession of blows given to a chisel-pointed tool which is partially turned round between each blow as already described, and known as percussive boring.

170. Hand-power Drilling Machines. -Hand-power drilling machines have been employed to a limited extent; they are operated by two or three men turning a handle and flywheel conveying to the mechanism the necessary power to cause the drilling-bit to strike two or three blows on the rock for each turn of the handle. In this class of machine the force of the blow is limited, while a considerable amount of power is absorbed by the machine itself through the friction of its working parts. As a machine does not create power, a hand-power boring machine must be lacking in efficiency and economy. Although machines of this particular class have not attained a high reputation, still they may be regarded as the first step in the transition from hand to machine drilling. There can be no doubt that the Sommeiller drilling machine used for piercing the rock during the construction of the Mont Cenis Tunnel is the prototype of the rock drills now in use.

171. Different Types and Requirements of a Rock-drilling Machine. -The numerous types of machine rock drills in use at the present time vary in many particulars, but chiefly in the details of construction rather than in the principle on which they work. Durability is of great importance in a rock drill; any defect in the principle of the mechanism or construction of the parts subjected to considerable strain materially affects the working expenses. The weight should be reduced where not necessary for efficiency and the fitting of the working parts, an important element which may, if not properly designed and manufactured, involve the outlay of considerable expenditure for repairs. Great strength in certain parts of rock drills.