tral division. The drilling operations of this division will be taken as representative of all the dry rock work. An approximation, based on over fifty million yards, would indicate that the rock from this division would classify by volume as consisting of eighty per cent. of soft rock and twenty per cent. of hard rock. Reference to the general geological formations already described will make clear the distinctions of hard and soft rock.

The average number of drills employed when the whole division was in active operating condition was, well or drop drills, 153; tripod percussion drills, 231.

Based on the drilling of 6,412,000 linear feet of blast holes bored for blasting 12,863,000 cubic yards of material, the performance of the well drill averaged six and one-quarter feet in depth of hole per hour, with an average labor operating cost of 6.68 cents per linear foot. The tripod drills averaged, including the drilling of toe holes and working in the hardest rock, five and one-eighth feet per drill hour, at an operating labor cost of 8.96 cents per linear foot. In drilling the hardest trap rock in Culebra Cut, based on cutting 6,297 linear feet, the average cutting speed of tripod drills was 2.47 linear feet per drill hour.

An average of 100 horizontal toe holes fifteen feet in depth were drilled each working day. Six hundred vertical holes nineteen feet deep were bored in the same time. These records of performance are of a period when the organization and working conditions had been developed to a condition productive of high economic and operative efficiency.

The variable nature of the materials drilled was particularly adapted, in a great portion of the excavation, to the use of well drills, owing to the mechanical design and operation of this type of machine being effective in boring holes through an overlay of earth, and through the seams containing loose materials, sand and gravel, so frequently encountered. These conditions and the proportionate depths of rock and

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earth are exemplified in the record of operation of twelve well drills on the Pacific division. During twelve months 86,827 linear feet were drilled; of this footage, 50,889 feet were through the overlay of earth, and 35,938 feet through a friable rock,-a relative proportion of about ten of earth to seven of rock. The average performance of each drill was about twentythree linear feet of hole per day. A pipe casing was used in the drilled hole to prevent the overlay of earth from caving into the hole.

From the inception of the canal work in 1904, until the end of 1912, 725 rock drills of the different types had been purchased for use on the canal work at a cost of $288,376. This number of drills approximately served to complete the canal, as operations were at the height of activity and the maximum quantity of plant in use in 1912. From this period the quarrying and other rock sections were ceasing operations owing to completion of work; this led to the reduction and retirement of drilling plant.

The crushed stone used in the concrete construction of the locks and other structures at Gatun was produced at the Porto Bello quarries on the Atlantic side of the isthmus. The crushed stone used in similar structures in the Pacific division, as the locks at Pedro Miguel and Miraflores, was produced from the quarry at Ancon Hill.

The cost of quarrying, covering the production of over 1,300,000 cubic yards, averaged 95.57 cents per cubic yard at the Porto Bello quarries, and the unit-cost of drilling was 4.4 cents.

In the production of about 1,700,000 cubic yards from the Ancon quarry, the average cost of quarrying was 53.85 cents per cubic yard, of which five cents was the unit-cost of drilling. Both quarries were of igneous formation of basalt or trap. Percussion tripod drills of three and fiveeighth-inch piston diameter were in quite general use at Porto Bello; at Ancon, in addition to the tripod drills, a number of well drills were in use. The well drill,

because of its slower and less powerful blow and the larger area of holes drilled, penetrates these verv close and hard rocks but slowly.

Subaqueous rock drilling was performed in the Atlantic entrance of the canal and in the Pacific entrance. The tidal conditions of the Atlantic give a range between rise and fall of about fourteen inches; on the Pacific side the extreme range is about twenty-four feet.

The rock formation on the Atlantic side is of soft, easily worked coral formation. On the Pacific side the rock is of volcanic origin, of fine texture and of medium hardness. The nature of the coral deposits to be Idrilled and the most favorable tidal conditions permitted of very economical results being obtained, on the Atlantic side, with an improvised drilling plant.

An old hull, used on the French works, was fitted with eight old well drills and used as a drill barge. The individual boiler was removed and steam for actuating the drills was supplied from a central boiler. These drills were placed four on each side of the barge. The two lines of drills were twenty-two feet apart, and the drills in each line fifteen feet apart. The total cost of this improvised installation was $4,000. An average of eight holes was drilled, loaded and fired each day.

In mining 174,580 cubic yards, of which 83,800 cubic yards was an overlay of earth, the cost was 4.5 cents per cubic yard, or if all expense is charged to the rock, the cost would be 8.7 cents per cubic yard. The cost per cubic yard of 191,872 yards of subaqueous rock without an overlay of earth was 7.9 cents.

The subaqueous drilling plant in the Pacific entrance of the canal followed in general design and process the standard American system as carried out on the Great Lakes and the St. Lawrence River. There were minor modifications in the spud mechanism to meet the tidal variation in level.

The greater physical difficulties to be met with in the removal of the under-water rock on the Pacific side required a more

elaborate and substantial plant than was needed on the Atlantic side. These physical features were the great range in tidal levels, the harder rock formations, and the larger volume to be removed.

The hull of the drill barge was of steel, 112 feet long and 36 feet 8-inch beam. Two longitudinal and six transverse bulkheads divided the hull into twentyone watertight compartments. Two of these compartments amidships were utilized for water storage and other of the compartments contained six fuel oil tanks, each of forty-barrel capacity. Four timber spuds, twenty-four inches square, were located towards the corners of the hull. These spuds were each controlled and lifted by an independent pair of engines connected by gears with steel racking on the spuds. The function of these engines had a controlling influence on the performance of the plant. The engines were under steam pressure which exerted a downward thrust on the spuds or lifting effect on the hull while the drills were working. This thrust was of a force sufficient to keep the vessel above her line of normal flotation.

The proportion of the weight of the vessel on the spuds served to keep the vessel anchored or fixed in position over the holes being drilled as the water level changed with the stage of the tide. The mechanical principle involved is the condition of equilibrium established when the weight or downward pressure on the spuds equals the resistance of the steam pressure. The rise or fall of the water in which the vessel floats disturbs this equilibrium, with the resultant effect that the engines automatically reëstablish this equilibrium concurrently with the increase or decrease of load on the engines, the engines are overhauled against the steam pressure, or because of the steam pressure force the spuds downward.

Three drill frames or towers carrying percussion drills were in line over the gunwale on one side of the vessel. The frames were supported on tracks and were movable in the direction of the length of the vessel by

connection with an endless chain operated by hydraulic power. The frames had a travel of eighty-five feet. The drill frames or towers, about forty feet in height, were provided with guides, on the outboard side, in which were operated sliding saddles or crossheads to which the percussion drills were bolted. The length of these slides, which approximated the height of the towers, was the length or range (less the length of the crosshead) of the feed or travel of the rock drill. This long range was of advantage, in this design, in providing for feeding the drill according to the depth of the rock cutting, and the changing positions of the drill as affected by the stage or level of the tide. The crosshead to which the drills were bolted was of sufficient weight to resist the lifting force exerted by the drill when striking. The up and down movement or feed of this crosshead was controlled by a steam-operated hoisting winch located on the base of each tower and connected by a steel hoisting cable running up over the tower and down to the crosshead.

The drills, three in number, were of five and one-half-inch piston diameter, steam actuated, and of the Ingersoll-Rand type of submarine drill. The drills in their action and design were similar to the familiar form of percussion rock drill as mounted on tripods, the main differences being in greater power, weight, and corresponding structural increase in dimensions and strength. Drill steels or bars to sixty-five feet in length were used. Steam was supplied to the drills and hoists from a central boiler through swivel and slip pointed pipes. The vessel was manoeuvered on four Manila cables attached to kedges. The average performance of the plant, taken from 24 months of continuous operation, was as follows:

During this time the drills worked 22,854 hours (i.e., the total of hours in work on the separate drills-drill hours) and drilled 286,005 feet of hole of an average depth of 16.5 feet each. The drills averaged thirteen feet of hole per drill per hour. The

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maximum over a period of one month was 21.64 feet per drill hour, and the minimum over a like period 5.9 feet per drill hour. The unit average cost, over a period of twentyfour months of operating, drilling and blasting, was 39.62 cents per cubic yard.

In forming the Pacific entrance two methods of drilling the under-water rock were employed, by the drill barge as described and by the well drill. At places where rock below the surface required to be drilled, there was an overlay of earth that came above the water level. Well drills were used in these situations. The rock was the same as that drilled by the submarine drills on the drill barge. The operations of the well drills and of the submarine drills in the same formation demonstrated the relative speed of boring of both types of machines.

During nine months of operations with an average number of 11.2 drills working, the total linear feet drilled was 52,777; of this footage, 19,756 feet, or about thirtyseven and one-half per cent., were through the overlay of earth.

Assuming twenty-six eight-hour days to the month, the performance of each drill averaged 4712.2 feet in nine months, which would equal 523.5 feet per month and 2.52 feet per drill per hour as the average work through earth and rock. The submarine percussion drills averaged in two years' work thirteen feet per drill per hour.

This speed of penetration of the submarine drill is far in excess of the performance of any other type employed on the canal.

The Ingersoll-Rand Company supplied the majority of the drills used on the canal works. This same firm supplied 300 tripod drills of three and one-quarter-inch piston diameter to the French Company. For the commission the firm supplied 184 drills of three and five-eighth-inch piston diameter. Of this number twenty-five were of the tappet valve type, the remainder of the air thrown valve type, or the auxiliary valve form. Fifty drills, tripod mounted, of four and one-half-inch piston diameter, twenty-five of each being

of the air thrown and tappet valve form, were also furnished. These drills, because of their weight, were not as handy as the three and five-eighth-inch size, any gain due to their greater striking force being somewhat offset by the inconvenience in shifting position in starting new holes. Of the smaller tripod machines there were fourteen of two and one-half-inch piston diameter and fifty-eight of the two and onequarter-inch piston diameter. In addition there were two very heavy tripod drills of five-inch piston diameter, and six unmounted submarine drills of five and onehalf-inch piston diameter used on the drill barge in the Pacific entrance of the canal. A rock drill, in common with all other mechanical means of converting energy into useful work, will perform an amount of work that is constant where the factors of force, time, speed, resistance and other influencing causes remain constant. In the actual cutting of a drill hole the force expended is in direct proportion to the area of the hole, and the time of cutting in inverse proportion. An increase in the number of blows in a given time affects a corresponding increase in penetration. A decrease in the area of the hole augments this in proportion to the lessened area. Increase of force to the blows will proportionately further increase the cutting speed. These are the fundamental theoretical conditions governing the performance of a percussion rock drill.

Drill holes range from less than one-half inch in diameter to eight inches or more, as in well drilling. The highest speed of striking is in the small hammer drills, which strike upward of 2,000 blows per minute. The striking speed successively diminishes from the tripod mounted hammer drill, the smaller to larger sizes of direct steam or air-actuated drills, to the largest size of submarine drills striking about 250 blows a minute, and culminating in the slow acting well drill striking less than sixty blows a minute.

The striking force of the different types runs from the light rapid blows of the small

hammer drills to the powerful steam-hammer-like blows of the largest subaqueous drilling machines.

To obtain reliable records of the performance of any type or class of drilling machine it is essential that more or less stable conditions obtain in the formations drilled. The extreme variations in formation and the heterogeneous composition of the rock masses, on the isthmus, brought about by the effects of volcanoes and earthquakes, heat and cold, coral formations, water and other agents of denudation and transport and the sequence of natural phenomena, permit only of most general deductions based on the average collective performance of numbers of drills working in constantly changing and differing conditions. These general conclusions are evident from what has already been stated and require no and require no elucidation for their apprehension.

It may be affirmed that as an economic factor the rock drill was indispensable to the creation of the canal, and its work the physical requisite preliminary to the disruption and displacement of the vast masses upheaved by the great natural convulsion of the surface of the earth which formed the barrier to the reëstablishment of an interoceanic passageway connecting the Atlantic and Pacific as existent in geologic time.

The first use of compressed air to any extent as a means of power transmission was for the operation of drills in mines and tunnels. This followed the development of the steam drill. The superior efficiency of the steam drill as compared to hand drilling having been demonstrated, its use in mining work was advocated, and the use of steam being obviously objectionable, some other elastic fluid had to be utilized, with the natural result that the air compressor was developed to meet the demand for a supply of this fluid in the form of compressed air, which not only has none of the disadvantages of steam, but instead, positive advantages by way of added ventilating effect, allowing the machines to re