942 THE COLLIERY GUARDIAN. May 19, 1916. _________________________________________________________________________________________________________________________________ single-pole circuit breakers, an ammeter, and a double- pole switch arranged to be locked open. If the source of power is close by the mine, the shot- firing panel may well be placed in the power house. If this is not feasible, or if the source of power is too far from the mine, the shot-firing panel should be provided with suitable housing, and with means for disconnect- ing it entirely from the source of power. Distributing Circuits. The conductors of distributing circuits should always be covered with insulating material, and unless the mine is dry, rubber-insulated wire should be used. The size of the wire depends upon the available firing voltage, the number of detonators to be shot, the method of con- necting the detonators, and the length and arrangement of the circuits. A change in any of these factors will, if the conductors have no factor of safety in their dimen- sions, make a difference in the size of wire that it is desirable to use. Therefore, it is necessary to assume values for some of the variables entering into the pro- blem ; and in order to make recommendations that will be generally applicable, the values must be assumed great enough to cover the most difficult case. The Bureau recommends that shot-fining conductors in rooms be not smaller in size than No. 14 (B. and S.) gauge, those in cross entries not smaller than No. 10 (B. and S.) gauge, and those used between entries not smaller than No. 8 (B. and S.) gauge. The recom- mendations for the size of wire for the remainder of the shot-firing circuit—the, line connecting the source of power to the first entry to be shot—are based upon the assumption that these above-mentioned minimum values are used and upon the following assumption as to the dimensions and arrangement of the mine entries and rooms:—Maximum length of cross entry, 3,000ft.; distance between cross entries, 500 ft.; number of pairs of cross entries, six; maximum length of rooms, 400 ft.; distance between rooms, 50 ft.; maximum number of shots to be fired, 800. It is further assumed that three shots will be fired in each room, and that the shots will be connected in multiple, as the Bureau recommends. Lengths of various sizes of wire between source of power and first entry allowable when shooting with 250 volts and 500 volts are given in the table :— Allowable Length of Whies. Distance from source of power to first entry for Size of wire (----------A-----------> (B. & S. gauge). 250 volts. 500 volts. 8 .... 1,500 .... 4,500 6 .... 2,500 .... 7,000 4 .... 4,000 .... 11,000 The allowable length may be increased somewhat if the cross entries are less than 3,000 ft. long, if the rooms are less than 400 ft. long, or if wires larger than those assumed are used in the rooms, in the entries, or between the entries. A decrease in the number of shots will not increase materially the allowable length of con- ductors between the first entry and the source of power. Insulation of Conductors. This will depend on the voltage used for firing and on the conditions of the mine as to dampness. In order to ensure satisfactory operation, the conductors of the main circuit should be installed on insulators. The voltage on the branch circuits will rarely exceed about 100 volts, except for the brief interval between the discharge of the shots and the opening of the firing switch in the shot-firing cabin. Therefore, it is probable that systems which do not use insulators on the branch circuits will operate successfully under favourable conditions. Except for the interval mentioned above, the potential in the rooms will not exceed 5 volts approximately, and therefore porcelain or glass insulators are not necessary if the rooms are dry and the wires are supported on dry wood. It is desirable to instal the shot-firing conductors in entries where there are no other wires, and wherever the conductors of the shot-firing system cross live wires adequate precaution should be taken to prevent leakage of current to the shot-firing wires. An excellent method that has been adopted in some mines to prevent such leakage is the installation of switches in the shot-firing circuit on either side of any other circuit that it crosses. These switches can be kept open ordinarily, preferably by locking in that position, and closed when the rest of the switches in the underground system are closed. Power Wires not Recommended. The use of power wires as part of the shot-firing system is not recommended because it precludes the use of safeguards that the Bureau engineers consider desirable, thus reducing the factor of safety of the system. If power wires are used, it is scarcely practic- able to have more than one open switch between the detonator and the firing system, unless switches are for installing the conductors and that competent wiro- men are available for doing the work. On this basis the cost of an outside shot-firing equip- ment, exclusive of the source of power, but including a shot-firing panel, lightning gap, eight cross entry switches, the necessary surface main line, underground main line, underground branch circuits, 150 room equip- ments, and labour of installation has been estimated as follows for a mine like that assumed, equipped as just stated :—■ Dois. Complete with rubber-covered wire and room switches ........................about 1,700 Complete with rubber-covered wire without room switches ...................about 1,500 Complete with weatherproof wire and room switches .......................about 1,400 Complete with weatherproof wire and no room switches .........................about 1,200 installed at the entrance of each room. Thus the dis- charge of shots during working hours is possible, and has been reported. The use of power wires also precludes the use of a large gap in the circuit as a protection against lightning discharges, which gap is recommended as being simpler and more effective than lightning arresters. Protection Against Lightning. A suitable gap may be provided by inserting in each side of ‘the circuit, near the bottom of the shaft or the slope, or about 100 ft. inbye from the mouth of the drift, a flexible conductor about 6 ft. long. Each of these conductors can be permanently joined to the end of one of the solid incoming conductors, and the other end of each flexible conductor can be provided with a lug for fastening to one of a pair of screw studs, in which the _____________________________ outcoming solid conductors should terminate. A similar pair of dummy studs should be installed 6 ft. distant from the live studs, so as to provide a place for fastening the flexible conductors out of circuit. The dummy studs should be provided with a locked clamp or similar device for locking the flexible conductors out of circuit. Shooting from Grounded System. It is dangerous to connect one side of a detonator to earth before it is to be fired. Shooting from a grounded system does not necessitate making a connection of this sort, because connection between the source of power and the shot-firing system should not be made until it is time to fire the shots. There is therefore no objection to firing from a grounded system if the underground shot-firing circuits are entirely separate from the power circuits. Switches. It is recommended that between the detonators and the source of power there be at least four switches, or their equivalent, .situated as follows :—One at -each cross entry, to protect the entry from any current leakage that might take place at any other point, a 6 ft. gap at the mine entrance to. protect against lightning and to form one of three breaks in the main circuit; one for connect- ing the shot-firing system to the source of power; and one on the shot-firing panel for finally closing the circuit and causing the shots to discharge. If there are power wires on the cross entries beyond the entry switches, means should be provided for opening the firing circuit at each room. This recommendation is made to protect the miner from the effects of leakage between the entry power wires and the shot-firing circuit. All switches, except room switches, should be designed so that they can be locked open. Means should be provided to ensure that power is applied to the shot-firing system only once, and for a brief interval (not to exceed three seconds). If the shots will fire at all, they will do so within three seconds from the time the switch is closed. A second closing of the firing switch serves no useful purpose, and may cause flashes in the rooms if the conductors of the firing circuits have been brought in contact as the result of the shooting. Operation of the System. The failure of shots to fire appears to be due largely to the fact that the electrical connection between room wires and detonators is not properly made. It is there- fore recommended that these connections be made, after the miners have left the mine, by men specially dele- gated for the duty. This practice has been followed in some mines with satisfactory results. Experiments that the Bureau has just completed indicate that fewer failures to fire will occur if the deto- nators are connected in multiple in the rooms than if they are connected in series. Operation of Switches. The operation of all underground shot-firing switches should be in the hands of duly authorised men, who should attend to this duty after they have connected the detonators. It is recommended that one man be made responsible for the operation and upkeep of the shot-firing system, and that all other men having duties in connection with the system be under his supervision. After these men have left the mine they should examine the check board to ensure that all men are out of the mine before the switch that connects the shot-firing system to the source of power is closed. The shot- firing panel may then be connected to the source of power, and the shots fired by closing the switch on the shot-firing panel. This switch should be closed only once, and should not be kept closed for more than five seconds. All working places where shots have been fired should be examined for fire and missfires as soon as possible after the shots have been fired. All switches in the shot-firing system should be opened before the miners are allowed to go into the mine. The man who fires the shots should, before going off duty, record in a book provided for the purpose the number and location of all shots that have failed to fire. Cost of Installation. In order to give a concrete example of the cost of installing an outside shot-firing system, the following assumptions have been made :—-Number of active cross entries, 8; number of rooms to be wired, 150; distance from shot-firing panel to pit mouth, 500 ft.; distance from pit mouth to first entry, 2,000 ft.; distance from first entry to face, 2,000ft.; average length of entries, 1,000 ft.; average length of rooms, 200ft. It is also assumed that the character of the roof or side of entries is such as to offer favourable conditions HORSE v. MOTOR LORRIES. The scarcity of men and horse-drawn lorries is causing increased attention to be directed to- the advantages resulting from the employment of motor lorries for delivering heavy goods. In the coal trade these vehicles have already been adopted to some degree in this country and more extensively in America, where opinion is generally in their favour, especially for districts where the roads are well paved or metalled and the journeys are long. With regard to the size of the lorries, the consensus of opinion seems in favour of one to two tons capacity, though even larger sizes are considered more economical than horse haulage under favourable condi- tions. For large steam contracts and quick delivery, or where the coal is delivered to domestic consumers through chutes in the street pavement, the motor lorry is a great advantage, though it is regarded as more diffi- cult to get in and out of yards where the coal has to be delivered into cellars. Some users, too, look upon the motor lorry as a good advertisement, even though the cost of running should be a little higher than horse traction. With regard to the question of ,cost, however, it is interesting to note that, according to the following data collected some few months ago by Arnold, the motor lorry costs less to operate than horse-drawn vehicles. A manufacturing firm making light and heavy machinery desired to ascertain whether three horse- drawn trucks could be supplanted by one automobile truck, and what advantages would accrue financially, or otherwise, if the change were made. A careful analysis of the existing conditions established the fact that a five-ton motor lorry would best meet the requirements of the service in question. Several demon- strations were arranged, a careful log being kept of the performance of the automobile trucks and of the horse- drawn vehicles. A comparison of the summaries of the two logs showed that the motor truck had 73'66 ton-miles to its credit, while the corresponding figure for one horse-drawn truck, during practically the same amount of time, was 23*25 ton-miles. In the case of the three two-horse- drawn trucks this figure would be 69*75 ton-miles. These results indicated that, from the standpoint of material handled, one motor truck has a slight advantage over the three horse-drawn trucks. This conclusion was further borne out by the results obtained from other demon s t r a t ion s. In order to arrive at an understanding regarding the financial side of the question, a comparison was made of the initial investments, annual fixed charges, and operating costs. The prime cost of the motor truck was 4,800 dols., and that of three two-horse trucks (including six horses) 3,154 dols. The total fixed charges were 849*80 dols. and 645*94 dols. respectively, and operating costs totalled 2,525 dols. and 5,240dols. respectively, the total annual cost showing a saving, in favour of the automobile, truck, of 2,511 dols., an amount that would more than pay the additional initial investment required for its purchase. This saving is largely made possible by the fact that, in the substitution of the motor truck for the horse-drawn equipment, two drivers and helpers will be dispensed with, resulting in an annual saving of 2,420 dols. in labour alone. The following table shows the daily cost of operation, the cost of operation per ton of material handled, the cost of operation per mile of material hauled, and the cost of operation per ton-mile performed by the motor truck compared with the horse-drawn equipment. The saving effected in each of these items by substituting the motor truck for the horse-drawn equipment is also shown. Cost per day. Dols. Cost per Cost per Cost per ton mile. Dols. ton. Do’s. mile. Dols. Five-ton motor truck 10*82 . .. 0’22 . .. 0'60 ... 0*146 One two-hors -truck... 6’29 . .. 0*295 . .. 0*632 ... 0*227 Three two-horse trucks 18*86 — — — Saving by using motor truck 8*04 . .. 0*075 . .. 0*032 ... 0*081 Other advantages that would be gained by using the automobile truck in place of the horse-drawn vehicles are : Saving in wear and tear of the shop floors and streets, greater cleanliness of same, less housing facilities required, thus permitting the greater part of the present stables to be torn down or used for other purposes, power- driven auxiliary equipment on truck to assist in loading and unloading heavy material, and ability to make occasional long hauls not possible with horse-drawn equipment. The investigation of the transportation conditions at the plant under discussion showed that a material advantage, both financially and otherwise, would result from substituting one five-ton motor truck for the three two-horse-drawn trucks previously employed.