GAS, COAL 



593 



These dimensions are applicable to the case where the body of gas is transmitted 

 through pipes without being let off in its way by burners, that is, to the mains which 

 conduct the gas to the places where it is to be used. If the main sends off branches 

 for burners, then for the same length the diameter may be reduced, or for like 

 diameter the length may be greater. For example, if a pipe of 5'32 inches which 

 transmits 2,000 cubic feet through a length of 2,000 feet, gives off, in this space, 1,000 

 cubic feet of gas ; then the remainder of the pipe, having the same diameter, can 



continue to transmit the gas through a length of 2,450 feet = ( 1 V, with undi- 



minished pressure for the purposes of lighting. Inversely, the diameter should be 

 progressively reduced in proportion to the number of jets sent off in the length of 

 the pipe. 



Suppose, for instance, the gasholder to discharge 2,000 cubic feet per hour, and the 

 last point of the jets to be at a distance of 4,000 feet. Suppose also that from the 

 gasholder to the first point of lighting, the gas proceeds through 1000 feet of close pipe 

 the diameter of the pipe will be here 4'47 inches; in the second 1,000 feet of length, 

 suppose the pipe to give off, at equal distances, 1,000'cubic feet of gas, the diameter in 

 this length (calculated at 1,500 cubic feet for 1,000 feet long) = 3-87 inches; in the 

 third extent of 1,000 feet, 600 cubic feet of gas will be given off, and the diameter 

 (reckoning 700 cubic feet for 1,000 feet long) will be 2-65 inches ; in the fourth and 

 last space (for 200 cubic feet in 1,000 feet long) the pipe has a diameter of only an 

 inch and a half, for which, in practice, a two-inch cast-iron pipe is substituted ; this 

 being the smallest used in mains, into which branch pipes can be conveniently 

 inserted. 



The same relations hold with regard to branch pipes through which the gas is 

 transmitted into buildings and other places to be illuminated. If such pipes make fre- 

 quent angular turnings, whereby they retard the motion of the gas, they must be a 

 third or a half larger in diameter, The smallest tubes of distribution are never less 

 than one fourth of an inch in the bore. 



Where, from one central gas-work, a very great quantity of light is required in 

 particular localities, there ought to be placed near these spots gasholders of distribu- 

 tion, which being filled during the slack hours of the day are ready to supply the 

 burners at night without making any considerable demand upon the original main 

 pipe. Suppose the first main be required to supply 8,000 cubic feet in the hour, for 

 an illumination of 8 hours, at the distance of 2,000 feet, a pipe lOf inches in diameter 

 would be necessary ; but if two or three gasholders of distribution, or station gas- 

 holders be had recourse to, into which the gas during the course of 24 hours would 

 flow through the same distance continuously from the central gas-works, the quantity 

 required per hour from them would be only one third of 8,000 = 2666-6 cubic feet; 

 consequently, the diameter for such a pipe is only 6*15 inches. 



All the principal as well as branch pipes, whose interior diameter exceeds an inch 

 and a half, are made of cast iron, from 6 to 8 feet long, with elbow pipes cast in them 

 where it is necessary. These pipe-lengths are shown in fig. 1055, having at one end 

 a wide socket a, and at the other a nozzle b, which fits the former. After inserting 

 the one in the other in their proper horizontal position, a coil of hemp, soaked with ' 

 tar is driven home at the junction; then a luting of clay^ is applied at the mouth, 

 within which a ring of lead is cast into the socket, which is driven tight home with a 

 mallet and blunt chisel. 



The pipes should be proved by a force-pump before being received into the gas- 

 works ; two or three lengths of them should be joined before laying them down, and 

 tliey should be placed at least two feet below the surface, to prevent their being 

 affected by changes of temperature, which would loosen the joints. The tubes for 



VouII. QQ 



