DISTRIBUTION OF GAS THROUGH MAINS. 167 



nearest approach to such a situation is advisable for obvious reasons. The 

 erection of the works upon a marshy ground will be attended with more 

 expense, but this is of minor importance to the attainment of a good situa- 

 tion with respect to the level. A map of the town must be obtained, or a 

 survey made of the different streets and thoroughfares ; running levels must 

 be taken through them at several points, and their respective heights marked 

 with reference to the level of the works as a datum ; upon this map ah 1 the 

 mains must be drawn, also their branches, valves, and governors. Their 

 arrangement must be such as to allow of a perfect circulation of the gas, and 

 a nearly uniform pressure at the highest and lowest point. All the pipes 

 upon the same level should be joined into one another, and no valves used 

 but such as are necessary to shut off the gas for repair of mains. To supply 

 a higher level a governor should be placed at the summit of the lower level, 

 with the lower main leading into it. The pipe or pipes for supplying the 

 higher parts should proceed from the regulating vessel. A cellar may be ap- 

 propriated for the reception of this vessel. One leading main should be 

 taken direct from the works to an equilibrium cylinder situated at some point 

 from which several streets diverge, and no supply taken from this main until 

 it reaches the cylinder. Branches suitable to the supply of each division of 

 the district should lead from this cylinder. The supply of gas to the cylinder 

 should be so regulated as to cause the gas to flow along the branches at an 

 even pressure of about five-tenths of an inch. If the cylinder be at any con- 

 siderable distance from the works, a smaller main with increased pressure may 

 lead to it, its size being sufficient to equalize the discharge. 



Supposing a district were to be lighted requiring 1000 public or street 

 lamps, and 7000 private burners ; it is considered that each lamp on an 

 average will consume five cubic feet of gas every hour, therefore 40,000 cubic 

 feet will be required to light the district for an hour, and the leading main 

 must be capable of delivering that quantity into the equilibrium cylinder in 

 that time. We will suppose that the cylinder is situated at the distance of 440 

 yards from the gasometers in the works. By referring to the Tables, it will 

 be seen that a twelve-inch main, with a pressure of three inches, will serve. 

 But in determining the size of this main the probable increase of lamps must 

 be taken into consideration ; and as that will depend so much upon circum- 

 stances in every instance, the judgment of the engineer alone can serve to 

 regulate the additional area. If the increase should be beyond that which 



