DISTRIBUTION OF GAS THROUGH MAINS. 1 83 



since the expense of laying down a pipe of half an inch more than the exact 

 diameter will be scarcely felt, and for which the decrease of leakage will 

 amply compensate. For the discharge of the quantity of gas through a 

 sixteen-inch pipe given in the Tables, probably a pipe of fifteen inches diameter 

 would serve, and this would be the extent of the variation. 



In mains rising above the horizontal line the quantity of gas delivered by 

 them will be greater, and in mains falling below that line it will be less. 

 In the first instance, the resistance offered to the flow of gas by the atmo- 

 spheric pressure will be lessened, and in the latter it will be increased, and 

 will cause a difference in the necessary pressure for the discharge of the gas 

 of one-tenth of an inch head of water for every ten feet rise or fall. For 

 example, if a six-inch main one mile long laid horizontally, supply 1963 cubic 

 feet of gas in an hour, with a pressure of five-tenths of an inch head of 

 water, it would supply the same quantity with a pressure of four-tenths of an 

 inch at an elevation of ten feet above the level of the curb of the gas-holder ; 

 and likewise, if the point to be supplied were ten feet below the works, the 

 pressure given must be equal to six-tenths of an inch. From this it is 

 obvious, that sections of the town to be lighted are necessary, as well as a 

 map. The quantity of elevation or depression of the main must be measured 

 from a horizontal line drawn at the level of the governor to the end of that 

 main, because the differences of altitude at any intermediate parts of the 

 length will counteract one another. 



The effect of bends and angles in the main, upon the quantity of gas de- 

 livered, is essentially a matter of experiment : they may be considered as so 

 many mechanical obstructions. The results of the following experiments will 

 show, in some measure, what allowance to make for quadrant, semicircular, 

 and right-angle bends. A two-inch pipe thirty feet long, perfectly horizontal 

 and free from obstructions, delivered 2898 cubic feet of gas in one hour, with 

 a pressure of five-tenths of an inch head of water. The same pipe, discon- 

 nected in the middle of its length, and returned by a semicircular bend to the 

 point at which it left the gasometer, delivered 2754 cubic feet in the same 

 time, being a difference of nearly one-twentieth in the whole quantity. The 

 semicircular bend was removed and a quadrant bend substituted, making the 

 two fifteen-foot lengths of pipe form a right angle with one another ; the quan- 

 tity delivered was 2834 cubic feet in the hour, a difference of about -^th of 

 the first discharge. Again, the pipes were disconnected, and a right-angle 



