STREET-MAINS. 1 59 



STREET-MAINS. 



THE term main is applied to all cast-iron conduit-pipes that serve to convey 

 gas from the works to the place or district to be lighted, and especially applied 

 to those pipes from which smaller ramifications branch. The diameters of the 

 mains vary from 1^ to 15 or 18 inches, according to the quantity of gas re- 

 quired to be supplied, and the distance it has to flow. 



The 1^-inch mains are cast four feet six inches long, the two and three- 

 inch mains about six feet long, and all the other sizes nine feet, with a socket 

 at one end, and a plain bead at the other. 



When they arrive at the works, before they are suffered to be laid, every 

 length must be proved ; that is, water should be forced into them until the 

 internal pressure is equal to a column of water 250 or 300 feet high, and if 

 any moisture appears on the outside having a certain direction or shape, the 

 length must be rejected ; for although so minute a fissure may at first be gas- 

 tight, yet after being exposed under the ground to wet and frost, it would 

 soon increase and become leaky. 



In warm weather, if the pipe is proved by cold water, a dew-point will be 

 formed, which, by condensing the vapour in the atmosphere upon the outside 

 of the pipe, will often deceive the workmen, and lead to the rejection of those 

 that are sound. It is therefore necessary during the summer months to 

 prove pipes with water of the same temperature as the atmosphere. 



An experienced workman in the habit of proving pipes will distinguish a 

 perfect pipe from a faulty one with considerable correctness, by the difference 

 in sound produced by blows of a hammer. A perfect pipe will ring when 

 struck, while that having a crack will jar; by this means also a difference in 

 the thickness of metal may be detected. 



Sockets. 



Nos. 1, 2 and 3, in the adjoining figures, represent the sections of sockets 

 of different-sized pipes to a scale of three inches to one foot. No. 1. is that 



