Chelsea was on fire again. Already the scene of one of the biggest fires in the history of the country in 1908, the city was engulfed anew on Oct. 14, 1973. A new inferno had begun just a few hundred yards from the original fire. The scope of the conflagration is difficult to imagine, with 18 city blocks burned right down to the ground. More than 600 people lost their jobs when businesses went up in flames along with homes as more than 300 buildings burned. Help from 67 Massachusetts fire departments poured into Chelsea, and with a mile-long area on fire, there was plenty of work to go around. No one was killed.