A fleet of six Cambridge fire trucks hustled over to Mather Hall last night at 9 p.m. with sirens screaming. 'While a ladder was raised to the roof, and while students were shouting "I smell it but can't see it," firemen spent five minutes in locating and dousing the blase. The undramatically small leaf are, found in a window well next to the entrance of Mather Hall C, was probably caused by a misplaced cigarette.